An overview of HTTP From MDN

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An overview of HTTP - HTTP | MDN

MDN 上的文档还是很棒的。

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Client: the user-agent: The browser is always the entity initiating the request.

The Web server: A server presents only as a single machine virtually. With HTTP/1.1 and the Host header, they may even share the same IP address.

Proxies:

• caching (the cache can be public or private, like the browser cache)
• filtering (like an antivirus scan, 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)

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Basic aspects of HTTP

HTTP is simple: HTTP messages are human readable.

HTTP is extensible: New HTTP headers can be introduced by a simple agreement between a client and a server.

HTTP is stateless, but not sessionless: While the core of HTTP itself is stateless, HTTP cookies allow the use of stateful sessions.

HTTP and connections: Keep-Alive and call a better transport protocol.

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What can be controlled by HTTP

• Cache
• Relaxing
• the origin constraint
• Authentication
• Proxy and tunneling
• Sessions

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HTTP flow

• Open a TCP connection
• Send an HTTP message
• Read the response sent by the server
• Close or reuse the connection for further requests

If HTTP pipelining is activated, several requests can be sent without waiting for the first response to be fully received. HTTP pipelining has proven difficult to implement in existing networks, where old pieces of software coexist with modern versions. HTTP pipelining has been superseded in HTTP/2 with more robust multiplexing requests within a frame.

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HTTP Messages

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APIs based on HTTP

1. The most commonly used API based on top of HTTP is the XMLHttpRequest API, which can be used to exchange data between a user agent and a server.

2. The modern Fetch APIprovides the same features with a more powerful and flexible feature set.

3. Another API, server-sent events, is a one-way service that allows a server to send events to the client, using HTTP as a transport mechanism.

Using the EventSource interface, the client opens a connection and establishes event handlers. The client browser automatically converts the messages that arrive on the HTTP stream into appropriate Event objects, delivering them to the event handlers that have been registered for the events' type if known, or to the onmessage event handler if no type-specific event handler was established.

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Conclusion

HTTP is an extensible protocol that is easy to use. The client-server structure, combined with the ability to simply add headers, allows HTTP to advance along with the extended capabilities of the Web.

Though HTTP/2 adds some complexity, by embedding HTTP messages in frames to improve performance, the basic structure of messages has stayed the same since HTTP/1.0. Session flow remains simple, allowing it to be investigated, and debugged with a simple HTTP message monitor.