go/context

Go Context

Context is the first parameter, flows through call chains, never lives in structs.

Decision Framework: Context Creation

Create Context	Use Case
context.Background()	Top-level (main, tests, init)
context.TODO()	Placeholder when unclear
WithTimeout(parent, d)	Operations with time limits
WithCancel(parent)	Manual cancellation needed
WithDeadline(parent, t)	Absolute deadline
WithValue(parent, k, v)	Request-scoped data
WithoutCancel(parent)	Detach from parent cancellation (Go 1.21+)

Pattern 1: HTTP Handler with Timeout

func (h *Handler) GetUser(w http.ResponseWriter, r *http.Request) {
    ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
    defer cancel()

    user, err := h.repo.FindByID(ctx, userID)
    if errors.Is(err, context.DeadlineExceeded) {
        http.Error(w, "request timeout", http.StatusGatewayTimeout)
        return
    }
}

Pattern 2: Type-Safe Context Values

type contextKey string
const traceIDKey contextKey = "trace_id"

func WithTraceID(ctx context.Context, id string) context.Context {
    return context.WithValue(ctx, traceIDKey, id)
}

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

Rules: custom type for keys, accessor functions for type safety, context values = request-scoped immutable data only.

When to Use Context Values vs Parameters

Context Values	Explicit Parameters
Request metadata (trace IDs, correlation IDs)	Business logic parameters
Auth credentials (user ID, tokens)	Function behavior config
Cross-cutting concerns (logging, tracing)	Domain data and entities

Pattern 3: Detach from Cancellation (Go 1.21+)

// Post-response async work that must not cancel with the HTTP request
go h.mailer.SendConfirmation(context.WithoutCancel(r.Context()), order)

Preserves parent's values (trace ID) but outlives parent's cancellation.

Anti-Patterns

• Storing context in struct — always pass as first parameter
• String keys — use custom types to prevent collisions
• Ignoring cancellation — check ctx.Done() in loops
• Background in inner functions — pass parent context through

Verification

• context.Context is the first parameter in every function that accepts one
• No context.Context stored in struct fields
• Custom key types used for context values (not bare string)
• WithTimeout/WithCancel always paired with defer cancel()