golang-context

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.

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.

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)
}

• 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.

• → 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

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