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context.Context is how Go propagates cancellation, deadlines, and request-scoped values through call chains.

Every Go function that does I/O or blocks should accept a ctx context.Context as its FIRST argument.

Creating contexts:

Patterns:

The key must be your own unexported type, not a bare string or int — two packages that both use "user_id" would silently overwrite each other's value, and go vet flags the bare-string form.

context.WithValue is for request-scoped data only — request ID, trace span, auth user. Don't use it as a function-arg replacement.

The cardinal rule: if a goroutine outlives its caller, it should accept a context and check ctx.Done(). Goroutine leaks are the #1 Go bug at scale.