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Oracle® Call Interface Programmer's Guide
10g Release 1 (10.1)

Part Number B10779-01
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OCI Programming Advanced Topics

This chapter contains these topics:

Overview of OCI Multithreaded Development

Threads are lightweight processes that exist within a larger process. Threads share the same code and data segments but have their own program counters, machine registers, and stacks. Global and static variables are common to all threads, and a mutual exclusivity mechanism is required to manage access to these variables from multiple threads within an application.

Once spawned, threads run asynchronously with respect to one another. They can access common data elements and make OCI calls in any order. Because of this shared access to data elements, a synchronized mechanism is required to maintain the integrity of data being accessed.

The mechanism to manage data access takes the form of mutexes (mutual exclusivity locks), that is implemented to ensure that no conflicts arise between multiple threads accessing shared. In OCI, mutexes are granted for each environment handle.

The thread safety feature of the Oracle database server and OCI libraries allows developers to use the OCI in a multithreaded environment. Thread safety ensures code can be reentrant, with multiple threads making OCI calls without side effects.


Thread safety is not available on every operating system. Check your Oracle system-specific documentation for more information.

In a multithreaded UNIX environment, OCI calls are not allowed in a user signal handler, except for OCIBreak().

Advantages of OCI Thread Safety

The implementation of thread safety in OCI has the following advantages:

OCI Thread Safety and Three-Tier Architectures

In addition to client/server applications, where the client can be a multithreaded program, a typical use of multithreaded applications is in three-tier, or client-agent-server, architectures. In this architecture the client is concerned only with presentation services. The agent, or application server, processes the application logic for the client application. Typically, this relationship is a many-to-one relationship, with multiple clients sharing the same application server.

The server tier in this scenario is a database. The applications server, or agent, is very well suited to being a multithreaded application server, with each thread serving a single client application. In an Oracle environment this application server is an OCI or precompiler program.

Implementing Thread Safety

In order to take advantage of thread safety, an application must be running on a thread-safe operating system. The application specifies that it is running in a multithreaded environment by making an OCIEnvNlsCreate() call with OCI_THREADED as the value of the mode parameter.

All subsequent calls to OCIEnvNlsCreate() must also be made with OCI_THREADED.


Applications running on non-thread-safe operating systems must not pass a value of OCI_THREADED to OCIInitialize() or OCIEnvNlsCreate().

If an application is single-threaded, whether or not the operating system is thread-safe, the application must pass a value of OCI_DEFAULT to OCIInitialize() or OCIEnvNlsCreate(). Single-threaded applications that run in OCI_THREADED mode may incur lower performance.

If a multithreaded application is running on a thread-safe operating system, the OCI library will manage mutexes for the application for each environment handle. An application can override this feature and maintain its own mutex scheme by specifying a value of OCI_NO_MUTEX in the mode parameter of the OCIEnvCreate() call.

The following scenarios are possible, depending on how many connections exist in each environment handle, and how many threads are spawned in each connection.

  1. If an application has multiple environment handles, with a single thread in each, mutexes are not required.
  2. If an application running in OCI_THREADED mode maintains one or more environment handles, with multiple connections, it has these options:
    • Pass a value of OCI_NO_MUTEX for the mode of OCIEnvNlsCreate(). The application must mutex OCI calls made on the same environment handle. This has the advantage that the mutex scheme can be optimized to the application design. The programmer must also insure that only one OCI call is in process on the environment handle connection at any given time.
    • Pass a value of OCI_DEFAULT for the mode of OCIEnvNlsCreate(). The OCI library automatically gets a mutex on every OCI call on the same environment handle.


      The bulk of processing of an OCI call happens on the server, so if two threads using OCI calls go to the same connection, then one them can be blocked while the other finishes processing at the server.

      Use one error handle per thread in an application, since OCI errors can be over-written by other threads.

Mixing 7.x and Later Release OCI Calls

If an application is mixing later release and 7.x OCI calls, and the application has been initialized as thread-safe (with the appropriate calls of the later release), it is not necessary to call opinit() to achieve thread safety. The application will get 7.x behavior on any subsequent 7.x function calls.

The OCIThread Package

The OCIThread package provides a number of commonly used threading primitives. It offers a portable interface to threading capabilities native to various operating systems, but does not implement threading on operating systems that do not have native threading capability.

OCIThread does not provide a portable implementation, but it serves as a set of portable covers for native multithreaded facilities. Therefore, operating systems that do not have native support for multithreading will only be able to support a limited implementation of the OCIThread package. As a result, products that rely on all of OCIThread's functionality will not port to all operating systems. Products that must port to all operating systems must use only a subset of OCIThread's functionality.

The OCIThread API consists of three main parts. Each part is described briefly here. The following subsections describe each in greater detail.

Initialization and Termination

The types and functions described in this section are associated with the initialization and termination of the OCIThread package. OCIThread must be initialized before any of its functionality can be used.

The observed behavior of the initialization and termination functions is the same regardless of whether OCIThread is in single-threaded or multithreaded environment. Table 9-1 lists functions for thread initialization and termination.

Table 9-1 Initialization and Termination Multithreading Functions  
Function Purpose


Performs OCIThread process initialization.


Initializes OCIThread context.


Terminates the OCIThread layer and frees context memory.


Tells the caller whether the application is running in a multithreaded environment or a single-threaded environment.

See Also:

"Thread Management Functions" for complete descriptions of each thread function

OCIThread Context

Most calls to OCIThread functions use the OCI environment or user session handle as a parameter. The OCIThread context is part of the OCI environment or user session handle and it must be initialized by calling OCIThreadInit(). Termination of the OCIThread context occurs by calling OCIThreadTerm().


The OCIThread context is an opaque data structure. Do not attempt to examine the contents of the context.

Passive Threading Primitives

The passive threading primitives deal with the manipulation of mutex, thread ID's, and thread-specific data. Since the specifications of these primitives do not require the existence of multiple threads, they can be used both in multithreaded and single-threaded operating systems. Table 9-2 lists functions used to implement passive threading.

Table 9-2 Passive Threading Primitives  
Function Purpose


Allocates and initializes a mutex.


Destroys and deallocates a mutex.


Acquires a mutex for the thread in which it is called.


Releases a mutex.


Allocates and initializes a key.


Destroys and deallocates a key.


Gets the calling thread's current value for a key.


Sets the calling thread's value for a key.


Allocates and initializes a thread ID.


Destroys and deallocates a thread ID.


Sets on thread ID to another.


Nulls a thread ID.


Retrieves a thread ID for the thread in which it is called.


Determines if two thread IDs represent the same thread.


Determines if a thread ID is NULL.


The OCIThreadMutex datatype is used to represent a mutex. This mutex is used to ensure that:

Mutex pointers can be declared as parts of client structures or as stand-alone variables. Before they can be used, they must be initialized using OCIThreadMutexInit(). Once they are no longer needed, they must be destroyed using OCIThreadMutexDestroy().

A thread can acquire a mutex by using OCIThreadMutexAcquire(). This ensures that only one thread at a time is allowed to hold a given mutex. A thread that holds a mutex can release it by calling OCIThreadMutexRelease().


The datatype OCIThreadKey can be thought of as a process-wide variable with a thread-specific value. This means that all threads in a process can use a given key, but each thread can examine or modify that key independently of the other threads. The value that a thread sees when it examines the key will always be the same as the value that it last set for the key. It will not see any values set for the key by other threads. The datatype of the value held by a key is a dvoid * generic pointer.

Keys can be created using OCIThreadKeyInit(). Key value are initialized to NULL for all threads.

A thread can set a key's value using OCIThreadKeySet(). A thread can get a key's value using OCIThreadKeyGet().

The OCIThread key functions will save and retrieve data specific to the thread. When clients maintain a pool of threads and assign them to different tasks, it may not be appropriate for a task to use OCIThread key functions to save data associated with it.

Here is a scenario of how things can fail: A thread is assigned to execute the initialization of a task. During initialization, the task stores data in the thread using OCIThread key functions. After initialization, the thread is returned back to the threads pool. Later, the threads pool manager assigns another thread to perform some operations on the task, and the task needs to retrieve the data it stored earlier in initialization. Since the task is running in another thread, it will not be able to retrieve the same data. Application developers that use thread pools have to be aware of this.


OCIThreadKeyDestFunc is the type of a pointer to a key's destructor routine. Keys can be associated with a destructor routine when they are created using OCIThreadKeyInit(). A key's destructor routine will be called whenever a thread with a non-NULL value for the key terminates. The destructor routine returns nothing and takes one parameter, the value that was set for key when the thread terminated.

The destructor routine is guaranteed to be called on a thread's value in the key after the termination of the thread and before process termination. No more precise guarantee can be made about the timing of the destructor routine call; no code in the process may assume any post-condition of the destructor routine. In particular, the destructor is not guaranteed to execute before a join call on the terminated thread returns.


OCIThreadId datatype is used to identify a thread. At any given time, no two threads will ever have the same OCIThreadId, but OCIThreadId values can be recycled; once a thread dies, a new thread may be created that has the same OCIThreadId value. In particular, the thread ID must uniquely identify a thread T within a process, and it must be consistent and valid in all threads U of the process for which it can be guaranteed that T is running concurrently with U. The thread ID for a thread T must be retrievable within thread T. This is done using OCIThreadIdGet().

The OCIThreadId type supports the concept of a NULL thread ID: the NULL thread ID will never be the same as the ID of an actual thread.

Active Threading Primitives

The active threading primitives deal with manipulation of actual threads. Because specifications of most of these primitives require multiple threads, they work correctly only in the enabled OCIThread; In the disabled OCIThread, they always return an error. The exception is OCIThreadHandleGet(); it may be called in a single-threaded environment and has no effect.

Active primitives can only be called by code running in a multithreaded environment. You can call OCIThreadIsMulti() to determine whether the environment is multithreaded or single-threaded. Table 9-3 lists functions used to implement active threading.

Table 9-3 Active Threading Primitives  
Function Purpose


Allocates and initializes a thread handle.


Destroys and deallocates a thread handle.


Creates a new thread.


Allows the calling thread to join with another.


Closes a thread handle.


Retrieves a thread handle.


Datatype OCIThreadHandle is used to manipulate a thread in the active primitives: OCIThreadJoin() and OCIThreadClose(). A thread handle opened by OCIThreadCreate() must be closed in a matching call to OCIThreadClose(). A thread handle is invalid after the call to OCIThreadClose().

Connection Pooling in OCI

Connection pooling is the use of a group (the pool) of reusable physical connections by several sessions, in order to balance loads. The management of the pool is done by OCI, not the application. Applications that can use connection pooling include middle-tier applications for Web application servers and e-mail servers.

A sample usage of this feature is in a Web application server connected to a back-end Oracle database. Suppose that a Web application server gets several concurrent requests for data from the database server. The application can create a pool (or a set of pools) in each environment during application initialization.

OCI Connection Pooling Concepts

Oracle has several transaction monitor capabilities such as the fine-grained management of database sessions and connections. This is done by separating the notion of database sessions (user handles) from connections (server handles). Using these OCI calls for session switching and session migration, it is possible for an application server or transaction monitor to multiplex several sessions over fewer physical connections, thus achieving a high degree of scalability by pooling of connections and back-end Oracle server processes.

