CS 101

Core Application Components

Plan for Today

  • Recall: requests go to a server, which returns a response
  • Recall: servers store information in databases
  • Today: how do applications work?
    • Distributed Systems
    • Databases
    • APIs
    • Code in the Real-World

Another note about ads

Distributed Systems: Theory

  • Problem: more memory is expensive; more CPU is expensive
  • Idea: link a bunch of cheap computers together into a "giant" computer
  • Have each computer solve a tiny part of the problem
  • Have each computer store part of the problem
  • Challenges of Distributed Systems:
    • Sometimes, computers fail
    • Need to store information across multiple computers
    • Need to wait for all computers to finish calculations


Source: Wikipedia
  • Idea: make a faster CPU that can execute many instructions at the same time
  • Each computer solves part of the problem
  • Famous example: WordCount

Distributed System: Databases

Source: Wikipedia
  • (Usually) stored across many computers (distributed system)
    • Added benefit: can spatially disperse the knowledge
  • Recall: like a giant Excel sheet, but with millions/billions/trillions of rows
  • Databases are made of tables. Each table represents one object
  • Relationships between different tables
  • Example: Table of superheroes, and another table of Marvel Movies. Each superhero is in a Marvel movie.


  • Amazon Web Services
  • How Amazon makes money ($17.46 billion in revenue in 2017)
  • Amazon manages servers (cheaper for small websites) - easily scales
  • Amazon S3 - storage by Amazon (e.g. Piazza)
  • Easy way to host a website - make it available on the internet


  • Key concept in computer science: abstraction
  • You can use something without knowing how it works
  • API: Application Programming Interface
  • Set of commands that allow access to data
  • Many companies make their APIs available to developers:

Formatting Data

  • Data has to be organized well to easily extract information
  • APIs need to return data in a well-organized way
  • Commonly use JSON (JavaScript Object Notation); alternative is XML
  • Structured way to determine what information exists and its form
  • "class": {
    	"name": "CS101",
    	"students": ["Ashley", "Shreya"],
    	"location": {"building": "Gates", "number": "B12", "capacity": 50}

Code in the Real-World

  • Writing code "in production" presents lots of challenges
  • Talk with a neighbor about the challenges a large company like Facebook would have to overcome

Code in the Real-World

  • How do we know that code works? Testing
    • Unit tests
    • Integration tests
    • Development servers
  • How do we handle multiple people editing code at the same time? Version Control (GitHub)

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