Go Context Usage

context.Context carries security credentials, tracing information, deadlines, and cancellation signals across API and process boundaries. Go programs pass contexts explicitly along the entire function call chain.

Based on Go Wiki CodeReviewComments - Contexts.

Context as First Parameter

Functions that use a Context should accept it as their first parameter:

// Good: Context is first parameter func F(ctx context.Context, / other arguments /) error { // ... }

func ProcessRequest(ctx context.Context, req Request) (Response, error) { // ... }

This is a strong convention in Go that makes context flow visible and consistent across codebases.

Don't Store Context in Structs

Do not add a Context member to a struct type. Instead, pass ctx as a parameter to each method that needs it:

// Bad: Context stored in struct type Worker struct { ctx context.Context // Don't do this // ... }

func (w *Worker) Process() error { // Uses w.ctx - context lifetime unclear }

// Good: Context passed to methods type Worker struct { // ... }

func (w *Worker) Process(ctx context.Context) error { // Context explicitly passed - lifetime clear }

Exception: Methods whose signature must match an interface in the standard library or a third-party library may need to work around this.

Don't Create Custom Context Types

Do not create custom Context types or use interfaces other than context.Context in function signatures:

// Bad: Custom context type type MyContext interface { context.Context GetUserID() string }

func Process(ctx MyContext) error { ... }

// Good: Use standard context.Context func Process(ctx context.Context) error { userID := GetUserID(ctx) // Extract from context value // ... }

Where to Put Application Data

If you have application data to pass around, consider these options in order of preference:

Function parameters - Most explicit and type-safe Receiver - For data that belongs to the type Globals - For truly global configuration (use sparingly) Context value - Only if it truly belongs there (request-scoped data) // Good: Explicit parameter func ProcessOrder(ctx context.Context, userID string, order *Order) error { // userID is explicit }

// Good: Context value for request-scoped data func ProcessOrder(ctx context.Context, order *Order) error { // Request ID from context is appropriate - it's request-scoped reqID := RequestIDFromContext(ctx) // ... }

Context values are appropriate for:

Request IDs and trace IDs Authentication/authorization info that flows with requests Deadlines and cancellation signals

Context values are not appropriate for:

Optional function parameters Data that could be passed explicitly Configuration that doesn't vary per-request 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:

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

// Good: 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:

// Good: Main function or initialization func main() { ctx := context.Background() if err := run(ctx); err != nil { log.Fatal(err) } }

// Good: Top-level background task 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:

// Prefer: Accept context even for "simple" operations func LoadConfig(ctx context.Context) (*Config, error) { // Even if not using ctx now, accepting it allows future // additions without API changes }

Common Patterns Deriving Contexts // Add timeout ctx, cancel := context.WithTimeout(ctx, 5*time.Second) defer cancel()

// Add cancellation ctx, cancel := context.WithCancel(ctx) defer cancel()

// Add deadline ctx, cancel := context.WithDeadline(ctx, time.Now().Add(time.Hour)) defer cancel()

// Add value (use sparingly) ctx = context.WithValue(ctx, requestIDKey, reqID)

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

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
        return
    }
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
}

json.NewEncoder(w).Encode(result)

}

Quick Reference Pattern Guidance Parameter position Always first: func F(ctx context.Context, ...) Struct storage Don't store in structs; pass to methods Custom types Don't create; use context.Context interface Application data Prefer parameters > receiver > globals > context values Request-scoped data Appropriate for context values Sharing context Safe - contexts are immutable context.Background() Only for non-request-specific code Default Pass context even if you think you don't need it See Also go-concurrency: Goroutine patterns, cancellation, and coordination go-error-handling: Handling context cancellation errors go-interfaces: Interface design patterns for context-accepting APIs

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