The connection pool itself is normally configured with a shared pool of physical connections, translating to a back-end server pool containing an identical number of dedicated server processes.

The number of physical connections is less than the number of database sessions in use by the application.The number of physical connections and back-end server processes are also reduced by using connection pooling. Thus many more database sessions can be multiplexed.

Similarities and Differences from Shared Server

Connection pooling on the middle-tier is similar to what shared server offers on the back end. Connection pooling makes a dedicated server instance behave like a shared server instance by managing the session multiplexing logic on the middle tier.

The pooling of dedicated server processes including incoming connections into the dedicated server processes is controlled by the connection pool on the middle tier. The main difference between connection pooling and a shared server is that in the latter case, the connection from the client is normally to a dispatcher in the database instance. The dispatcher is responsible for directing the client request to an appropriate shared server. On the other hand, the physical connection from the connection pool is established directly from the middle-tier to the dedicated server process in the back-end server pool.

Connection pooling is beneficial only if the middle tier itself is multithreaded. Each thread can maintain a session to the database. The actual connections to the database are maintained by the connection pool and these connections (including the pool of dedicated database server processes) are shared among all the threads in the middle tier.

Stateless Sessions Versus Stateful Sessions

Stateless sessions are serially reusable across mid-tier threads. After a thread is done processing a database request on behalf of a three-tier user, the same database session can be reused for the purpose of a completely different request on behalf of a completely different three-tier user.

Stateful sessions to the database, on the other hand, are not serially reusable across mid-tier threads because they may have some particular state associated with a particular three-tier user. Examples of such state may be: open transactions, fetch state from a statement, PL/SQL package state, and so on. This makes the session not reusable for a different request for the duration that such state persists.

Note: Stateless sessions too may have open transactions, open statement fetch state, and so on. However, such a state persists for a relatively short duration (only during the processing of a particular three-tier request by a mid-tier thread) which allows the session to be serially reused for a different three-tier user (when such state is cleaned up).

Note: Stateless sessions are typically used in conjunction with Statement Caching.

What connection pooling offers is stateless connections and stateful sessions. Users who need to work with stateless sessions, see "Session Pooling in OCI".

Multiple Connection Pools

This advanced concept can be used for different database connections. Multiple connection pools can also be used when different priorities are assigned to users. Different service level guarantees can be implemented using connection pooling.

The following figure illustrates connection pooling:

Figure 9-1 OCI Connection Pooling

Text description of lnoci043.gif follows

Text description of the illustration lnoci043.gif

OCI Calls for Connection Pooling

The steps in using connection pooling in your application are:

Allocate the Pool Handle

Connection pooling requires that the pool handle OCI_HTYPE_CPOOL be allocated by OCIHandleAlloc(). Multiple pools can be created for a given environment handle.

For a single connection pool, here is an allocation example:

OCICPool *poolhp;
OCIHandleAlloc((dvoid *) envhp, (dvoid **) &poolhp, OCI_HTYPE_CPOOL, 
                      (size_t) 0, (dvoid **) 0));

Create the Connection Pool

The function OCIConnectionPoolCreate() initializes the connection pool handle. It has these IN parameters:

All the preceding attributes can be configured dynamically. So the application has the flexibility of reading the current load (number of open connections and number of busy connections) and tuning these attributes appropriately.

If the pool attributes (connMax, connMin, connIncr) are to be changed dynamically, OCIConnectionPoolCreate() must be called with mode set to OCI_CPOOL_REINITIALIZE.

The OUT parameters poolName and poolNameLen will contain values to be used in subsequent OCIServerAttach() and OCILogon2() calls in place of the database name and the database name length arguments.

There is no limit on the number of pools that can be created by an application. Middle tier applications can take advantage of this feature and create multiple pools to connect to the same server or to different servers, to balance the load based on the specific needs of the application.

Here is an example of this call:

OCIConnectionPoolCreate((OCIEnv *)envhp,
                   (OCIError *)errhp, (OCICPool *)poolhp,
                   &poolName, &poolNameLen,
                   (text *)database,strlen(database),
                   (ub4) conMin, (ub4) conMax, (ub4) conIncr,
                   (text *)pooluser,strlen(pooluser),
                   (text *)poolpasswd,strlen(poolpasswd),

Logon to the Database

The application will need to log on to the database for each thread, using one of the following interfaces.

The OCI_MIGRATE flag will be set internally in any case. Credentials can be set to OCI_CRED_RDBMS or OCI_CRED_PROXY. If the credentials are set to OCI_CRED_PROXY, only username needs to be set on the session handle. (no explicit primary session needs to be created and OCI_ATTR_MIGSESSION need not be set).

Deal with SGA Limitations in Connection Pooling

With OCI_CPOOL mode (connection pooling), the session memory (UGA) in the back-end database will come out of the SGA. This may require some SGA tuning on the back-end database to have a larger SGA if your application consumes more session memory than the SGA can accommodate. The memory tuning requirements for the back-end database will be similar to configuring the LARGE POOL in case of a shared server back end except that the instance is still in dedicated mode.

See Also:

Oracle Database Performance Tuning Guide for more information, see the section on configuring Shared Server

If you are still running into the SGA limitation, you must consider:

The application must avoid using dedicated database links on the back end with connection pooling.

If the back end is a dedicated server, effective connection pooling will not be possible because sessions using dedicated database links will be tied to a physical connection rendering that same connection unusable by other sessions. If your application uses dedicated database links and you do not see effective sharing of back-end processes among your sessions, you must consider using shared database links.

See Also:

For more information about distributed databases, see the section on shared database links in Oracle Database Administrator's Guide

Logoff from the Database

Corresponding to the logon calls, these are the interfaces to use to log off from the database in connection pooling mode.

Destroy the Connection Pool

Use OCIConnectionPoolDestroy() to destroy the connection pool.

Free the Pool Handle

The pool handle is freed using OCIHandleFree().

These last three actions are illustrated in this code fragment:

 for (i = 0; i < MAXTHREADS; ++i)
    checkerr(errhp, OCILogoff((dvoid *) svchp[i], errhp));
  checkerr(errhp, OCIConnectionPoolDestroy(poolhp, errhp, OCI_DEFAULT));
  checkerr(errhp, OCIHandleFree((dvoid *)poolhp, OCI_HTYPE_CPOOL));

See Also:

Examples of OCI Connection Pooling

Examples of connection pooling in tested complete programs can be found in cdemocp.c and cdemocpproxy.c in directory demo.

Session Pooling in OCI

Session pooling means that the application will create and maintain a group of stateless sessions to the database. These sessions will be handed over to thin clients as requested. If no sessions are available, a new one may be created. When the client is done with the session, the client will release it to the pool. Thus, the number of sessions in the pool can increase dynamically.

Some of the sessions in the pool may be 'tagged' with certain properties. For instance, a user may request for a default session, set certain attributes on it, then label it or 'tag' it and return in to the pool. That user, or some other user, can require a session with the same attributes, and thus request for a session with the same tag. There may be several sessions in the pool with the same tag. The 'tag' on a session can be changed or reset.

See Also:

"Using Tags in Session Pools" for a discussion of using tags.

Proxy sessions, too, can be created and maintained through this interface.

The behavior of the application when no free sessions are available and the pool has reached it's maximum size, will depend on certain attributes. A new session may be created or an error returned, or the thread may just block and wait for a session to become free.

The main benefit of this type of pooling will be performance. Making a connection to the database is a time-consuming activity, especially when the database is remote. Thus, instead of a client spending time connecting to the server, authenticating its credentials, and then receiving a valid session, it can just pick one from the pool.

Functionality of OCI Session Pooling

Session pooling has the following features:

Homogeneous and Heterogeneous Session Pools

A session pool can be either homogeneous or heterogeneous. Homogeneous session pooling means that sessions in the pool are alike with respect to authentication (have the same username and password and privileges). Heterogeneous session pooling means that you must provide authentication information because the sessions can have different security attributes and privileges.

Using Tags in Session Pools

The tags provide a way for users to customize sessions in the pool. A client may get a default or untagged session from a pool, set certain attributes on the session (such as NLS settings), and return the session to the pool, labeling it with an appropriate tag in the OCISessionRelease() call.

The user, or some other user, may request a session with the same tags in order to have a session withe the same attributes, and can do so by providing the same tag in the OCISessionGet() call.

See Also:

"OCISessionGet()" for a further discussion of tagging sessions.

OCI Handles for Session Pooling

Two handle types have been added for session pooling:


This is the session pool handle. It is allocated using OCIHandleAlloc(). It needs to be passed to OCISessionPoolCreate(), and OCISessionPoolDestroy(). It has the attribute type OCI_HTYPE_SPOOL.

An example of the OCIHandleAlloc() call follows:

OCISPool *spoolhp; 
OCIHandleAlloc((dvoid *) envhp, (dvoid **) &spoolhp, OCI_HTYPE_SPOOL, 
                      (size_t) 0, (dvoid **) 0));

For an environment handle, multiple session pools can be created.


This is the authentication information handle. It is allocated using OCIHandleAlloc(). It is passed to OCISessionGet(). It supports all the attributes that are supported for user session handle. Please refer to user session handle attributes for more information. The authentication information handle has the attribute type OCI_HTYPE_AUTHINFO.

An example of the OCIHandleAlloc() call follows:

OCIAuthInfo *authp;        
OCIHandleAlloc((dvoid *) envhp, (dvoid **) &authp, OCI_HTYPE_AUTHINFO, 
                      (size_t) 0, (dvoid **) 0));

See Also:

Using OCI Session Pooling

The steps in writing a simple session pooling application which uses a username and password are:

OCI Calls for Session Pooling

Here are the usages for OCI calls for session pooling.

Allocate the Pool Handle

Session pooling requires that the pool handle OCI_HTYPE_SPOOL be allocated by calling OCIHandleAlloc().

Multiple pools can be created for a given environment handle. For a single session pool, here is an allocation example:

OCISPool *poolhp; 
OCIHandleAlloc((dvoid *) envhp, (dvoid **) &poolhp, OCI_HTYPE_SPOOL, (size_t) 0,
               (dvoid **) 0));

Create the Pool Session

The function OCISessionPoolCreate() can be used to create the session pool. Here is an example of how to use this call:

OCISessionPoolCreate(envhp, errhp, poolhp, (OraText **)&poolName, 
              (ub4 *)&poolNameLen, database, 
              (ub4)strlen((const signed char *)database),
              sessMin, sessMax, sessIncr,
              (OraText *)appusername,
              (ub4)strlen((const signed char *)appusername),
              (OraText *)apppassword,
              (ub4)strlen((const signed char *)apppassword),

Logon to the Database

These are the interfaces that can be used to logon to the database in session pooling mode.

Logoff from the Database

Corresponding to the preceding logon calls, these are the interfaces to use to log off from the database in session pooling mode.

