golang-context

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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. Propagate the same context through the entire request lifecycle: HTTP handler → service → DB → external APIs — any link that starts a fresh context keeps working after the client is gone.
2. Take ctx as the first parameter, named ctx context.Context — the fixed position is what makes context-aware APIs recognizable at a glance and what linters check.
3. Pass context through function parameters instead of storing it in a struct — the struct outlives the request that filled it, so later calls reuse a context that is already cancelled or belongs to someone else.
4. Pass context.TODO() rather than a nil context — nil panics on the first Done() or Value() call, far from the caller that passed it.
5. Call cancel() on all control-flow paths for WithCancel/WithTimeout/WithDeadline, unless ownership of the context and cancel function is explicitly returned or transferred — an uncalled cancel() keeps the child attached to its parent and leaks its timer until the parent finishes.
6. Create context.Background() only at top-level entry points (main, init, tests). Deeper in the call chain — especially mid-request — it detaches the work from the caller's deadline and cancellation, the propagation break shown below.
7. Use context.TODO() as a placeholder when a context is needed but none exists yet — it marks the gap for a later fix instead of hiding it behind a Background() that looks deliberate.
8. Declare context value keys as unexported types — with a plain string key, two packages using "user" silently overwrite each other.
9. Carry only request-scoped metadata in context values, never function parameters — values retrieved through Value() lose compile-time typing and disappear from the function signature.
10. Use context.WithoutCancel (Go 1.21+) when spawning background work that must outlive the parent request — otherwise the handler returning cancels the audit log or cleanup just started.

Creating Contexts

Situation	Use
Entry point (main, init, test)	context.Background()
Function needs context but caller doesn't provide one yet	context.TODO()
Inside an HTTP handler	r.Context()
Need cancellation control	context.WithCancel(parentCtx)
Need a deadline/timeout	context.WithTimeout(parentCtx, duration)

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

Enforce with Linters

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