Go: What is context in Go?

Context — a standard library type (context.Context) that carries deadlines, cancellation signals, and request-scoped values across goroutine boundaries.

Core interfaces:

• Deadline() — returns the time when work should be cancelled.
• Done() — returns a channel that is closed when the context is cancelled.
• Err() — returns Canceled if the context was cancelled, or DeadlineExceeded if the deadline passed.
• Value(key) — retrieves a value associated with the context.

Creating contexts:

1// Background context (root, no deadline)
2ctx := context.Background()
3
4// Context with timeout (5 seconds)
5ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
6defer cancel()
7
8// Context with deadline (specific time)
9deadline := time.Now().Add(10 * time.Second)
10ctx, cancel := context.WithDeadline(context.Background(), deadline)
11defer cancel()
12
13// Context with cancellation (manual)
14ctx, cancel := context.WithCancel(context.Background())
15defer cancel()
16
17// Context with value
18ctx = context.WithValue(context.Background(), "userID", "12345")

Using context in HTTP handlers:

1func handler(w http.ResponseWriter, r *http.Request) {
2    ctx := r.Context() // Get context from request
3    
4    // Pass to downstream services
5    result, err := fetchUserData(ctx, userID)
6    if err != nil {
7        // Check if context was cancelled
8        if ctx.Err() == context.Canceled {
9            return // Client disconnected
10        }
11    }
12}

When to use context:

• HTTP server requests (cancel downstream calls if client disconnects).
• Database queries with timeouts.
• Microservice communication.
• Passing authentication tokens or request IDs across goroutines.

Common mistakes:

• Not calling cancel() (causes goroutine leak).
• Storing context in a struct field (pass as first parameter instead).
• Using context.Background() or context.TODO() in library code (use the context passed to you).
• Using context for unrelated values (only use for request-scoped data).
• Creating context with WithCancel but never calling cancel().