I was wondering how HTTP is stateless while its built over TCP which is stateful ? I'm still begineer backend engineer and I dont have solid understanding of this topics. I tried to search for explanations but I'm not sure if this question has been asked before.

CodePudding user response：

There are transport layer (TCP) states and application layer (HTTP) states.

When talking about TCP being stateful one is talking about transport layer states. TCP is stateful because a transport layer state consisting of current sequence numbers etc is needed to provide the reliability guarantees of TCP, i.e. ordering of packets, removing of duplicates, acknowledgements and retransmission. Thus a state spanning over multiple "units" (packets) is needed.

In HTTP this unit is the HTTP message, i.e. the HTTP request from the client and the HTTP response from the server. When talking about HTTP being stateless it means that there is no state inside the HTTP protocol needed which spans multiple such messages: a response strictly follows a request and there is no state covering multiple requests or responses - all requests are independent from each other from the perspective of HTTP.

Within web applications itself though some state usually is needed, like for a user session. These states are implemented on top of HTTP, usually with cookies shared between the requests. These states are then independent from a specific HTTP request and also independent from the underlying TCP connection.