Introduction to Web applications:

Lecture Notes for CS349W
Fall Quarter 2008
John Ousterhout

• Web applications introduce dynamic behavior in 3 ways:
  • Web pages may contain dynamically-generated content (e.g., from a database): the page may contain different information each time it is displayed.
    • When a page is referenced, a program runs on the server to generate the page; these programs can be written in various languages, including Ruby, PHP, Java, and Perl.
    • Almost all Web applications use a relational database for storing the dynamic data.
  • Data entered by the user (typically in forms) can be sent back to the server to modify the back-end data.
  • Programs in Javascript can run in the browser to achieve additional dynamic effects:
    • Pieces of a page can be updated individually without redisplaying the entire page, using AJAX.
    • Javascript can produce various other dynamic effects, such as drag-and-drop, direct manipulation, visual effects, etc.
• How does a Web application compare to a traditional installed GUI application?

  Traditional                           Web
  ----------------------------------------------------------------------------
  All application state in one place    State split between server and browser
  Data kept in main memory              Data stored in database
  Low latency between display and app   Higher latency between browser and
                                        server
  Stateful                              Stateless
  Manipulate screen directly            HTML document model
  Single language (C/C++, Java)         Multiple languages/syntaxes:
                                            HTML
                                            CSS
                                            Server-side language (e.g. Ruby
                                               or Java)
                                            Javascript
                                            SQL for database
  One user for one application          Multiple users managed by a single
                                        server
  

• Although it's possible to create interesting interactive applications with the Web paradigm, it isn't easy:
  • Many languages and formats to learn.
  • The state of the application is split between the browser and server:
    • A single server handles many browsers, and it must manage the state for all of them, each logically independent.
    • Only a limited amount of state can be kept on the browser, and it is volatile (it tends to get lost when new pages are displayed).
    • Information kept on the browser isn't very secure:
      • Hard to keep it from leaking to other Web applications in the same browser.
      • The server can't trust most information coming from the browser because it can be forged.
    • Frequent communication between browser and server can make applications slow. But frequent communication is necessary to get a high level of interactivity.
  • User-interface capabilities not as sophisticated as for traditional applications (no pixel-level access)
  • For many applications the server must be highly scalable, which adds to complexity:
    • The first step is to replicate the Web front-ends using a load-balancer to distribute incoming connections.
    • Unfortunately, this means that connections from the same browser may go to different servers at different times; if server state is kept in main memory, it may be hard for a new server to find existing state.
    • For very large applications, such as Amazon or Facebook, the back-end database becomes a scalability bottleneck and has to be partitioned manually.
  • Users generally expect high reliability from Web applications: server crashes should not be visible to users.
  • Security issues are more complex for Web applications: bad guys can get between the user interface and the back end.
  • The most common mode of development is for every project to build everything from scratch, starting with the languages and database.
    • There are relatively few reusable building blocks to make it easier to build Web applications.
    • Existing frameworks don't raise the level of programming very much.
• On the other hand, Web applications give you some things that traditional applications don't:
  • Immediate universal accessibility: no downloads or installs.
  • Instant sharing: people and organizations can work together.
  • No porting issues with server software (but, must face browser compatibility issues).
  • Easy to make frequent upgrades to applications; encourages agile programming.
  • Easy to measure all usage of application.
  • Can test new features on a subset of user committee, measure impact.