How does context propagate through an HTTP request lifecycle in Go?

Since Go 1.7, every *http.Request carries a context accessible via r.Context(). This context is cancelled when the client disconnects or the server shuts down. Middleware and handlers should pass this context to all downstream calls — database queries, outbound HTTP calls, gRPC — so they all cancel when the request does.

// The request context is cancelled when the client disconnects
func userHandler(w http.ResponseWriter, r *http.Request) {
    ctx := r.Context() // inherits client-disconnect cancellation

    // Add a per-request deadline
    ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
    defer cancel()

    // Pass ctx to ALL downstream blocking calls
    userID := chi.URLParam(r, "id")
    user, err := userService.FindByID(ctx, userID)  // DB call respects deadline
    if err != nil {
        if errors.Is(err, context.DeadlineExceeded) {
            http.Error(w, "request timed out", http.StatusGatewayTimeout)
            return
        }
        http.Error(w, "internal error", http.StatusInternalServerError)
        return
    }
    json.NewEncoder(w).Encode(user)
}

// Making an outbound HTTP call with the request context
func fetchExternalData(ctx context.Context, url string) ([]byte, error) {
    req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
    if err != nil {
        return nil, fmt.Errorf("creating request: %w", err)
    }
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        return nil, fmt.Errorf("executing request: %w", err)
    }
    defer resp.Body.Close()
    return io.ReadAll(resp.Body)
}

// Middleware: attach request-scoped values to context
func requestIDMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        id := r.Header.Get("X-Request-ID")
        if id == "" {
            id = uuid.New().String()
        }
        ctx := context.WithValue(r.Context(), requestIDKey, id)
        w.Header().Set("X-Request-ID", id)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

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