Go: What is context?

1ctx := context.Background()                    // root
2ctx, cancel := context.WithCancel(parent)       // manual
3ctx, cancel := context.WithTimeout(parent, 5s)  // timeout
4ctx, cancel := context.WithDeadline(parent, t)  // deadline
5ctx = context.WithValue(parent, "key", "val")  // value
6defer cancel()

1func handler(w http.ResponseWriter, r *http.Request) {
2    ctx := r.Context()
3    
4    // Timeout for DB query
5    dbCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
6    defer cancel()
7    
8    var user User
9    if err := db.QueryRowContext(dbCtx, "SELECT * FROM users WHERE id=$1", id).Scan(&user); err != nil {
10        if errors.Is(err, context.DeadlineExceeded) {
11            http.Error(w, "timeout", http.StatusGatewayTimeout)
12        } else {
13            http.Error(w, "error", http.StatusInternalServerError)
14        }
15        return
16    }
17    json.NewEncoder(w).Encode(user)
18}