Destroy the Session Pool

OCISessionPoolDestroy() must be called to destroy the session pool. Here is an example of how this call can be made:

OCISessionPoolDestroy(poolhp, errhp, OCI_DEFAULT);

Free the Pool Handle

OCIHandleFree() must be called to free the session pool handle. Here is how this call can be made:

OCIHandleFree((dvoid *)poolhp, OCI_HTYPE_SPOOL);


The application has to ensure either a commit or rollback is done before a session is released to the session pool. OCI does not reset the state of the session.

Example of OCI Session Pooling

Here is an example of session pooling in a tested, complete program:

See Also:

cdemosp.c in directory demo

When to Use Connection Pooling, Session Pooling, or Neither

If database sessions are not reusable by mid-tier threads (that is, they are stateful) and the number of back-end server processes may cause scaling problems on the database, use OCI connection pooling.

If database sessions are reusable by mid-tier threads (that is, they are stateless) and the number of back-end server processes may cause scaling problems on the database, use OCI session pooling.

If database sessions are not reusable by mid-tier threads (that is, they are stateful) and the number of back-end server processes will never be large enough to potentially cause any scaling issue on the database, there is no need to use any pooling mechanism.


Having non-pooled sessions/connections will result in tearing down and recreation of the database session/connection for every mid-tier user request. This can cause severe scaling problems on the database side and excessive latency for the fulfillment of the request. Hence, it is strongly recommended that one of the pooling strategies be adopted for mid-tier applications based on whether the database session is stateful or stateless.

In connection pooling, the pool element is a connection and in session pooling, the pool element is a session.

As with any pool, the pooled resource is locked by the application thread for a certain duration until the thread has done its job on the database and the resource is released. The resource is unavailable to other threads during its period of use. Hence, application developers need to be aware that any kind of pooling works effectively with relatively short tasks. If the application is performing a long running transaction for example, it may deny the pooled resource to other sharers for long periods of time leading to starvation. Hence, pooling should be used in conjunction with short tasks and the size of the pool should be sufficiently large to maintain the desired concurrency of transactions.

Also, note that with

  1. OCI Connection Pool
    1. Connections to the database are pooled. Sessions are created and destroyed by the user. Each call to the database will pick up an appropriate available connection from the pool.
    2. The application is multiplexing several sessions over fewer physical connections to the database. The users can tune the pool configuration to achieve required concurrency.
    3. The life-time of the application sessions is independent of the life-time of the cached pooled connections.
  2. OCI Session Pool

    Sessions and connections are pooled by OCI. The application gets sessions from the pool and release sessions back to the pool.

Functions for Session Creation

The choices are:

  1. OCILogon()

    This is the simplest way to get an OCI Session. The advantage is ease of obtaining an OCI service context. The disadvantage is that you cannot perform any advance OCI operations like session migration, proxy authentication, using a connection pool, or a session pool.

    See Also:

    "Application Initialization, Connection, and Session Creation"

  2. OCILogon2()

    This includes the functionality of OCILogon() to get a session. This session may be a new one with a new underlying connection, or one that is started over a virtual connection from an existing connection pool, or one from an existing session pool. The mode parameter value that the function is called with determines its behavior.

    The user cannot modify the attributes (except OCI_ATTR_ STMTCACHESIZE) of the service context returned by OCI.

    See Also:


  1. OCISessionBegin()

    This supports all the various options of an OCI session such as proxy authentication, getting a session from a connection pool or a session pool, external credentials, and migratable sessions. This is the lowest level call where all handles are needed to be explicitly allocated and all attributes set, and OCIServerAttach() is to be called prior to this call.

    See Also:


  2. OCISessionGet()

    This is now the recommended method to get a session. This session may be a new one with a new underlying connection, or one that is started over a virtual connection from an existing connection pool, or one from an existing session pool. The mode parameter value that the function is called with determines its behavior. This works like OCILogon2() but additionally allows you to specify tags for obtaining specific sessions from the pool.

    See Also:


    Choosing Between Different Types of OCI Sessions

    The choices are:

    • Basic OCI Sessions

      This works by using user name and password over a dedicated OCI server handle. This is the no-pool mechanism. See earlier notes of when to use it.

      If authentication is obtained via external credentials, then user name or password is not required.

    • Session Pool Sessions

      These sessions are from the session pool cache. Some sessions may be tagged. These are stateless sessions. Each OCISessionGet() and OCISessionRelease() call gets and releases a session from the session cache. This saves the server from creating and destroying sessions.

      See the earlier notes on connection pool sessions versus session pooling sessions versus no-pooling sessions.

    • Connection Pool Sessions

      These are sessions created using OCISessionGet() and OCISessionBegin() calls from an OCI Connection Pool. There is no session cache as these are stateful sessions. Each call creates a new session and the user is responsible for terminating these sessions.

      The sessions are automatically migratable between the server handles of the connection pool. Each session can have user name and password or be a proxy session. See the earlier notes on connection pool sessions versus session pooling sessions versus no-pooling sessions.

    • Sessions Sharing a Server Handle

      You can multiplex several OCI Sessions over a few physical connections. The application does this manually by having the same server handle for these multiple sessions. It is preferred to have the session multiplexing details be left to OCI by using the OCI Connection Pool APIs.

    • Proxy Sessions

      This is useful if the password of the client needs to be protected from the middle-tier. Proxy sessions can also be part of OCI Connection Pool and OCI Session Pool.

    • Migratable Sessions

    With transaction handles being migratable, there should be no need for applications to use this older feature, in light of OCI Connection Pooling.

    See Also:

    "OCI Session Management"

    Statement Caching in OCI

    Statement caching refers to the feature that provides and manages a cache of statements for each session. In the server, it means that cursors are ready to be used without the need to parse the statement again. Statement caching can be used with connection pooling and with session pooling, and will improve performance and scalability. It can be used without session pooling as well. The OCI calls that implement statement caching are:

    • OCIStmtPrepare2()
    • OCIStmtRelease()

    Statement Caching without Session Pooling in OCI

    Users perform the usual OCI steps to logon. The call to obtain a session will have a mode that specifies whether statement caching is enabled for the session. Initially the statement cache will be empty. Developers will try to find a statement in the cache using the statement text. If the statement exists the API will return a previously prepared statement handle, otherwise it will return an newly prepared statement handle.

    The application developer can perform binds and defines and then simply execute and fetch the statement before returning the statement back to the cache. In the latter case, where the statement handle was not found, the developer will need to set different attributes on the handle in addition to the other steps.

    OCIStmtPrepare2() will also take a mode which will determine if the developer wants a prepared statement handle or a null statement handle if the statement is not found in the cache.

    The pseudo code will look like:

    OCISessionBegin( userhp, ... OCI_STMT_CACHE)  ;
    OCIAttrset(svchp, userhp, ...);  /* Set the user handle in the service context 
    OCIStmtPrepare2(svchp, &stmthp, stmttext, key, ...);
    OCIBindByPos(stmthp, ...);
    OCIDefineByPos(stmthp, ...);
    OCIStmtExecute(svchp, stmthp, ...);
    OCIStmtFetch(svchp, ...);
    OCIStmtRelease(stmthp, ...);

    Statement Caching with Session Pooling in OCI

    The concepts remain the same, except that the statement cache is enabled at the session pool layer rather than at the session layer.

    The attribute OCI_ATTR_SPOOL_STMTCACHESIZE represents the statement cache size for the entire session pool. It is set on the OCI_HTYPE_SPOOL handle. The value of OCI__ATTR_SPOOL_STMTCACHESIZE can be changed at any time. This attribute can be used to enable or disable statement caching at the pool level, after creation, just as attribute OCI_ATTR_STMTCACHESIZE (on the service context) is used to enable or disable statement caching at the session level. This change will be reflected on individual sessions in the pool, when they are handed to a user. Tagged sessions are an exception to this behavior. This is explained later.

    Enabling or disabling of statement caching is allowed on individual pooled sessions similar to non-pooled sessions.

    A user can enable statement caching on a session retrieved from a non-statement cached pool in an OCISessionGet() or OCILogon2() call by specifying OCI_SESSGET_STMTCACHE or OCI_LOGON2_STMTCACHE, respectively, in the mode argument.

    When you ask for a session from a session pool, the statement cache size will default to that of the pool. This may also mean enabling or disabling statement caching in that session. For example, if a pooled session (session A) has statement caching enabled, and statement caching is turned off in the pool, and a user asks for a session, and session A is returned, then statement caching will be turned off in Session A. As another example, if Session A in a pool does not have statement caching enabled, and statement caching at the pool level is turned on, then before returning session A to a user, statement caching on Session A with size equal to that of the pool is turned on.

    This will not hold true if a tagged session is asked for and retrieved. In this case, the size of the statement cache will not be changed. Consequently, it will not be turned on or off. Moreover, if the user specifies mode OCI_SESSGET_STMTCACHE in the OCISessionGet() call, this will be ignored if the session is tagged. In our earlier example, if Session A was tagged, then it is returned as is to the user.

    Rules for Statement Caching in OCI

    Here are some notes to follow:

    • Use the function OCIStmtPrepare2() instead of OCIStmtPrepare(). If you are using OCIStmtPrepare(), you are strongly urged not to use a statement handle across different service contexts. Doing so will raise an error if the statement has been obtained by OCIStmtPrepare2(). Migration of a statement handle to a new service context actually closes the cursor associated with the old session and therefore no sharing is achieved. Client-side sharing is also not obtained, because OCI will free all buffers associated with the old session when the statement handle is migrated.
    • You are urged to keep one service context for each session and use statement handles only for that service context. That will be the preferred and recommended model and usage.
    • A call to OCIStmtPrepare2(), even if the session does not have a statement cache, will also allocate the statement handle and therefore applications using only OCIStmtPrepare2() must not call OCIHandleAlloc() for the statement handle.
    • A call to the OCIStmtPrepare2() must be followed with OCIStmtRelease() after the user is done with the statement handle. If statement caching is used, this will release the statement to the cache. If statement caching is not used, the statement will be deallocated. Do not call OCIHandleFree() to free the memory.
    • If the call to OCIStmtPrepare2() is made with the OCI_PREP2_CACHE_SEARCHONLY mode and a NULL statement was returned (statement was not found), the subsequent call to OCIStmtRelease() is not required and must not be performed.
    • Do not call OCIStmtRelease() for a statement that was prepared using OCIStmtPrepare().
    • The statement cache has a maximum size (number of statements) which can be modified by an attribute on the service context, OCI_ATTR_STMTCACHESIZE. The default value is 20.
    • You can choose a to tag a statement at the release time so that the next time you can request a statement of the same tag. The tag will be used to search the cache. An untagged statement (tag is null) is a special case of a tagged statement. Two statements are considered different if they only differ in their tags, or if one is untagged and the other is not.

      See Also:

      For information about the functions for statement caching,

    OCI Statement Caching Code Example

    Here is an example of statement caching:

    See Also:

    Please refer to ocisc.c in directory demo for a working example of statement caching.

    User-Defined Callback Functions in OCI

    The Oracle Call Interface has the capability to execute user-specific code in addition to OCI calls. This functionality can be used for:

    • Adding tracing and performance measurement code to enable users to tune their applications.
    • Performing pre- or post-processing code for specific OCI calls.
    • Accessing other data sources with OCI by using the native OCI interface for Oracle databases and directing the OCI calls to use user callbacks for non-Oracle data sources.

