Contexts with a deadline in Go

A context carries cancellation down a call tree, and every wait respects it.

func fetch(ctx context.Context, delay time.Duration) (string, error) {
    select {
    case <-time.After(delay):
        return "fetched", nil
    case <-ctx.Done():
        return "", ctx.Err()
    }
}

func run() (string, error) {
    ctx, cancel := context.WithTimeout(context.Background(),
        20*time.Millisecond)
    defer cancel()
    return fetch(ctx, 200*time.Millisecond)
}

How it works

1. WithTimeout cancels itself when the clock runs out.
2. ctx.Done() is the channel to select on.
3. ctx.Err() says whether it was canceled or timed out.

Keywords and builtins used here

• case
• defer
• error
• func
• return
• select
• string

The run, in numbers

Lines

15

Characters to type

336

Tokens

98

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 56 seconds.

Type this snippet

Step 2 of 3 in Context, step 12 of 25 in Concurrency.

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