golang-context

Adapted from samber/cc-skills-golang@golang-context. Pair with golang-common-practice for shared verification guidance.

Go context.Context Best Practices

context.Context is Go's mechanism for propagating cancellation signals, deadlines, and request-scoped values across API boundaries and between goroutines. Think of it as the "session" of a request — it ties together every operation that belongs to the same unit of work.

Best Practices Summary

1. The same context MUST be propagated through the entire request lifecycle: HTTP handler → service → DB → external APIs
2. ctx MUST be the first parameter, named ctx context.Context
3. NEVER store context in a struct — pass explicitly through function parameters
4. NEVER pass nil context — use context.TODO() if unsure
5. cancel() MUST be called on all control-flow paths for WithCancel/WithTimeout/WithDeadline, unless ownership of the context and cancel function is explicitly returned or transferred
6. context.Background() MUST only be used at the top level (main, init, tests)
7. Use `context.TODO()` as a placeholder when you know a context is needed but don't have one yet
8. NEVER create a new context.Background() in the middle of a request path
9. Context value keys MUST be unexported types to prevent collisions
10. Context values MUST only carry request-scoped metadata — NEVER function parameters
11. Use `context.WithoutCancel` (Go 1.21+) when spawning background work that must outlive the parent request

Creating Contexts

Context Propagation: The Core Principle

The most important rule: propagate the same context through the entire call chain. When you propagate correctly, cancelling the parent context cancels all downstream work automatically.

// ✗ Bad — creates a new context, breaking the chain
func (s *OrderService) Create(ctx context.Context, order Order) error {
    return s.db.ExecContext(context.Background(), "INSERT INTO orders ...", order.ID)
}

// ✓ Good — propagates the caller's context
func (s *OrderService) Create(ctx context.Context, order Order) error {
    return s.db.ExecContext(ctx, "INSERT INTO orders ...", order.ID)
}

Deep Dives

• [Cancellation, Timeouts & Deadlines](./references/cancellation.md) — How cancellation propagates: WithCancel for manual cancellation, WithTimeout for automatic cancellation after a duration, WithDeadline for absolute time deadlines. Patterns for listening (<-ctx.Done()) in concurrent code, AfterFunc callbacks, and WithoutCancel for operations that must outlive their parent request (e.g., audit logs).

• [Context Values & Cross-Service Tracing](./references/values-tracing.md) — Safe context value patterns: unexported key types to prevent namespace collisions, when to use context values (request ID, user ID) vs function parameters. Trace context propagation: OpenTelemetry trace headers, correlation IDs for log aggregation, and marshaling/unmarshaling context across service boundaries.

• [Context in HTTP Servers & Service Calls](./references/http-services.md) — HTTP handler context: r.Context() for request-scoped cancellation, middleware integration, and propagating to services. HTTP client patterns: NewRequestWithContext, client timeouts, and retries with context awareness. Database operations: always use *Context variants (QueryContext, ExecContext) to respect deadlines.

Cross-References

• → See the golang-concurrency skill for goroutine cancellation patterns using context
• → See the golang-database skill for context-aware database operations (QueryContext, ExecContext)
• → See the golang-observability skill for trace context propagation with OpenTelemetry
• → See the golang-design-patterns skill for timeout and resilience patterns

Enforce with Linters

Many context pitfalls are caught automatically by linters: govet, staticcheck. → See the golang-lint skill for configuration and usage.