    The OCI callback feature has been added by providing support for calling user code before or after executing the OCI calls. Functionality has also been provided to allow the user-defined code to be executed instead of executing the OCI code.

    The user callback code can also be registered dynamically without modifying the source code of the application. The dynamic registration is implemented by loading up to five user-created dynamically linked libraries after the initialization of the environment handle during the OCIEnvCreate() call. These user-created libraries (such as Dynamic Link Libraries (DLLs) on NT, or shared libraries on Solaris register the user callbacks for the selected OCI calls transparently to the application.

    Sample Application

    For a listing of the complete demonstration programs that illustrate the OCI user callback feature, see Appendix B, "OCI Demonstration Programs".

    Registering User Callbacks in OCI

    An application can register user callback libraries with the OCIUserCallbackRegister() function. Callbacks are registered in the context of the environment handle. An application can retrieve information about callbacks registered with a handle with the OCIUserCallbackGet() function.

    See Also:

    For detailed descriptions of these functions and their parameters, refer to the descriptions of OCIUserCallbackGet() and OCIUserCallbackRegister()

    A user-defined callback is a subroutine that is registered against an OCI call and an environment handle. It can be specified to be either an entry callback, a replacement callback, or an exit callback.

    • If it is an entry callback, it is called when the program enters the OCI function.
    • Replacement callbacks are executed after entry callbacks. If the replacement callback returns a value of OCI_CONTINUE, then a subsequent replacement callback or the normal OCI-specific code is executed. If a replacement callback returns anything other than OCI_CONTINUE, subsequent replacement callbacks and the OCI code does not execute.
    • After a replacement callback returns something other than OCI_CONTINUE, or an OCI function successfully executes, program control transfers to the exit callback (if one is registered).

    If a replacement or exit callback returns anything other than OCI_CONTINUE, then the return code from the callback is returned from the associated OCI call.

    A user callback can return OCI_INVALID_HANDLE when either an invalid handle or an invalid context is passed to it.


    If any callback returns anything other than OCI_CONTINUE, then that return code is passed to the subsequent callbacks. If a replacement or exit callback returns a return code other than OCI_CONTINUE, then the final (not OCI_CONTINUE) return code is returned from the OCI call.


    A user callback is registered using the OCIUserCallbackRegister() call.

    See Also:

    See OCIUserCallbackRegister() for the syntax of this call.

    Currently, OCIUserCallbackRegister() is only registered on the environment handle. The user's callback function of typedef OCIUserCallback is registered along with its context for the OCI call identified by the OCI function code, fcode. The type of the callback, whether entry, replacement, or exit, is specified by the when parameter.

    For example, the stmtprep_entry_dyncbk_fn entry callback function and its context dynamic_context, are registered against the environment handle hndlp for the OCIStmtPrepare() call by calling the OCIUserCallbackRegister() function with the following parameters.

    OCIUserCallbackRegister( hndlp, 
                             (OCIUcb*) NULL);

    User Callback Function

    The user callback function has to follow the following syntax:

    typedef sword (*OCIUserCallback)
         (dvoid *ctxp,      /* context for the user callback*/
          dvoid *hndlp,     /* handle for the callback, env handle for now */
          ub4 type,         /* type of handlp, OCI_HTYPE_ENV for this release */
          ub4 fcode,        /* function code of the OCI call */
          ub1 when,         /* type of the callback, entry or exit */
          sword returnCode, /* OCI return code */
          ub4 *errnop,      /* Oracle error number */
          va_list arglist); /* parameters of the oci call */

    In addition to the parameters described in the OCIUserCallbackRegister() call, the callback is called with the return code, errnop, and all the parameters of the original OCI as declared by the prototype definition.

    The return code is always passed in as OCI_SUCCESS and *errnop is always passed in as 0 for the first entry callback. Note that *errnop refers to the content of errnop because errnop is an IN/OUT parameter.

    If the callback does not want to change the OCI return code, then it must return OCI_CONTINUE, and the value returned in *errnop is ignored. If on the other hand, the callback returns any other return code than OCI_CONTINUE, the last returned return code becomes the return code for the call. At the this point, the value of *errnop returned is set in the error handle, or in the environment handle if the error information is returned in the environment handle because of the absence of the error handle for certain OCI calls such as OCIHandleAlloc().

    For replacement callbacks, the returnCode is the non-OCI_CONTINUE return code from the previous callback or OCI call and *errnop is the value of the error number being returned in the error handle. This allows the subsequent callback to change the return code or error information if needed.

    The processing of replacement callbacks is different in that if it returns anything other than OCI_CONTINUE, then subsequent replacement callbacks and OCI code is bypassed and processing jumps to the exit callbacks.

    Note that if the replacement callbacks return OCI_CONTINUE to allow processing of OCI code, then the return code from entry callbacks is ignored.

    All the original parameters of the OCI call are passed to the callback as variable parameters and the callback must retrieve them using the va_arg macros. The callback demonstration programs provide examples.

    See Also:

    See Appendix B, "OCI Demonstration Programs"

    A null value can be registered to de-register a callback. That is, if the value of the callback (OCIUserCallback) is NULL in the OCIUserCallbackRegister() call, then the user callback is de-registered.

    When using the thread-safe mode, the OCI program acquires all mutexes before calling the user callbacks.

    UserCallback Control Flow

    This pseudocode describes the overall processing of a typical OCI call:

     Acquire mutexes on handles;
     retCode = OCI_SUCCESS;
     errno = 0;
     for all ENTRY callbacks do
         EntryretCode = (*entryCallback)(..., retcode, &errno, ...);
         if (retCode != OCI_CONTINUE)
             set errno in error handle or environment handle;
             retCode = EntryretCode;
      for all REPLACEMENT callbacks do
       retCode = (*replacementCallback) (..., retcode, &errno, ...);
       if (retCode != OCI_CONTINUE)
           set errno in error handle or environment handle
           goto executeEXITCallback;
       retCode = return code for XyzCall; /* normal processing of OCI call */
       errno = error number from error handle or env handle;
       for all EXIT callbacks do
           exitRetCode = (*exitCallback)(..., retCode, &errno,...);
           if (exitRetCode != OCI_CONTINUE)
               set errno in error handle or environment handle;
               retCode = exitRetCode;
        release mutexes;
        return retCode

    UserCallback for OCIErrorGet()

    If the callbacks are a total replacement of the OCI code, then they usually maintain their own error information in the call context and use that to return error information in bufp and errnop parameters of the replacement callback of the OCIErrorGet() call.

    If on the other hand, the callbacks are either partially overriding OCI code, or just doing some other post processing, then they can use the exit callback to modify the error text and errnop parameters of the OCIErrorGet() by their own error message and error number. Note that the *errnop passed into the exit callback is the error number in the error or the environment handle.

    Errors from Entry Callbacks

    If an entry callback wants to return an error to the caller of the OCI call, then it must register a replacement or exit callback. This is because if the OCI code is executed, then the error code from the entry callback is ignored. Therefore the entry callback must pass the error to the replacement or exit callback through its own context.

    Dynamic Callback Registrations

    Because user callbacks are expected to be used for monitoring OCI behavior or to access other data sources, it is desirable that the registration of the callbacks be done transparently and non-intrusively. This is accomplished by loading user-created dynamically linked libraries at OCI initialization time. These dynamically linked libraries are called packages. The user-created packages register the user callbacks for the selected OCI calls. These callbacks can further register or de-register user callbacks as needed when receiving control at runtime.

    A makefile ( on Solaris) is provided with the OCI demonstration programs to create the package. The exact naming and location of this package is operating system dependent. The source code for the package must provide code for special callbacks that are called at OCI initialization and environment creation times.

    The loading of the package is controlled by setting an operating system environment variable, ORA_OCI_UCBPKG. This variable names the packages in a generic way. The packages must be located in the $ORACLE_HOME/lib directory.

    Loading Multiple Packages

    The ORA_OCI_UCBPKG variable can contain a semicolon separated list of package names. The packages are loaded in the order they are specified in the list.

    For example, previously one specified the package as:

    setenv ORA_OCI_UCBPKG mypkg

    Now, you can still specify the package as earlier, but in addition multiple packages can be specified as:

    setenv ORA_OCI_UCBPKG "mypkg;yourpkg;oraclepkg;sunpkg;msoftpkg"

    All these packages are loaded in order. That is, mypkg is loaded first and msoftpkg is loaded last.

    A maximum of five packages can be specified.


    The sample makefile creates on a Solaris or ociucb.dll on an NT system. To load the ociucb package, the environmental variable ORA_OCI_UCBPKG must be set to ociucb. On Solaris, if the package name ends with .so, OCIInitialize() fails. The package name must end with .so.1.0.

    For further details about creating the dynamic link libraries, read the Makefiles provided in the demo directory for your operating system. For further information on user-defined callbacks, see your operating system-specific documentation on compiling and linking applications.

    Package Format

    Previously a package had to specify the source code for the OCIEnvCallback() function. Now the OCIEnvCallback() function is obsolete. Instead, the package source must provide two functions. The first function has to be named as packagename suffixed with the word Init. For example, if the package is named foo, then the source file (for example, but not necessarily, foo.c) must contain a fooInit() function with a call to OCISharedLibInit() function specified exactly as:

    sword fooInit(metaCtx, libCtx, argfmt, argc, argv)
          dvoid *         metaCtx;         /* The metacontext */
          dvoid *         libCtx;          /* The context for this package. */
          ub4             argfmt;          /* package argument format */
          sword           argc;            /* package arg count*/
          dvoid *         argv[];          /* package arguments */
      return  (OCISharedLibInit(metaCtx, libCtx, argfmt, argc, argv,

    The last parameter of the OCISharedLibInit() function, fooEnvCallback(), in this case, is the name of the second function. It can be named anything, but by convention it can be named packagename suffixed with the word EnvCallback.

    This function is a replacement for OCIEnvCallback(). Now all the dynamic user callbacks must be registered in this function. The function must be of type OCIEnvCallbackType, which is specified as:

    typedef sword (*OCIEnvCallbackType)(OCIEnv *env, ub4 mode,
                                        size_t xtramem_sz, dvoid *usrmemp,
                                        OCIUcb *ucbDesc);

    When an environment handle is created, then this callback function is called at the very end. The env parameter is the newly created environment handle.

    The mode, xtramem_sz, and usrmemp are the parameters passed to the OCIEnvCreate() call. The last parameter, ucbDesc, is a descriptor that is passed to the package. The package uses this descriptor to register the user callbacks as described later.

    A sample ociucb.c file is provided in the demo directory. The makefile is also provided (on Solaris) in the demo directory to create the package. Please note that this may be different on other operating systems. The demo directory also contains full user callback demo programs (cdemoucb.c, cdemoucbl.c,) illustrating this.

    User Callback Chaining

    User callbacks can both be registered statically in the application itself or dynamically at runtime in the DLLs. A mechanism is needed to allow the application to override a previously registered callback and then later invoke the overridden one in the newly registered callback to preserve the behavior intended by the dynamic registrations. This can result in chaining of user callbacks.

