go-context

Go Context Usage

Compatibility: Baseline Go 1.27 (see COMPATIBILITY.md). context.WithoutCancel, context.AfterFunc, context.WithTimeoutCause and context.WithDeadlineCause require Go 1.21+; t.Context() in tests, Go 1.24+.

Resource Routing

• references/PATTERNS.md - Read when deriving contexts, checking cancellation, handling HTTP request contexts, or using typed context-value keys.

Context as First Parameter

Functions that use a Context accept it as their first parameter. Do not add a Context member to a struct type (containedctx in the gate reports it); pass ctx to each method that needs it. Exception: methods whose signature must match an interface in the standard library or a third-party library. Do not create custom Context types or use interfaces other than context.Context in function signatures.

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Where to Put Application Data

Parameters first, then the receiver, then a global; a context value carries only request-scoped data that crosses APIs — a request or trace ID, the authenticated principal — never an optional parameter, per-process configuration, or a deadline, which is derived with WithTimeout, WithDeadline, or WithCancel.

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Common Patterns

Deriving Contexts

defer cancel() immediately after creating a derived context. Inside a loop a defer waits for the function to return, so every iteration's context stays live until then: cancel at the end of each iteration, or move the iteration into its own function that defers cancel().

ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
defer cancel()

for _, item := range items {
    itemCtx, cancelItem := context.WithTimeout(ctx, time.Second)
    err := send(itemCtx, item)
    cancelItem() // per iteration, not deferred
    if err != nil {
        return err
    }
}

Cancellation With a Reason

With context.WithCancelCause, WithTimeoutCause, or WithDeadlineCause, ctx.Err() still returns Canceled or DeadlineExceeded; only context.Cause(ctx) returns the reason supplied, so a caller that must tell this cancellation or timeout from an upstream one reads Cause. WithCancelCause returns a CancelCauseFunc, released on the success path with defer cancel(nil); WithTimeoutCause and WithDeadlineCause return a plain CancelFunc (defer cancel()), and their cause is set only when the timer fires.

context.WithoutCancel(ctx) keeps the values but drops cancellation; use it for work that must outlive the request (audit log, cleanup) and give that work its own timeout.

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Related Skills

• go-resilience: total and attempt time budgets, cancellation-aware backoff, durable retry ownership.
• go-concurrency: goroutine cancellation, select timeouts, errgroup.
• Returning ctx.Err() or context.Cause(ctx): Cancellation With a Reason above.
• go-interfaces: APIs that take context alongside interfaces.
• go-logging: loggers and request IDs carried in context.