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HTTP - HyperText Transfer Protocol

The HTTP is an application layer protocol is the foundation of the web.

  • Used to transfer and manipulate data on the web;
  • This data is called "resources";
  • Executes over TCP/IP;
  • Its a client-server model, meaning that a client opens a connection to make a request, and waits until it receives a response from the server;
  • Stateless protocol, meaning that each time a request is made, the server has no knowledge of the previous request;
  • Sessionful protocol, meaning that the server can hold information about the client, by using cookies;

The target of an HTTP request is called a "resource". HTTP does not limit the nature of a resource; it merely defines an interface that might be used to interact with resources. Each resource is identified by a Uniform Resource Identifier (URI), as described in Section 2.7 of [RFC7230].

Uniform Interface

HTTP provides a uniform interface for interacting with a resource, regardless of its type, nature, or implementation, via the manipulation and transfer of representations.

  • The HTTP is nor resource-specific, meaning that it can be used to transfer any type of data;
  • Requests are directed to resources using a generic interface, with standard semantics;
  • This helps to decouple the client and the server, allowing them to evolve independently;

HTTP Messages

There are two types of HTTP messages, requests and responses, each with its own format.

Requests

  • Method - defines the action to be performed;
  • Path - the path to the resource;
  • Version - the HTTP version;
  • Headers - optional headers, containing additional information about the request;
  • Body - contains the data to be sent to the server, optional;

HTTP Request

Request Methods

  • GET - requests a representation of the specified resource. Requests using GET should only retrieve data;
  • HEAD - same as GET, but without the response body, only the headers;
  • POST - used to submit an entity to the specified resource, often causing a change in state or side effects on the server;
  • PUT - replaces all current representations of the target resource with the request payload;
  • DELETE - deletes the specified resource;
  • PATCH - used to apply partial modifications to a resource;
  • ...

Propriedades

  • Safe - an HTTP method is safe if it doesn't alter the state of the server;
  • Idempotent - an HTTP method is idempotent if an identical request can be made once or several times in a row with the same effect while leaving the server in the same state. In other words, an idempotent method should not have any side effects — unless those side effects are also idempotent;
  • Cacheable - a cacheable response is an HTTP response that can be cached, that is stored to be retrieved and used later, saving a new request to the server.
Method Safe Idempotent Cacheable
GET Yes Yes Yes
HEAD Yes Yes Yes
POST No No Yes
PUT No Yes No
DELETE No Yes No
PATCH No No No

Responses

  • Version - the HTTP version;
  • Status code - a 3-digit number that indicates the status of the response;
  • Status message - a textual description of the status code;
  • Headers - optional headers, containing additional information about the response;
  • Body - contains the data to be sent to the client, optional.

HTTP Response

Status Codes

  • 1xx - Informational responses;
  • 2xx - Successful responses;
  • 3xx - Redirection messages;
  • 4xx - Client error responses;
  • 5xx - Server error responses.

More relevant status codes:

  • 200 - OK - the request has succeeded;
  • 201 - Created - the request has succeeded and a new resource has been created as a result;
  • 301 - Moved Permanently - the target resource has been assigned a new permanent URI and any future references to this resource should use one of the enclosed URIs;
  • 304 - Not Modified - indicates that the resource has not been modified since the version specified by the request headers If-Modified-Since or If-None-Match;
  • 400 - Bad Request - the server cannot or will not process the request due to an apparent client error;
  • 401 - Unauthorized - similar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided;
  • 403 - Forbidden - the request was a valid request, but the server is refusing to respond to it;
  • 404 - Not Found - the requested resource could not be found but may be available in the future;
  • 500 - Internal Server Error - a generic error message, given when an unexpected condition was encountered and no more specific message is suitable.

Web Resources

The target of an HTTP request is called a "resource". HTTP does not limit the nature of a resource; it merely defines an interface that might be used to interact with resources. Each resource is identified by a Uniform Resource Identifier (URI), as described in Section 2.7 of [RFC7230].

  • Resources are identified by a URI;
  • URI have an uniform format: scheme:[//authority][/]path[?query][#fragment];
    • Scheme - the protocol used to access the resource, like http or https or mailto.
    • Authority - composed by three components: [user:password@]host[:port]:
      • The userinfo, which is optional, may contain a username and password used for authentication;
      • The host, which identifies the network location of the resource;
      • The port, which is optional and defaults to the standard port if not specified;
    • Path - the path to the resource;
    • Query - the query string, which is optional;
    • Fragment - the fragment identifier, which is optional;