    For this purpose, the OCIUserCallbackGet() function is provided to find out which function and context is registered for an OCI call.

    See Also:

    See OCIUserCallbackGet() for the syntax of this call

    Accessing Other Data Sources Through OCI

    Because Oracle is the predominant database accessed, applications can take advantage of the OCI interface to access non-Oracle data by using the user callbacks to access them. This allows an application written in OCI to access Oracle data without any performance penalty. To access non-Oracle data sources, drivers can be written that access the non-Oracle data in user callbacks. Because OCI provides a very rich interface, there is usually a straightforward mapping of OCI calls to most data sources. This solution is better than writing applications for other middle layers such as ODBC that introduce performance penalties for all data sources. Using OCI does not incur any penalty for the common case of accessing Oracle data sources, and incurs the same penalty that ODBC does for non-Oracle data sources.

    Restrictions on Callback Functions

    There are certain restrictions on the usage of callback functions, including OCIEnvCallback():

    • A callback cannot call other OCI functions except OCIUserCallbackRegister(), OCIUserCallbackGet(), OCIHandleAlloc(), OCIHandleFree(). Even for these functions, if they are called in a user callback, then callbacks on them are not called to avoid recursion. For example, if OCIHandleFree() is called in the callback for OCILogoff(), then the callback for OCIHandleFree() is disabled during the execution of the callback for OCILogoff().
    • A callback cannot modify OCI data structures such as the environment or error handles.
    • A callback cannot be registered for OCIUserCallbackRegister() call itself, or for any of the following:
      • OCIUserCallbackGet()
      • OCIEnvCreate()
      • OCIInitialize()
      • OCIEnvInit()

    Example of OCI Callbacks

    For example, lets suppose that there are five packages each registering entry, replacement, and exit callbacks for OCIStmtPrepare call. That is, the ORA_OCI_UCBPKG variable is set as:

    setenv ORA_OCI_UCBPKG "pkg1;pkg2;pkg3;pkg4;pkg5" 

    In each package pkgN (where N can be 1 through 5), the pkgNInit() and PkgNEnvCallback() functions are specified as:

    pkgNInit(dvoid *metaCtx, dvoid *libCtx, ub4 argfmt, sword argc, dvoid **argv)
      return OCISharedLibInit(metaCtx, libCtx, argfmt, argc, argv, pkgNEnvCallback);

    The pkgNEnvCallback() function registers the entry, replacement, and exit callbacks as:

    pkgNEnvCallback(OCIEnv *env, ub4 mode, size_t xtramemsz,
                                    dvoid *usrmemp, OCIUcb *ucbDesc)
      OCIHandleAlloc((dvoid *)env, (dvoid **)&errh, OCI_HTYPE_ERROR, (size_t) 0,
            (dvoid **)NULL);
      OCIUserCallbackRegister(env, OCI_HTYPE_ENV, errh, pkgN_entry_callback_fn,
      OCIUserCallbackRegister(env, OCI_HTYPE_ENV, errh, pkgN_replace_callback_fn,
      OCIUserCallbackRegister(env, OCI_HTYPE_ENV, errh, pkgN_exit_callback_fn,
            pkgNctx, OCI_FNCODE_STMTPREPARE, OCI_UCBTYPE_EXIT, ucbDesc);
      return OCI_CONTINUE;

    Finally, in the source code for the application, user callbacks can be registered with the NULL ucbDesc as:

      OCIUserCallbackRegister(env, OCI_HTYPE_ENV, errh, static_entry_callback_fn,
      OCIUserCallbackRegister(env, OCI_HTYPE_ENV, errh, static_replace_callback_fn,
      OCIUserCallbackRegister(env, OCI_HTYPE_ENV, errh, static_exit_callback_fn,

    When the OCIStmtPrepare() call is executed, the callbacks are called in the following order:

          OCI code for OCIStmtPrepare call 


    The exit callbacks are called in the reverse order of the entry and replacement callbacks

    The entry and exit callbacks can return any return code and the processing continues to the next callback. However, if the replacement callback returns anything other than OCI_CONTINUE, then the next callback (or OCI code if it is the last replacement callback) in the chain is bypassed and processing jumps to the exit callback. For example, if pkg3_replace_callback_fn() returned OCI_SUCCESS, then pkg4_replace_callback_fn(), pkg5_replace_callback_fn(), and the OCI processing for the OCIStmtPrepare call is bypassed. Instead, pkg5_exit_callback_fn() is executed next.

    OCI Callbacks from External Procedures

    There are several OCI functions that can be used as callbacks from external procedures.

    See Also:

    These functions are listed in Chapter 19, "OCI Cartridge Functions". For information about writing C subroutines that can be called from PL/SQL code, including a list of which OCI calls can be used, and some example code, refer to the Oracle Database Application Developer's Guide - Fundamentals.

    Application Failover Callbacks in OCI

    Application failover callbacks can be used in the event of the failure of one database instance, and failover to another instance. Because of the delay which can occur during failover, the application developer may want to inform the user that failover is in progress, and request that the user stand by. Additionally, the session on the initial instance may have received some ALTER SESSION commands. These will not be automatically replayed on the second instance. Consequently, the developer may wish to replay these ALTER SESSION commands on the second instance.

    See Also:

    Oracle Net Services Reference Guide for more detailed information about application failover

    Failover Callback Overview

    To address the problems described earlier, the application developer can register a failover callback function. In the event of failover, the callback function is invoked several times during the course of reestablishing the user's session.

    The first call to the callback function occurs when Oracle first detects an instance connection loss. This callback is intended to allow the application to inform the user of an upcoming delay. If failover is successful, a second call to the callback function occurs when the connection is reestablished and usable. At this time the client may wish to replay ALTER SESSION commands and inform the user that failover has happened. If failover is unsuccessful, then the callback is called to inform the application that failover will not take place. Additionally, the callback is called each time a user handle besides the primary handle is reauthenticated on the new connection. Since each user handle represents a server-side session, the client may wish to replay ALTER SESSION commands for that session.

    An initial attempt at failover may not always successful. The OCI provides a mechanism for retrying failover after an unsuccessful attempt.

    See Also:

    See "Handling OCI_FO_ERROR" for more information about this scenario

    Failover Callback Structure and Parameters

    The basic structure of a user-defined application failover callback function is as follows:

    sb4 appfocallback_fn ( dvoid      * svchp, 
                           dvoid      * envhp, 
                           dvoid      * fo_ctx, 
                           ub4        fo_type, 
                           ub4        fo_event );

    An example is provided in the section "Failover Callback Example" for the following parameters:


    The first parameter, svchp, is the service context handle. It is of type dvoid *.


    The second parameter, envhp, is the OCI environment handle. It is of type dvoid *.


    The third parameter, fo_ctx, is a client context. It is a pointer to memory specified by the client. In this area the client can keep any necessary state or context. It is passed as a dvoid *.


    The fourth parameter, fo_type, is the failover type. This lets the callback know what type of failover the client has requested. The usual values are:

    • OCI_FO_SESSION, which indicates that the user has requested only session failover.
    • OCI_FO_SELECT, which indicates that the user has requested select failover as well.

    The last parameter is the failover event. This indicates to the callback why it is being called. It has several possible values:

    • OCI_FO_BEGIN indicates that failover has detected a lost connection and failover is starting.
    • OCI_FO_END indicates successful completion of failover.
    • OCI_FO_ABORT indicates that failover was unsuccessful, and there is no option of retrying.
    • OCI_FO_ERROR also indicates that failover was unsuccessful, but it gives the application the opportunity to handle the error and retry failover.

      See Also:

      "Handling OCI_FO_ERROR" for more information about this value

    • OCI_FO_REAUTH indicates that a user handle has been reauthenticated. To find out which, the application checks the OCI_ATTR_SESSION attribute of the service context handle (which is the first parameter).

    Failover Callback Registration

    For the failover callback to be used, it must be registered on the server context handle. This registration is done by creating a callback definition structure and setting the OCI_ATTR_FOCBK attribute of the server handle to this structure.

    The callback definition structure must be of type OCIFocbkStruct. It has two fields: callback_function, which contains the address of the function to call, and fo_ctx which contains the address of the client context.

    An example of callback registration is included as part of the example in the next section.

    Failover Callback Example

    The following code shows an example of a simple user-defined callback function definition and registration.

    Part 1, Failover Callback Definition

    sb4  callback_fn(svchp, envhp, fo_ctx, fo_type, fo_event )
    dvoid * svchp;
    dvoid * envhp;
    dvoid *fo_ctx;
    ub4 fo_type;
    ub4 fo_event;
    switch (fo_event) 
       case OCI_FO_BEGIN:
         printf(" Failing Over ... Please stand by \n");
         printf(" Failover type was found to be %s \n",
                         ((fo_type==OCI_FO_SESSION) ? "SESSION" 
                         :(fo_type==OCI_FO_SELECT) ? "SELECT"
                         : "UNKNOWN!")); 
         printf(" Failover Context is :%s\n", 
                        (fo_ctx?(char *)fo_ctx:"NULL POINTER!"));
       case OCI_FO_ABORT:
         printf(" Failover aborted. Failover will not take place.\n");
       case    OCI_FO_END:
           printf(" Failover ended ...resuming services\n");
       case OCI_FO_REAUTH:
           printf(" Failed over user. Resuming services\n");
         printf("Bad Failover Event: %d.\n",  fo_event);
       return 0;

    Part 2, Failover Callback Registration

    int register_callback(srvh, errh)
    dvoid *srvh; /* the server handle */
    OCIError *errh; /* the error handle */
      OCIFocbkStruct failover;                 /*  failover callback structure */
      /* allocate memory for context */
      if (!(failover.fo_ctx = (dvoid *)malloc(strlen("my context.")+1)))
      /* initialize the context. */
      strcpy((char *)failover.fo_ctx, "my context.");
      failover.callback_function = &callback_fn;
      /* do the registration */
      if (OCIAttrSet(srvh, (ub4) OCI_HTYPE_SERVER,
                    (dvoid *) &failover, (ub4) 0,
                    (ub4) OCI_ATTR_FOCBK, errh)  != OCI_SUCCESS)
      /* successful conclusion */
      return (0);

    Handling OCI_FO_ERROR

    A failover attempt is not always successful. If the attempt fails, the callback function receives a value of OCI_FO_ABORT or OCI_FO_ERROR in the fo_event parameter. A value of OCI_FO_ABORT indicates that failover was unsuccessful, and no further failover attempts are possible. OCI_FO_ERROR, on the other hand, provides the callback function with the opportunity to handle the error in some way. For example, the callback may choose to wait a specified period of time and then indicate to the OCI library that it must reattempt failover.


    This functionality is only available to applications linked with the 8.0.5 or later OCI libraries running against any Oracle server.

    Failover does not work if a LOB column is part of the select list.

    Consider the following timeline of events:

    Table 9-4 Time and Event  
    Time Event


    Database fails (failure lasts until T5).


    Failover triggered by user activity.


    User attempts to reconnect; attempt fails.


