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W2D3 – Networking with TCP (and HTTP)

Review

Asynchronous JS:

  • Async code is code that does not execute immediately, but at some specified later time or when a particular event is triggered.

Events in JS:

  • Events are actions or occurrences that happen in the system you are programming, which the system tells you about so you can respond to them as needed.

  • The system will give a signal when an event occurs, so that the appropriate response (that is, a callback function) is taken.

  • An Event Handler is a callback function that will be called when an event is triggered.

The event loop:

Networking – some basic definitions

Protocol:

  • A set of formal rules that guide the 'communication,' that is, the transfer of data between machines.

TCP (Transmission Control Protocol):

  • Connection must first be established between sender and receiver.
  • Messages are broken into packets. Each packet must contain information about who is sending the message, who should receive it, how many total messages there are, and the number of the current message.
  • Packet meta-data is used to re-assemble them into the complete message.
  • TCP is what's known as a reliable protocol, because the sender is informed whether each packet was received at the destination. This information is used by the sender to re-send any missing packets. An example of a popular protocol that is not reliable is called UDP (User Datagram Protocol).

IP (Internet Protocol):

  • Specifies the address of a machine on the network.
  • Includes information about routing, that is, an idea of how messages can be delivered to their correct destinations.
  • IPv4 the established standard, IPv6 slowly being deployed. An IPv4 address looks like this: 123.123.123.123

Building a TCP client and server with Node

We're going to build a TCP server that multiple clients can connect to at the same time. When a client connects to the server, all other connected clients are informed. When a client sends a message to the server, the server sends the message to all other connected clients.

  • Server needs to be set up to accept incoming connection requests

  • Client(s) needs to know where to connect to the server

  • Server needs to keep a list of all connected clients

See the working example in /code

A quick introduction to HTTP

HTTP (HyperText Transfer Protocol):

  • A protocol for sending and receiving files on the web. HTTP is the standard protocol web browsers use to communicate with web servers (but it's not the only one). HTTP is 'layered' on top of TCP.

  • HTTPS (the 'S' stands for 'Secure') is like HTTP, only encrypted.

URL (Uniform Resource Locator):

  • Specifies the address of a particular resource, and is made up of several parts:

    https://www.youtube.com:443/watch?v=SMxx9XEF6m0

    https:// – the protocol (in this case not HTTP, but HTTPS, which is like HTTPS only encrypted)

    www.youtube.com – the domain name or host

    :443 – the port. 443 is the default port for HTTPS, 80 is the default port for HTTP.

    /watch – the path to the resource.

    ?v=SMzz9XEF6m0 – query parameters (we'll learn more about these next week).

Request / Response cycle:

  • HTTP(S) requests include a URL and a method. The URL defines the destination of the request, the method defines what kind of request is being made.

  • One of the most common methods is called a GET request. GET requests are for fetching a resource over HTTP(S).

  • Included in every response from an HTTP(S) server is a status code. Some common status codes are:

    Status code Meaning
    200 Everything's alright
    404 Resource not found
    500 Internal server error

References and further readings

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