CO3404 Distributed Systems
CO3404 Lecture 4 - JavaScript Functions

Lecture Documents¶

CO3404 Lecture 5.pdf

Written Notes¶

Calling an API¶

Response from backend maybe slow which in a synchronous frontend will block the single javascript thread, causing the UI to become unresponsive for the user. However, the JavaScript fetch function does not block the UI, effectively running in a separate thread (technically a C++ function call to manage a thread).

It is also important to ensure that the backend server thread is not being blocked given it runs on JavaScript (Express.JS) especially in the event of lengthy requests or lookups.

flowchart LR
    Code["function level1() {
      level2();
    }
    function level2() {
      level3();
    }
    function level3() {
      setTimeout(callbackDemo, 2000);
    }
    const callbackDemo = function() {
      console.log('In callbackDemo');
    }
    level1();"]

    subgraph CS[Call Stack]
        CS_cb[callbackDemo]
    end

    subgraph APIs
        API_setTimeout[setTimeout]
        API_console[console]
    end

    subgraph EQ[Event Queue FIFO]
        EQ_cb[callbackDemo callback]
    end

    MH[Event Loop]

    CS_cb --> API_console
    CS_cb -.-> API_setTimeout
    API_setTimeout -.-> EQ_cb
    EQ_cb --> MH
    MH --> CS_cb

CO3404 Lecture 6 - Adding a Database to the Stack