    Failover callback invoked with OCI_FO_ERROR.


    Failover callback enters predetermined sleep period.


    Database comes back up again.


    Failover callback triggers new failover attempt; it is successful.


    User successfully reconnects

    The callback function triggers the new failover attempt by returning a value of OCI_FO_RETRY from the function.

    The following example code shows a callback function which might be used to implement the failover strategy similar to the scenario described earlier. In this case the failover callback enters a loop in which it sleeps and then reattempts failover until it is successful:

    /* the user defined failover callback  */
    sb4  callback_fn(svchp, envhp, fo_ctx, fo_type, fo_event )
    dvoid * svchp;
    dvoid * envhp;
    dvoid *fo_ctx;
    ub4 fo_type;
    ub4 fo_event;
       OCIError *errhp;
       OCIHandleAlloc(envhp, (dvoid **)&errhp, (ub4) OCI_HTYPE_ERROR,
                  (size_t) 0, (dvoid **) 0);
       switch (fo_event) 
       case OCI_FO_BEGIN:
         printf(" Failing Over ... Please stand by \n");
         printf(" Failover type was found to be %s \n",
                ((fo_type==OCI_FO_NONE) ? "NONE"
                 :(fo_type==OCI_FO_SESSION) ? "SESSION" 
                 :(fo_type==OCI_FO_SELECT) ? "SELECT"
                 :(fo_type==OCI_FO_TXNAL) ? "TRANSACTION"
                 : "UNKNOWN!")); 
         printf(" Failover Context is :%s\n", 
                (fo_ctx?(char *)fo_ctx:"NULL POINTER!"));
       case OCI_FO_ABORT:
         printf(" Failover aborted. Failover will not take place.\n");
       case    OCI_FO_END:
           printf("\n Failover ended ...resuming services\n");
       case OCI_FO_REAUTH:
           printf(" Failed over user. Resuming services\n");
       case OCI_FO_ERROR:
         /* all invocations of this can only generate one line. The newline
          * will be put at fo_end time.
         printf(" Failover error gotten. Sleeping...");
         printf("Retrying. ");
         return (OCI_FO_RETRY);
         printf("Bad Failover Event: %d.\n",  fo_event);
       return 0;

    The following is sample output from a program containing this failover callback function:

    executing select...
    7369    SMITH    CLERK
    7499    ALLEN    SALESMAN
     Failing Over ... Please stand by 
     Failover type was found to be SELECT 
     Failover Context is :My context.
     Failover error gotten. Sleeping...Retrying.  Failover error gotten. 
    Sleeping...Retrying.  Failover error gotten. Sleeping...Retrying.  Failover 
    error gotten. Sleeping...Retrying.  Failover error gotten. Sleeping...Retrying.  
    Failover error gotten. Sleeping...Retrying.  Failover error gotten. 
    Sleeping...Retrying.  Failover error gotten. Sleeping...Retrying.  Failover 
    error gotten. Sleeping...Retrying.  Failover error gotten. Sleeping...Retrying. 
     Failover ended ...resuming services
    7521    WARD    SALESMAN
    7566    JONES    MANAGER
    7654    MARTIN    SALESMAN
    7698    BLAKE    MANAGER
    7782    CLARK    MANAGER
    7788    SCOTT    ANALYST
    7839    KING    PRESIDENT
    7844    TURNER    SALESMAN
    7876    ADAMS    CLERK
    7900    JAMES    CLERK
    7902    FORD    ANALYST

    OCI and Streams Advanced Queuing

    The OCI provides an interface to Streams Advanced Queuing (Streams AQ) feature. Streams AQ provides message queuing as an integrated part of the Oracle server. Streams AQ provides this functionality by integrating the queuing system with the database, thereby creating a message-enabled database. By providing an integrated solution Streams AQ frees application developers to devote their efforts to their specific business logic rather than having to construct a messaging infrastructure.


    In order to use Streams Advanced Queuing, you must be using the Enterprise Edition

    See Also:

    OCI Streams Advanced Queuing Functions

    The OCI library includes several functions related to Streams Advanced Queuing:

    • OCIAQEnq()
    • OCIAQDeq()
    • OCIAQListen()
    • OCIAQEnqArray()
    • OCIAQDeqArray()

    You can enqueue an array of messages to a single queue. The messages all share the same enqueue options, but each message in the array can have different message properties. You can also dequeue an array of messages from a single queue. For transaction group queues, you can dequeue all messages for a single transaction group using one call.

    See Also:

    "Streams Advanced Queuing and Publish-Subscribe Functions", contains complete descriptions of these functions and their parameters

    OCI Streams Advanced Queuing Descriptors

    The following descriptors are used by OCI Streams AQ operations:

    • OCIAQEnqOptions
    • OCIAQDeqOptions
    • OCIAQMsgProperties
    • OCIAQAgent

    You can allocate these descriptors with respect to the service handle using the standard OCIDescriptorAlloc() call. The following code shows examples of this:

    OCIDescriptorAlloc(svch, &enqueue_options, OCI_DTYPE_AQENQ_OPTIONS, 0, 0 ); 
    OCIDescriptorAlloc(svch, &dequeue_options, OCI_DTYPE_AQDEQ_OPTIONS, 0, 0 ); 
    OCIDescriptorAlloc(svch, &message_properties, OCI_DTYPE_AQMSG_PROPERTIES, 0, 0);
    OCIDescriptorAlloc(svch, &agent, OCI_DTYPE_AQAGENT, 0, 0 ); 

    Each descriptor has a variety of attributes which can be set or read.

    See Also:

    These attributes are described in more detail in "Advanced Queuing Descriptor Attributes"

    Streams Advanced Queuing in OCI Versus PL/SQL

    The following tables compare functions, parameters, and options for OCI Streams AQ functions and descriptors, and PL/SQL AQ functions in the DBMS_AQ package.

    Table 9-5 AQ Functions  
    PL/SQL Function OCI Function











    The following table compares the parameters for the enqueue functions:

    Table 9-6 Enqueue Parameters  
    DBMS_AQ.ENQUEUE Parameter OCIAQEnq() Parameter

    Note: OCIAQEnq() also requires the following additional parameters: svch, errh, payload_tdo, payload_ind, and flags.

    The following table compares the parameters for the dequeue functions:

    Table 9-7 Dequeue Parameters  
    DBMS_AQ.DEQUEUE Parameter OCIAQDeq() Parameter

    Note: OCIAQDeq() also requires the following additional parameters: svch, errh, dequeue_options, message_properties, payload_tdo, payload, payload_ind, and flags.

    The following table compares parameters for the listen functions:

    Table 9-8 Listen Parameters  
    DBMS_AQ.LISTEN Parameter OCIAQListen() Parameter

    Note: OCIAQListen() also requires the following additional parameters: svchp, errhp, agent_list, num_agents, wait, agent, and flags.

    The following table compares parameters for the array enqueue functions:

    Table 9-9 Array Enqueue Parameters  
    DBMS_AQ.ENQUEUE_ARRAY Parameter OCIAQEnqArray() Parameter

    Note: OCIAQEnqArray() also requires the following additional parameters: svch, errh, payload_tdo, payload_ind, ctxp, enqcbfp, and flags.

    The following table compares parameters for the array dequeue functions:

    Table 9-10 Array Dequeue Parameters  
    DBMS_AQ.DEQUEUE_ARRAY Parameter OCIAQDeqArray() Parameter

    Note: OCIAQDeqArray() also requires the following additional parameters: svch, errh, msgprop, payload_tdo, payload_ind, ctxp, deqcbfp, and flags.

    The following table compares parameters for the agent attributes:

    Table 9-11 Agent Parameters  
    PL/SQL Agent Parameter OCIAQAgent Attribute






    The following table compares parameters for the message properties:

    Table 9-12 Message Properties  
    PL/SQL Message Property OCIAQMsgProperties Attribute
























    The following table compares enqueue option attributes:

    Table 9-13 Enqueue Option Attributes  
    PL/SQL Enqueue Option OCIAQEnqOptions Attribute






    The following table compares dequeue option attributes:

    Table 9-14 Dequeue Option Attributes  
    PL/SQL Dequeue Option OCIAQDeqOptions Attribute














    Publish-Subscribe Notification in OCI

    The publish-subscribe notification feature allows an OCI application to receive client notifications directly, register an e-mail address to which notifications can be sent, register a HTTP URL to which notifications can be posted, or register a PL/SQL procedure to be invoked on a notification. Figure 9-2, "Publish-Subscribe Model" illustrates the process.

    Figure 9-2 Publish-Subscribe Model

    Text description of lnoci039.gif follows

    Text description of the illustration lnoci039.gif

    An OCI application can:

    • register interest in notifications in the AQ namespace and be notified when an enqueue occurs.
    • register interest in subscriptions to database events and receive notifications when the events are triggered.
    • manage registrations, such as disabling registrations temporarily or dropping the registrations entirely.
    • post, or send, notifications to registered clients.

    In all the preceding scenarios the notification can be received directly by the OCI application, or the notification can be sent to a pre-specified e-mail address, or it can be sent to a pre-defined HTTP URL, or a pre-specified database PL/SQL procedure can be invoked as a result of a notification.

    Registered clients are notified asynchronously when events are triggered or on an explicit AQ enqueue. Clients do not need to be connected to a database.

    See Also:

    Publish-Subscribe Registration Functions in OCI

    Registration can be done in two ways:

    • You register directly to the database. This way is simple and the registration will take effect immediately.
    • Open Registration. You register using LDAP, from which the database receives the registration request. This is useful when the client cannot have a database connection (the client wants to register for a database open event while the database is down), or if the client wants to register for the same event or events in multiple databases at one time.

    Let us next consider these two alternative ways of registration.

    Publish-Subscribe Register Directly to the Database

    The following steps are required in an OCI application to register and receive notifications for events. It is assumed that the appropriate event trigger or AQ queue has been set up. The initialization parameter COMPATIBLE must be set to 8.1 or higher.

    See Also:


    The publish-subscribe feature is only available on multithreaded operating systems.

    1. Execute OCIInitialize() with OCI_EVENTS mode to specify that the application is interested in registering for and receiving notifications. This starts a dedicated listening thread for notifications on the client.
    2. Execute OCIHandleAlloc() with handle type OCI_HTYPE_SUBSCRIPTION to allocate a subscription handle.
    3. Execute OCIAttrSet() to set the subscription handle attributes for:
      • OCI_ATTR_SUBSCR_NAME - subscription name
      • OCI_ATTR_SUBSCR_NAMESPACE - subscription namespace
      • OCI_ATTR_SUBSCR_CALLBACK - notification callback
      • OCI_ATTR_SUBSCR_CTX - callback context
      • OCI_ATTR_SUBSCR_PAYLOAD - payload buffer for posting
      • OCI_ATTR_SUBSCR_RECPT - recipient name
      • OCI_ATTR_SUBSCR_RECPTPROTO - protocol to receive notification with
      • OCI_ATTR_SUBSCR_RECPTPRES - presentation to receive notification with

      OCI_ATTR_SUBSCR_NAME, OCI_ATTR_SUBSCR_NAMESPACE and OCI_ATTR_SUBSCR_RECPTPROTO must be set before registering a subscription.



