Go Context Package Cheat Sheet
Creating cancelable, timeout-bound, and value-carrying contexts with context.Context for propagating deadlines and cancellation across goroutines.
Creating Contexts
context.Context flows through call chains as the first parameter, conventionally named ctx.
ctx := context.Background() // root context, never canceled, use in main/testsctx2 := context.TODO() // placeholder when unsure which context to use yet// Derived, cancelable contextctx3, cancel := context.WithCancel(ctx)defer cancel() // always call cancel to release resources, even if ctx finishes normally
Deadlines & Timeouts
WithTimeout and WithDeadline auto-cancel the context after a duration or absolute time.
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)defer cancel()req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)resp, err := http.DefaultClient.Do(req)if err != nil { if errors.Is(err, context.DeadlineExceeded) { log.Println("request timed out") } return err}// WithDeadline: cancel at a specific absolute timedeadline := time.Now().Add(10 * time.Second)ctx2, cancel2 := context.WithDeadline(context.Background(), deadline)defer cancel2()
Respecting Cancellation in Goroutines
Long-running work must select on ctx.Done() to react to cancellation.
func worker(ctx context.Context, jobs <-chan int) { for { select { case <-ctx.Done(): fmt.Println("worker stopping:", ctx.Err()) // context.Canceled or DeadlineExceeded return case job, ok := <-jobs: if !ok { return } process(job) } }}
Passing Request-Scoped Values
context.WithValue for request-scoped data only — never for optional function params.
type ctxKey stringconst requestIDKey ctxKey = "requestID" // unexported custom type avoids collisionsfunc withRequestID(ctx context.Context, id string) context.Context { return context.WithValue(ctx, requestIDKey, id)}func requestID(ctx context.Context) (string, bool) { id, ok := ctx.Value(requestIDKey).(string) return id, ok}
Context API Reference
The full public surface of the context package.
- context.Background()- empty root context for main(), init, tests
- context.TODO()- placeholder root context, signals 'context plumbing incomplete'
- context.WithCancel(parent)- returns ctx + cancel func to cancel manually
- context.WithTimeout(parent, d)- cancels automatically after duration d
- context.WithDeadline(parent, t)- cancels automatically at absolute time t
- context.WithValue(parent, k, v)- attaches a request-scoped key/value pair
- ctx.Done()- channel closed when the context is canceled or times out
- ctx.Err()- non-nil reason after Done() closes: Canceled or DeadlineExceeded
Always call the cancel function returned by WithCancel/WithTimeout/WithDeadline, typically via defer cancel(), even when the operation succeeds — skipping it leaks the internal timer/goroutine until the parent context itself is canceled.
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