How do you implement a hedged request pattern using select and goroutines?

A hedged request sends the same request to multiple backends simultaneously and returns the first successful response, cancelling the rest. It trades slightly higher resource use for dramatically lower tail latency — a technique from Google's Bigtable paper.

func hedgedFetch(ctx context.Context, urls []string) ([]byte, error) {
    ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
    defer cancel() // cancels all in-flight requests when we return

    type result struct{ data []byte; err error }
    ch := make(chan result, len(urls)) // buffered — goroutines can exit freely

    for _, url := range urls {
        url := url
        go func() {
            req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
            resp, err := http.DefaultClient.Do(req)
            if err != nil { ch <- result{err: err}; return }
            defer resp.Body.Close()
            data, err := io.ReadAll(resp.Body)
            ch <- result{data: data, err: err}
        }()
    }

    var errs []error
    for i := 0; i < len(urls); i++ {
        r := <-ch
        if r.err == nil {
            cancel() // cancel remaining requests
            return r.data, nil
        }
        errs = append(errs, r.err)
    }
    return nil, fmt.Errorf("all backends failed: %v", errs)
}

// Timed hedging — send to fallback only after 100ms
func timedHedge(ctx context.Context, primary, fallback string) ([]byte, error) {
    ch := make(chan result, 2)
    go func() { ch <- fetch(ctx, primary) }()
    select {
    case r := <-ch:
        if r.err == nil { return r.data, nil }
    case <-time.After(100 * time.Millisecond):
        go func() { ch <- fetch(ctx, fallback) }()
    }
    for i := 0; i < 2; i++ {
        if r := <-ch; r.err == nil { return r.data, nil }
    }
    return nil, errors.New("all requests failed")
}

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