      Setting OCI_ATTR_SUBSCR_CALLBACK and OCI_ATTR_SUBSCR_RECPT at the same time will cause an application error.

      OCI_ATTR_SUBSCR_PAYLOAD is required before posting to a subscription.

      See Also:

      For information on these attributes, see "Subscription Handle Attributes"

    4. If OCI_ATTR_SUBSCR_RECPTPROTO is set to OCI_SUBSCR_PROTO_OCI, then define the callback routine to be used with the subscription handle.

    5. If OCI_ATTR_SUBSCR_RECPTPROTO is set to OCI_SUBSCR_PROTO_SERVER, then define the PL/SQL procedure, to be invoked on notification, in the database.

    6. Execute OCISubscriptionRegister() to register with the subscriptions. This call can register interest in several subscriptions at the same time.

    Open Registration for Publish-Subscribe

    Prerequisites for this method are:

    • Registering using LDAP (open registration) requires the client to be an enterprise user.

      See Also:

      Oracle Advanced Security Administrator's Guide, sections on managing enterprise user security

    • The compatibility of the database has to be 9.0 or higher.
    • LDAP_REGISTRATION_ENABLED must be set to TRUE. This can be done this way:

      The default is FALSE.

    • LDAP_REG_SYNC_INTERVAL must be set to the time interval (in seconds) to refresh registrations from LDAP:

      The default is 0, which means do not refresh.

    • To force a database refresh of LDAP registration information immediately:

    The steps in open registration using Oracle Enterprise Security Manager (OESM) are:

    1. In each enterprise domain, create enterprise role, ENTERPRISE_AQ_USER_ROLE.
    2. For each database in the enterprise domain, add global role GLOBAL_AQ_USER_ROLE to enterprise role ENTERPRISE_AQ_USER_ROLE.
    3. For each enterprise domain, add enterprise role ENTERPRISE_AQ_USER_ROLE to privilege group cn=OracleDBAQUsers, under cn=oraclecontext, under the administrative context.
    4. For each enterprise user that is authorized to register for events in the database, grant enterprise role ENTERPRISE_AQ_USER_ROLE.

    Using OCI to Open Register with LDAP

    1. Call OCIInitialize() with mode set to OCI_EVENTS | OCI_USE_LDAP.
    2. Call OCIAttrSet() to set the following environment handle attributes for accessing LDAP:
    • OCI_ATTR_LDAP_HOST: the host name on which the LDAP server resides
    • OCI_ATTR_LDAP_PORT: the port on which the LDAP server is listening
    • OCI_ATTR_BIND_DN: the distinguished name to login to the LDAP server, usually the DN of the enterprise user
    • OCI_ATTR_LDAP_CRED: the credential used to authenticate the client, for example, the password for simple authentication (username/password)
    • OCI_ATTR_WALL_LOC: for SSL authentication, the location of the client wallet
    • OCI_ATTR_LDAP_AUTH: the authentication method code

      See Also:

      "OCI_ATTR_LDAP_AUTH" for a complete list of authentication modes

    • OCI_ATTR_LDAP_CTX: the administrative context for Oracle in the LDAP server
    • Call OCIHandleAlloc() with handle type OCI_HTYPE_SUBSCRIPTION, to allocate a subscription handle.
    • Call OCIDescriptorAlloc() with descriptor type OCI_DTYPE_SRVDN, to allocate a server DN descriptor.
    • Call OCIAttrSet() to set the server DN descriptor attributes for OCI_ATTR_SERVER_DN, the distinguished name of the database in which the client wants to receive notifications. OCIAttrSet() can be called multiple times for this attribute so that more than one database server is included in the registration
    • Call OCIAttrSet() to set the subscription handle attributes for:
    • OCI_ATTR_SUBSCR_NAME: subscription name
    • OCI_ATTR_SUBSCR_NAMESPACE: subscription namespace
    • OCI_ATTR_SUBSCR_CALLBACK: notification callback
    • OCI_ATTR_SUBSCR_CTX: callback context
    • OCI_ATTR_SUBSCR_PAYLOAD: payload buffer for posting
    • OCI_ATTR_SUBSCR_RECPT: recipient name
    • OCI_ATTR_SUBSCR_RECPTPROTO: protocol to receive notification
    • OCI_ATTR_SUBSCR_RECPTRES: presentation to receive notification with
    • OCI_ATTR_SUBSCR_SERVER_DN: the descriptor handles you populated in step 5
    • Call OCISubscriptionRegister() to register the subscriptions. The registration will take effect when the database accesses LDAP to pick up new registrations. The frequency of pick-ups is determined by the value of REG_SYNC_INTERVAL.

    OCI Functions Used to Manage Publish-Subscribe Notification

    The following functions are used to manage publish-subscribe notification.

    Table 9-15 Publish-Subscribe Functions  
    Function Purpose


    Disables a subscription.


    Enables a subscription.


    Posts a subscription.


    Registers a subscription.


    Unregisters a subscription.

    Notification Callback in OCI

    The client needs to register a notification callback that gets invoked when there is some activity on the subscription for which interest has been registered. In the AQ namespace, for instance, this occurs when a message of interest is enqueued.

    This callback is typically set through the OCI_ATTR_SUBSCR_CALLBACK attribute of the subscription handle.

    See Also:

    For information, see "Subscription Handle Attributes"

    The callback must return a value of OCI_CONTINUE and adhere to the following specification:

    typedef ub4 (*OCISubscriptionNotify) ( dvoid           *pCtx,
                                           OCISubscription *pSubscrHp,
                                           dvoid           *pPayload,
                                           ub4             iPayloadLen,
                                           dvoid           *pDescriptor,
                                           ub4             iMode);

    The parameters are described as follows:

    pCtx (IN)

    A user-defined context specified when the callback was registered.

    pSubscrHp (IN)

    The subscription handle specified when the callback was registered.

    pPayload (IN)

    The payload for this notification. For this release, only ub1 * (a sequence of bytes) for the payload is supported.

    iPayloadLen (IN)

    The length of the payload for this notification.

    pDescriptor (IN)

    The namespace-specific descriptor. Namespace-specific parameters can be extracted from this descriptor. The structure of this descriptor is opaque to the user and its type is dependent on the namespace.

    The attributes of the descriptor are namespace-specific. For Advanced Queuing, the descriptor is OCI_DTYPE_AQNFY. The attributes of this descriptor are:

    • Queue Name - OCI_ATTR_QUEUE_NAME
    • Consumer Name - OCI_ATTR_CONSUMER_NAME
    • Message Id - OCI_ATTR_NFY_MSGID
    • Message Properties - OCI_ATTR_MSG_PROP

      See Also:

      For more information about OCI and Advanced Queuing, refer to "OCI and Streams Advanced Queuing"

    iMode (IN)

    Call-specific mode. Valid value:

    • OCI_DEFAULT - executes the default call

    Notification Procedure

    The PL/SQL procedure that will be invoked when there is some activity on the subscription for which interest has been registered, has to be created in the database.

    This procedure is typically set through the OCI_ATTR_SUBSCR_RECPT attribute of the subscription handle.

    See Also:

    Publish-Subscribe Direct Registration Example

    This example shows how system events, client notification, and Advanced Queuing work together to implement publish subscription notification.

    The following PL/SQL code creates all objects necessary to support a publish-subscribe mechanism under the user schema, pubsub. In this code, the Agent snoop subscribes to messages that are published at logon events. Note that the user pubsub needs AQ_ADMINISTRATOR_ROLE and AQ_USER_ROLE privileges to use Advance Queuing functionality. The initialization parameter _SYSTEM_TRIG_ENABLED must be set to TRUE (the default) to enable triggers for system events.

    ----create queue table for persistent multiple consumers
    connect pubsub/pubsub;
    ---- Create or replace a queue table
      COMPATIBLE => '8.1.5');
    ---- Create a persistent queue for publishing messages
    ---- Create a queue for logon events
      COMMENT=>'Q for error triggers');
    ---- Start the queue
    ---- define new_enqueue for convenience
    create or replace procedure new_enqueue(queue_name  in varchar2,
                                            payload  in raw ,
    correlation in varchar2 := NULL,
    exception_queue in varchar2 := NULL)
      enq_ct     dbms_aq.enqueue_options_t;
      msg_prop   dbms_aq.message_properties_t;
      enq_msgid  raw(16);
      userdata   raw(1000);
      msg_prop.exception_queue := exception_queue;
      msg_prop.correlation := correlation;
      userdata := payload;
      DBMS_AQ.ENQUEUE(queue_name,enq_ct, msg_prop,userdata,enq_msgid);
    ---- add subscriber with rule based on current user name, 
    ---- using correlation_id
      subscriber :=$_agent('SNOOP', null, null);
      dbms_aqadm.add_subscriber(queue_name => 'pubsub.logon',
                                subscriber => subscriber,
                                rule => 'CORRID = ''ix'' ');
    ---- create a trigger on logon on database
    ---- create trigger on after logon
    create or replace trigger systrig2
         new_enqueue('pubsub.logon', hextoraw('9999'), dbms_standard.login_user);
    ---- create a PL/SQL callback for notification of logon 
    ---- of user 'ix' on database
    create or replace procedure plsqlnotifySnoop(
      context raw, reginfo$_reg_info, descr$_descriptor,
      payload raw, payloadl number)
     dbms_output.put_line('Notification : User ix Logged on\n');

    After the subscriptions are created, the client needs to register for notification using callback functions. The following sample code performs the necessary steps for registration. The initial steps of allocating and initializing session handles are omitted here for sake of clarity.

