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;

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.

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.

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;