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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.