/go-context

Context Patterns

Common patterns for deriving, checking, and propagating context.Context.

Contents

• Context Immutability
• When to Use context.Background()
• Deriving Contexts
• Checking Cancellation
• Respecting Cancellation in HTTP Handlers
• Context Value Best Practices
• Quick Reference

Context Immutability

Contexts are immutable. It's safe to pass the same ctx to multiple calls that share the same deadline, cancellation signal, credentials, and parent trace:

// Safe: same context to sequential calls
func ProcessBatch(ctx context.Context, items []Item) error {
    for _, item := range items {
        if err := process(ctx, item); err != nil {
            return err
        }
    }
    return nil
}

// Safe: same context to concurrent calls
func ProcessConcurrently(ctx context.Context, a, b *Data) error {
    g, ctx := errgroup.WithContext(ctx)
    g.Go(func() error { return processA(ctx, a) })
    g.Go(func() error { return processB(ctx, b) })
    return g.Wait()
}

When to Use context.Background()

Use context.Background() only for functions that are never request-specific:

func main() {
    ctx := context.Background()
    if err := run(ctx); err != nil {
        log.Fatal(err)
    }
}

func startBackgroundWorker() {
    ctx := context.Background()
    go worker(ctx)
}

Default to passing a Context even if you think you don't need to. Only use context.Background() directly if you have a good reason why passing a context would be a mistake:

func LoadConfig(ctx context.Context) (*Config, error) {
    // Even if not using ctx now, accepting it allows future
    // additions without API changes
}

Deriving Contexts

// Add timeout — cancel fires after duration elapses
ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
defer cancel()

// Add cancellation — caller controls when to cancel
ctx, cancel := context.WithCancel(ctx)
defer cancel()

// Add deadline — cancel fires at a specific wall-clock time
ctx, cancel := context.WithDeadline(ctx, time.Now().Add(time.Hour))
defer cancel()

// Add value (use sparingly — only for request-scoped data)
ctx = context.WithValue(ctx, requestIDKey, reqID)

Always defer cancel() immediately after creating a derived context. This ensures resources are released even if the function returns early.

Nested Derivation

Derived contexts form a tree. Cancelling a parent cancels all its children:

func handleRequest(ctx context.Context) error {
    // Parent timeout for the whole request
    ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
    defer cancel()

    // Tighter timeout for the database call
    dbCtx, dbCancel := context.WithTimeout(ctx, 5*time.Second)
    defer dbCancel()

    data, err := queryDB(dbCtx)
    if err != nil {
        return err
    }

    // Remaining time from parent context applies here
    return sendResponse(ctx, data)
}

Checking Cancellation

In Long-Running Loops

func LongRunningOperation(ctx context.Context) error {
    for {
        select {
        case <-ctx.Done():
            return ctx.Err()
        default:
            // Do work
        }
    }
}

Before Expensive Operations

Check cancellation before starting work that can't be interrupted:

func ProcessItems(ctx context.Context, items []Item) error {
    for _, item := range items {
        if ctx.Err() != nil {
            return ctx.Err()
        }
        if err := expensiveProcess(item); err != nil {
            return err
        }
    }
    return nil
}

Distinguishing Cancellation Causes

if err := ctx.Err(); err != nil {
    switch {
    case errors.Is(err, context.Canceled):
        // Caller explicitly cancelled (e.g., client disconnected)
    case errors.Is(err, context.DeadlineExceeded):
        // Timeout or deadline passed
    }
}

Respecting Cancellation in HTTP Handlers

func handler(w http.ResponseWriter, r *http.Request) {
    ctx := r.Context()

    result, err := slowOperation(ctx)
    if err != nil {
        if errors.Is(err, context.Canceled) {
            // Client disconnected — nothing to write
            return
        }
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    json.NewEncoder(w).Encode(result)
}

The r.Context() is cancelled when:

• The client closes the connection
• The request is cancelled by the client or HTTP/2 transport
• The ServeHTTP method returns

Context Value Best Practices

Use Unexported Key Types

type contextKey struct{}

var userIDKey contextKey

func WithUserID(ctx context.Context, id string) context.Context {
    return context.WithValue(ctx, userIDKey, id)
}

func UserIDFromContext(ctx context.Context) (string, bool) {
    id, ok := ctx.Value(userIDKey).(string)
    return id, ok
}

Using an unexported struct type as the key prevents collisions with keys from other packages — even if they use the same string or int value.

Provide Accessor Functions

Always wrap context.WithValue and ctx.Value in typed helper functions (as shown above) rather than exposing keys. This gives you type safety and a single place to change the implementation.

Quick Reference