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HTTP: Learning Outcomes

  • Know the different between GET, POST, PUT and DELETE
  • Understand the anatomy of a url
  • Know what HTTP means
  • Recognise what status codes stand for

HTTP

The Hypertext Transfer Protocol (HTTP) is the mechanism through which data is requested and provided on the World Wide Web.

When sending an API request, we use a URL to identify the specific server where we want to send the request (kind of like the address on an envelope) and the rest of the url specifies what it is we want the server to send back. These are called parameters or queries.

The image below labels the parts of a url. DNS uses the resource name to look up the server to which the request is addressed. Anything beyond the resource name is part of the query or filepath the specifies exactly what data or resources are being requested.

parts of a url image

Other transfer protocols

There are many other communication protocols, including FTP (file transfer protocol), SMTP (Simple Mail Transfer Protocol) and IMAP (Internet Message Access Protocol).

HTTPS and FTPS are secure versions of HTTP and FTP. They use SSL (Secure Sockets Layer), which encrypts the request and response. There is a close analogy between protocols and programming languages: protocols are to communications what programming languages are to computations.

HTTP methods

GET

  • 'gets' or grabs resources such as HTML, JavaScript, CSS, images and so on.
  • GET requests should not modify the resource in any way.
  • Request query values are sent as part of the url (so don't use GET when sending sensitive information!).

EXAMPLE: Looking at a particular topic on Wikipedia.

POST

  • Sends data to the server.
  • This data (e.g., from an HTML form) is processed to a specified path to modify or update a resource.
  • The data values are sent in the body of the request, in the format that the Content-Type header specifies. This way the server knows how to decode the information.
  • A Content-Length header is also required so that the server knows when it has received all of the information (a large amount of data might be split across multiple requests).
  • Unlike GET, POST data is not displayed in the url.

EXAMPLE: Submitting a comment on a message board.

PUT

  • Places some provided data at the exact path specified in the request. Usually used to create a new resource.
  • If something already exists at that path the server will overwrite whatever is there.

Note the distinction between PUT and POST: with POST the path specified in the request will handle the data and the server can put the data where it likes. A PUT request is basically a request to update the resource at the specified path with the data provided.

EXAMPLE: Uploading a file to a server.

DELETE

An obvious one - used when the client requests the server to delete a resource at the path specified.

HTTP status codes

Status codes accompany a server response to indicate whether the request was successful. They are made up of a three-digit number and a human-readable phrase, eg 200 OK or 404 Not Found. There are many status codes, and you don't need to memorise them, but it is useful to know the main groups.

You can view information about HTTP requests including status codes in your browser's Developer Tools, usually under a "Network" tab like here:

A complete list of Status Codes can be found here for reference: https://httpstatuses.com/

Codes starting with 1

These are known as 'informational' codes, indicating that the request was received and a provisional response has been supplied.

100 Continue
The server has received and approved the request headers and is giving permission to the client to send the request body. This is useful when the client wants to post a large amount of data.

101 Switching Protocols
The requester has asked the server to switch protocols and the server has agreed to do so

Codes starting with 2

These are success codes, indicating that the request was received and correctly processed.

200 OK
All is well. If a resource was requested it will be provided.

Codes starting with 3

These indicate some kind of redirection. The 302 status code is a common redirect code. Redirects cannot be handled using XHR requests because the browser takes care of them automatically. You will only get back what is at the redirected location.

Codes starting with 4

These codes indicate some kind of error with the request.

404 Not Found
The famous one! The client has requested a resource that the server doesn't know how to find.

Codes starting with 5

This group of responses indicates an error with the server.