Day 085 — net/http Server & ServeMux¶

Month 4 · Week 1 · ⬅ Day 084 · Day 086 ➡ · Journal index

🎯 Learning Objective¶

Stand up an HTTP server with the standard library and route requests with the Go 1.22 enhanced http.ServeMux — methods, path wildcards, and precedence.

📚 Topics¶

http.Server, ListenAndServe, http.NewServeMux, HandleFunc
Method+pattern routes, {id} / {path...} / {$}, r.PathValue

📖 Reading / Sources¶

📝 Notes¶

An http.Handler is anything with ServeHTTP(w, r). A *ServeMux is itself a Handler that dispatches to other handlers → [[http-handler]].
Go 1.22 patterns add method + wildcards: "GET /users/{id}". The method is optional; omit it to match any method. Read wildcards with r.PathValue("id"), never by slicing r.URL.Path.
{path...} is a trailing wildcard that greedily captures the remainder. {$} anchors an exact match (/things/{$} matches only /things/, not a subtree). A pattern ending in / without {$} matches the whole subtree.
Precedence: the most specific pattern wins regardless of registration order; truly conflicting patterns panic at registration.
The new mux returns 405 Method Not Allowed (with an Allow header) when a path matches but the method doesn't — the old mux silently 404'd.
Prefer constructing &http.Server{Addr, Handler, …Timeout} over the http.ListenAndServe shortcut so you can set timeouts later → [[http-timeouts]].
Set response headers before the first Write/WriteHeader; they freeze once the status line is sent. A bare Write implies WriteHeader(200).

💻 Code Examples¶

mux := http.NewServeMux()
mux.HandleFunc("GET /users/{id}", func(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "user %s\n", r.PathValue("id")) // read the wildcard
})
mux.HandleFunc("DELETE /users/{id}", func(w http.ResponseWriter, r *http.Request) {
    w.WriteHeader(http.StatusNoContent) // 204, no body
})
srv := &http.Server{Addr: ":8085", Handler: mux}
log.Fatal(srv.ListenAndServe())

Full code: examples/month-04/servemux/main.go · Run: go run ./examples/month-04/servemux

🏋️ Exercises / Practice¶

Exercise	Status	Link
Method routing + `{id}` PathValue, auto 404/405	✅	exercises/month-04/week-1/pathrouter

🐛 Mistakes Made¶

Tried to read the id with strings.Split(r.URL.Path, "/") — r.PathValue("id") is the right tool now and survives pattern changes.
Forgot the leading method: "/users/{id}" matched every method, so my DELETE ran the GET handler. Added explicit "GET …" / "DELETE …".

❓ Open Questions¶

When do two registered patterns "conflict" enough to panic vs. just rank by specificity? (Revisit the precedence rules in the docs.)

🧠 Active Recall (answer without looking)¶

Q: How do you read the {id} wildcard from "GET /users/{id}"?

A

`r.PathValue("id")` — it returns the matched segment as a string (empty if absent).

Q: What status does the Go 1.22 mux return when the path matches but the method doesn't?

A

`405 Method Not Allowed`, with an `Allow` header listing the methods that *are* registered for that path.

🪶 Feynman Reflection¶

A ServeMux is a switchboard: each incoming request is a call, and the mux connects it to the handler whose pattern best fits the method+path. Since Go 1.22 the switchboard understands verbs and named slots in the path, so I rarely need a third-party router for simple services.

🕳️ Knowledge Gaps¶

Subtree (/static/) vs exact ({$}) matching edge cases.
How ServeMux interacts with host-prefixed patterns (example.com/...).

✅ Summary¶

I can boot an http.Server, register method-aware routes on the stdlib mux, read path wildcards, and rely on automatic 404/405 handling.

⏭️ Next Steps / Prep for Tomorrow¶

Day 086: the http.Handler interface and the HandlerFunc adapter in depth.

Time spent	Difficulty	Confidence
90 min	🟦🟦⬜⬜⬜	🟦🟦🟦⬜⬜

Suggested commit: feat(examples): net/http server and ServeMux routing (day 085)