Go: What is context?

1// Root contexts
2ctx := context.Background()  // for main/init/tests
3ctx := context.TODO()        // when unsure
4
5// Derived contexts
6ctx, cancel := context.WithCancel(parent)     // manual cancel
7ctx, cancel := context.WithTimeout(parent, 5s) // timeout
8ctx, cancel := context.WithDeadline(parent, t) // deadline
9ctx = context.WithValue(parent, key, val)     // add value

1// HTTP handler with context propagation
2func handler(w http.ResponseWriter, r *http.Request) {
3    ctx := r.Context()
4    
5    // Add custom value
6    ctx = context.WithValue(ctx, "request_id", uuid.New())
7    
8    // Call service with timeout
9    svcCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
10    defer cancel()
11    
12    result, err := myService.DoWork(svcCtx)
13    if err != nil {
14        if errors.Is(err, context.DeadlineExceeded) {
15            http.Error(w, "timeout", http.StatusGatewayTimeout)
16        } else if errors.Is(err, context.Canceled) {
17            // Client disconnected, don't write response
18        } else {
19            http.Error(w, "error", http.StatusInternalServerError)
20        }
21        return
22    }
23    
24    json.NewEncoder(w).Encode(result)
25}