HTTP 04: Routing and Response

Follows HTTP 03: Frame The Body, which found the exact last byte of an incoming request, and hands off to HTTP Keep-Alive.

Before you start. This series builds a small copy of a web server: a program that reads a request coming in over a network and sends back the right answer. So far it can open a connection, read the words a browser sends, and know exactly where they stop; this stage decides who answers a finished request, and what a correct answer even looks like.

The idea in 30 seconds

A router is a table from an address to a handler, and ties are broken by scoring the match, never by which line was written first.
200 means it worked; 201 means it worked and something new now exists, at an address the reply tells you; 204 means it worked and there is deliberately nothing to send back.
A “go elsewhere” reply has to say two things separately - permanent or temporary, and must the next request keep the same method - which is why there are four different codes for it, not one.
404 means nothing exists here; 405 means the thing exists but not with that method, and a 405 must always say which methods do work instead.
A header built from something a visitor typed can smuggle a second, forged reply into the same connection, if nobody checks it for a stray line break first.
Build one function that every reply passes through, and make it refuse to send a reply that breaks one of these rules, and a whole class of bug stops shipping.

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