    static ub4 namespace = OCI_SUBSCR_NAMESPACE_AQ;
    static OCISubscription *subscrhpSnoop = (OCISubscription *)0;
    static OCISubscription *subscrhpSnoopMail = (OCISubscription *)0;
    static OCISubscription *subscrhpSnoopServer = (OCISubscription *)0;
    /* callback function for notification of logon of user 'ix' on database */
    static ub4 notifySnoop(ctx, subscrhp, pay, payl, desc, mode)
        dvoid *ctx;
        OCISubscription *subscrhp;
        dvoid *pay;
        ub4 payl;
        dvoid *desc;
        ub4 mode;
        printf("Notification : User ix Logged on\n");
      (void)OCIHandleFree((dvoid *)subscrhpSnoop,
                (ub4) OCI_HTYPE_SUBSCRIPTION);
        return 1;
    static void checkerr(errhp, status)
    OCIError *errhp;
    sword status;
      text errbuf[512];
      ub4 buflen;
      sb4 errcode;
      if (status == OCI_SUCCESS) return;
      switch (status)
        printf("Error - OCI_SUCCESS_WITH_INFO\n");
      case OCI_NEED_DATA:
        printf("Error - OCI_NEED_DATA\n");
      case OCI_NO_DATA:
        printf("Error - OCI_NO_DATA\n");
      case OCI_ERROR:
        OCIErrorGet ((dvoid *) errhp, (ub4) 1, (text *) NULL, &errcode,
                errbuf, (ub4) sizeof(errbuf), (ub4) OCI_HTYPE_ERROR);
        printf("Error - %s\n", errbuf);
        printf("Error - OCI_INVALID_HANDLE\n");
        printf("Error - OCI_STILL_EXECUTING\n");
      case OCI_CONTINUE:
        printf("Error - OCI_CONTINUE\n");
        printf("Error - %d\n", status);
    static void initSubscriptionHn (subscrhp,
    OCISubscription **subscrhp;
      char * subscriptionName;
      dvoid * func;
      ub4 recpproto;
      char * recpaddr;
      ub4 recppres;
        /* allocate subscription handle */
        (void) OCIHandleAlloc((dvoid *) envhp, (dvoid **)subscrhp,
            (ub4) OCI_HTYPE_SUBSCRIPTION,
            (size_t) 0, (dvoid **) 0);
        /* set subscription name in handle */
        (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
            (dvoid *) subscriptionName,
            (ub4) strlen((char *)subscriptionName),
            (ub4) OCI_ATTR_SUBSCR_NAME, errhp);
        /* set callback function in handle */
        if (func)
          (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
              (dvoid *) func, (ub4) 0,
              (ub4) OCI_ATTR_SUBSCR_CALLBACK, errhp);
        /* set context in handle */
        (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
            (dvoid *) 0, (ub4) 0,
           (ub4) OCI_ATTR_SUBSCR_CTX, errhp);
        /* set namespace in handle */
        (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
            (dvoid *) &namespace, (ub4) 0,
            (ub4) OCI_ATTR_SUBSCR_NAMESPACE, errhp);
        /* set receive with protocol in handle */
        (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
            (dvoid *) &recpproto, (ub4) 0,
            (ub4) OCI_ATTR_SUBSCR_RECPTPROTO, errhp);
        /* set recipient address in handle */
        if (recpaddr)
          (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
              (dvoid *) recpaddr, (ub4) strlen(recpaddr),
              (ub4) OCI_ATTR_SUBSCR_RECPT, errhp);
        /* set receive with presentation in handle */
        (void) OCIAttrSet((dvoid *) *subscrhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
            (dvoid *) &recppres, (ub4) 0,
            (ub4) OCI_ATTR_SUBSCR_RECPTPRES, errhp);
        printf("Begining Registration for subscription %s\n", subscriptionName);
        checkerr(errhp, OCISubscriptionRegister(svchp, subscrhp, 1, errhp,
    int main( argc, argv)
    int    argc;
    char * argv[];
        OCISession *authp = (OCISession *) 0;
    Initialize OCI Process/Environment
    Initialize Server Contexts
    Connect to Server
    Set Service Context
         /* Registration Code Begins */
    /* Each call to initSubscriptionHn allocates
               and Initialises a Registration Handle */
    /* Register for OCI notification */
        initSubscriptionHn(    &subscrhpSnoop,    /* subscription handle*/
        (char*) "PUBSUB.LOGON:SNOOP", /* subscription name 
            (dvoid*)notifySnoop,  /* callback function */
            OCI_SUBSCR_PROTO_OCI, /* receive with protocol */
            (char *)0, /* recipient address */
            OCI_SUBSCR_PRES_DEFAULT); /* receive with presentation */
    /* Register for email notification */
        initSubscriptionHn(    &subscrhpSnoopMail,    /* subscription handle */
         (char*) "PUBSUB.LOGON:SNOOP", /* subscription name */ /*
    <queue_name>:<agent_name> */
            (dvoid*)0, /* callback function */
            OCI_SUBSCR_PROTO_MAIL, /* receive with protocol */
            (char*)  "", /* recipient address */
            OCI_SUBSCR_PRES_DEFAULT); /* receive with presentation */
    /* Register for server to server notification */
        initSubscriptionHn(    &subscrhpSnoopServer,    /* subscription handle */
           (char*)  "PUBSUB.LOGON:SNOOP", /* subscription name */ /*
    <queue_name>:<agent_name> */
            (dvoid*)0, /* callback function */
            OCI_SUBSCR_PROTO_SERVER, /* receive with protocol */
             (char*) "pubsub.plsqlnotifySnoop", /* recipient address */
            OCI_SUBSCR_PRES_DEFAULT); /* receive with presentation */
        checkerr(errhp, OCITransCommit(svchp, errhp, (ub4) OCI_DEFAULT));
    The Client Process does not need a live Session for Callbacks
    End Session and Detach from Server
        OCISessionEnd ( svchp,  errhp, authp, (ub4) OCI_DEFAULT);
        /* detach from server */
        OCIServerDetach( srvhp, errhp, OCI_DEFAULT);
       while (1)    /* wait for callback */

    If user IX logs on to the database, the client is notified by e-mail, and the callback function notifySnoop is called. An e-mail notification will be sent to the address and the PL/SQL procedure plsqlnotifySnoop will also be called in the database.

    Publish-Subscribe LDAP Registration Example

    The following code fragment illustrates how to do LDAP registration. Please read all the program comments:

      /* TO use LDAP registration feature, OCI_EVENTS | OCI_USE_LDAP must be set 
         in OCIInitialize: */
      (void) OCIInitialize((ub4) OCI_EVENTS|OCI_OBJECT|OCI_USE_LDAP, (dvoid *)0,
                           (dvoid * (*)(dvoid *, size_t)) 0,
                           (dvoid * (*)(dvoid *, dvoid *, size_t))0,
                           (void (*)(dvoid *, dvoid *)) 0 );
      /* set LDAP attributes in the environment handle */
      /* LDAP host name */
      (void) OCIAttrSet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)"yow", 3
                        OCI_ATTR_LDAP_HOST, (OCIError *)errhp);
      /* LDAP server port */ 
      ldap_port = 389;
      (void) OCIAttrSet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&ldap_port,
                        (ub4)0, OCI_ATTR_LDAP_PORT, (OCIError *)errhp);
      /* bind DN of the client, normally the enterprise user name */
      (void) OCIAttrSet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)"cn=orcladmin",
                        12, OCI_ATTR_BIND_DN, (OCIError *)errhp);
      /* password of the client */
      (void) OCIAttrSet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)"welcome",
                        7, OCI_ATTR_LDAP_CRED, (OCIError *)errhp);
      /* authentication method is "simple", username/password authentication */
      ldap_auth = 0x01;
      (void) OCIAttrSet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&ldap_auth,
                        (ub4)0, OCI_ATTR_LDAP_AUTH, (OCIError *)errhp);
      /* adminstrative context: this is the DN above cn=oraclecontext */
      (void) OCIAttrSet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)"cn=acme,cn=com",
                        14, OCI_ATTR_LDAP_CTX, (OCIError *)errhp);
      /* retrieve the LDAP attributes from the environment handle */
      /* LDAP host */
      (void) OCIAttrGet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&buf, 
                        &szp,  OCI_ATTR_LDAP_HOST,  (OCIError *)errhp);
      /* LDAP server port */
      (void) OCIAttrGet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&intval, 
                        0,  OCI_ATTR_LDAP_PORT,  (OCIError *)errhp);
      /* client binding DN */
      (void) OCIAttrGet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&buf, 
                        &szp,  OCI_ATTR_BIND_DN,  (OCIError *)errhp);
      /* client password */
      (void) OCIAttrGet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&buf, 
                        &szp,  OCI_ATTR_LDAP_CRED,  (OCIError *)errhp);
      /* adminstrative context */
      (void) OCIAttrGet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&buf, 
                        &szp,  OCI_ATTR_LDAP_CTX,  (OCIError *)errhp);
      /* client authentication method */
      (void) OCIAttrGet((dvoid *)envhp, OCI_HTYPE_ENV, (dvoid *)&intval, 
                        0,  OCI_ATTR_LDAP_AUTH,  (OCIError *)errhp);
      /* to set up the server DN descriptor in the subscription handle */
      /* allocate a server DN descriptor, dn is of type "OCIServerDNs **", 
         subhp is of type "OCISubscription **" */
      (void) OCIDescriptorAlloc((dvoid *)envhp, (dvoid **)dn, 
                             (ub4) OCI_DTYPE_SRVDN, (size_t)0, (dvoid **)0);
      /* now *dn is the server DN descriptor, add the DN of the first database 
         that we want to register */
      (void) OCIAttrSet((dvoid *)*dn, (ub4) OCI_DTYPE_SRVDN, 
                        (dvoid *)"cn=server1,cn=oraclecontext,cn=acme,cn=com",
                        42, (ub4)OCI_ATTR_SERVER_DN, errhp);
      /* add the DN of another database in the descriptor */
      (void) OCIAttrSet((dvoid *)*dn, (ub4) OCI_DTYPE_SRVDN, 
                        (dvoid *)"cn=server2,cn=oraclecontext,cn=acme,cn=com",
                        42, (ub4)OCI_ATTR_SERVER_DN, errhp);
      /* set the server DN descriptor into the subscription handle */
      (void) OCIAttrSet((dvoid *) *subhp, (ub4) OCI_HTYPE_SUBSCRIPTION,
                     (dvoid *) *dn, (ub4)0, (ub4) OCI_ATTR_SERVER_DNS, errhp);
      /* now we will try to get the server DN information from the subscription
         handle */
      /* first, get the server DN descriptor out */
      (void) OCIAttrGet((dvoid *) *subhp, (ub4) OCI_HTYPE_SUBSCRIPTION, 
                        (dvoid *)dn, &szp, OCI_ATTR_SERVER_DNS, errhp);
      /* then, get the number of server DNs in the descriptor */
      (void) OCIAttrGet((dvoid *) *dn, (ub4)OCI_DTYPE_SRVDN, (dvoid *)&intval,
                        &szp, (ub4)OCI_ATTR_DN_COUNT, errhp);
      /* allocate an array of char * to hold server DN pointers returned by
         oracle */
        if (intval)
          arr = (char **)malloc(intval*sizeof(char *));
          (void) OCIAttrGet((dvoid *)*dn, (ub4)OCI_DTYPE_SRVDN, (dvoid *)arr,
                            &intval, (ub4)OCI_ATTR_SERVER_DN, errhp);
      /* OCISubscriptionRegister() calls have two modes: OCI_DEFAULT and 
         OCI_REG_LDAPONLY. If OCI_DEFAULT is used, there should be only one
         server DN in the server DN descriptor. The registration request will
         be sent to the database. If a database connection is not available,
         the registration request will be detoured to the LDAP server. On the 
         other hand, if mode OCI_REG_LDAPONLY is used the registration request
         will be directly sent to LDAP. This mode should be used when there are 
         more than one server DNs in the server DN descriptor, or we are sure
         that a database connection is not available.
         In this example, two DNs are entered; so we should use mode 
         OCI_REG_LDAPONLY in register. */
      OCISubscriptionRegister(svchp, subhp, 1, errhp, OCI_REG_LDAPONLY);
      /* as OCISubscriptionRegister(), OCISubscriptionUnregister() also has
         mode OCI_DEFAULT and OCI_REG_LDAPONLY. The usage is the same. */
      OCISubscriptionUnRegister(svchp, *subhp, errhp, OCI_REG_LDAPONLY);