If HTTP is stateless, how does a website keep me logged in?

The server hands your browser a credential after login, either an opaque session ID stored in a cookie or a signed token like a JWT. Your browser attaches it to every request, and the server uses it to reload your identity each time. The protocol stays stateless; the state rides along in the request.

Why can't a server just send data to a client whenever it wants?

In plain HTTP only the client can open a request, and most clients sit behind NATs and firewalls that block inbound connections anyway. To push data, the client first opens a channel outward, using WebSockets, server-sent events, or long polling, and the server delivers over that established connection.

What is the practical difference between HTTP/2 and HTTP/3?

HTTP/2 multiplexes many streams over one TCP connection, but one lost packet stalls every stream because TCP delivers bytes in order. HTTP/3 runs on QUIC over UDP, so streams recover from loss independently and the connection setup is faster. The gains show up most on lossy mobile networks.

Fundamentalschapter 2 of 6 · 4 lessons

Client-Server Model

Clients request, servers respond. The foundation of the web.

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You type an address and press enter. About 400 milliseconds later there is a page.

In that time your machine worked out which server to talk to, opened a connection across a few thousand kilometres, agreed on how to encrypt it, sent a few hundred bytes describing what it wanted, and a machine you will never see read a row out of a database and sent it back.

Most engineers can name message queues and microservices and cannot narrate those 400 milliseconds in order. The opening minutes of an interview tend to find that gap, because every architecture you will ever draw is stacked on top of this one exchange.

Lessons

4 in this chapter

The Request-Response LoopThe client always speaks first. The server can only answer.2 min
Before the First ByteDNS, TCP, and TLS all charge round trips before your API sees anything.2 min
HTTP Verbs and Status CodesThe method tells the server what kind of change you intend. Idempotency is the property that matters.2 min
State on a Stateless ProtocolHTTP forgets you between requests. Cookies and tokens are how apps remember anyway.2 min