HTTPWhat?!
What is HTTP and how does it work.
HTTP is a protocol which allows the fetching of resources, such as HTML documents. It is the foundation of any data exchange on the Web and it is a client-server protocol, which means requests are initiated by the recipient, usually a Web browser.
Clients and servers communicate by exchanging individual messages. The messages sent by the client are called requests and the messages sent by the server as an answer are called responses.
Components of HTTP-based systems
Client: the user-agent
The user-agent is any tool that acts on the behalf of the user. This role is primarily performed by the Web browser. The browser is always the entity initiating the request. It is never the server (though some mechanisms have been added over the years to simulate server-initiated messages).
To present a Web page, the browser sends an original request to fetch the HTML document that represents the page. It then parses this file, making additional requests corresponding to execution scripts, layout information (CSS) to display, and sub-resources contained within the page (usually images and videos). The Web browser then mixes these resources to present to the user a complete document, the Web page. Scripts executed by the browser can fetch more resources in later phases and the browser updates the Web page accordingly.
A Web page is a hypertext document. This means some parts of displayed text are links which can be activated (usually by a click of the mouse) to fetch a new Web page, allowing the user to direct their user-agent and navigate through the Web. The browser translates these directions in HTTP requests, and further interprets the HTTP responses to present the user with a clear response.
The Web server
On the opposite side of the communication channel, is the server, which serves the document as requested by the client. A server appears as only a single machine virtually: this is because it may actually be a collection of servers, sharing the load (load balancing) or a complex piece of software interrogating other computers (like cache, a DB server, or e-commerce servers), totally or partially generating the document on demand.
A server is not necessarily a single machine, but several server software instances can be hosted on the same machine.
Proxies
Between the Web browser and the server, numerous computers and machines relay the HTTP messages. Due to the layered structure of the Web stack, most of these operate at the transport, network or physical levels, becoming transparent at the HTTP layer and potentially making a significant impact on performance. Those operating at the application layers are generally called proxies. These can be transparent, forwarding on the requests they receive without altering them in any way, or non-transparent, in which case they will change the request in some way before passing it along to the server. Proxies may perform numerous functions:
- caching (the cache can be public or private, like the browser cache)
- filtering (like an antivirus scan or parental controls)
- load balancing (to allow multiple servers to serve the different requests)
- authentication (to control access to different resources)
- logging (allowing the storage of historical information)
HTTP flow
When a client wants to communicate with a server, either the final server or an intermediate proxy, it performs the following steps:
- Open a TCP connection: The TCP connection is used to send a request, or several, and receive an answer. The client may open a new connection, reuse an existing connection, or open several TCP connections to the servers.
- Send an HTTP message, for example:
GET / HTTP/1.1 Host: developer.mozilla.org Accept-Language: fr
3. Read the response sent by the server, such as:
HTTP/1.1 200 OK Date: Sat, 09 Oct 2010 14:28:02 GMT Server: Apache Last-Modified: Tue, 01 Dec 2009 20:18:22 GMT ETag: "51142bc1-7449-479b075b2891b" Accept-Ranges: bytes Content-Length: 29769 Content-Type: text/html <!DOCTYPE html... (here comes the 29769 bytes of the requested web page)
4. Close or reuse the connection for further requests.
HTTP Messages
There are two types of HTTP messages, requests and responses, each with its own format.
Requests
An example HTTP request:
Requests consist of the following elements:
- An HTTP method, usually a verb like
GET,POSTor a noun likeOPTIONSorHEADthat defines the operation the client wants to perform. Typically, a client wants to fetch a resource (usingGET) or post the value of an HTML form (usingPOST), though more operations may be needed in other cases. - The path of the resource to fetch; the URL of the resource stripped from elements that are obvious from the context, for example without the protocol (
http://), the domain (here,developer.mozilla.org), or the TCP port (here,80). - The version of the HTTP protocol.
- Optional headers that convey additional information for the servers.
- Or a body, for some methods like
POST, similar to those in responses, which contain the resource sent.
Responses
An example response:
Responses consist of the following elements:
- The version of the HTTP protocol they follow.
- A status code, indicating if the request was successful, or not, and why.
- A status message, a non-authoritative short description of the status code.
- HTTP headers, like those for requests.
- Optionally, a body containing the fetched resource.
