TIL
Every net/http request runs in its own goroutine
In Go’s net/http, the server starts a new goroutine for each incoming request. So your handlers run at the same time by default, even if you never wrote a single go statement.
This means any state that a handler touches across requests must be safe for concurrent use. If you share a mutable value (a map, a counter, a cached struct) without a mutex, you will get a data race.
The safe options: keep state per request (local variables, the request context), or protect shared state with a mutex. Before, I thought handlers ran one at a time. They don’t.