How does context.Context work and when do you use each context type?

context.Context carries deadlines, cancellation signals, and request-scoped values across API boundaries and goroutines. It is the standard way to propagate cancellation in Go services.

// Context hierarchy — child inherits cancellation from parent
ctx := context.Background() // root — never cancelled, never has deadline

// WithCancel — explicit cancellation
ctx1, cancel1 := context.WithCancel(ctx)
defer cancel1() // ALWAYS defer cancel to prevent goroutine leaks

// WithTimeout — automatically cancelled after duration
ctx2, cancel2 := context.WithTimeout(ctx, 5*time.Second)
defer cancel2()

// WithDeadline — cancelled at absolute time
deadline := time.Now().Add(10 * time.Second)
ctx3, cancel3 := context.WithDeadline(ctx, deadline)
defer cancel3()

// WithValue — carry request-scoped data (use sparingly)
type ctxKey string
ctx4 := context.WithValue(ctx, ctxKey("traceID"), "abc-123")
traceID := ctx4.Value(ctxKey("traceID")).(string)

// Propagate through function calls
func fetchData(ctx context.Context, url string) ([]byte, error) {
    req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
    resp, err := http.DefaultClient.Do(req) // cancels if ctx is done
    if err != nil { return nil, err }
    defer resp.Body.Close()
    return io.ReadAll(resp.Body)
}

// Checking cancellation in a long loop
func processItems(ctx context.Context, items []Item) error {
    for _, item := range items {
        select {
        case <-ctx.Done():
            return ctx.Err() // context.DeadlineExceeded or context.Canceled
        default:
            process(item)
        }
    }
    return nil
}

Rules: (1) always pass Context as the first argument, never store it in a struct (for long-lived objects use context.Background() stored at service init). (2) always call the cancel function to avoid goroutine leaks — even if the deadline or timeout fires naturally. (3) use context.WithValue only for truly request-scoped data (trace IDs, auth tokens) — not as a general parameter-passing mechanism.

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