What are the best practices for using Go's HTTP client in production?

The default http.DefaultClient is not suitable for production — it has no timeouts, uses a shared transport, and cannot be instrumented. Production code creates dedicated clients with explicit configuration.

// Production HTTP client — never use http.DefaultClient in libraries
func newHTTPClient() *http.Client {
    return &http.Client{
        Timeout: 30 * time.Second, // total request timeout including body read
        Transport: &http.Transport{
            MaxIdleConns:        100,
            MaxIdleConnsPerHost: 10,
            IdleConnTimeout:     90 * time.Second,
            TLSHandshakeTimeout: 10 * time.Second,
            // Keep-alive enabled by default
            DisableKeepAlives: false,
        },
    }
}

// Reuse clients — they manage connection pools internally
var apiClient = newHTTPClient()

// Always use context for cancellation and timeout
func callAPI(ctx context.Context, url string) ([]byte, error) {
    req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
    if err != nil {
        return nil, fmt.Errorf("creating request: %w", err)
    }
    req.Header.Set("Accept", "application/json")
    req.Header.Set("User-Agent", "myservice/1.0")

    resp, err := apiClient.Do(req)
    if err != nil {
        return nil, fmt.Errorf("executing request: %w", err)
    }
    defer resp.Body.Close()

    // Always read and close body — even on error status
    body, err := io.ReadAll(io.LimitReader(resp.Body, 10<<20)) // 10 MB limit
    if err != nil {
        return nil, fmt.Errorf("reading body: %w", err)
    }
    if resp.StatusCode >= 400 {
        return nil, fmt.Errorf("HTTP %d: %s", resp.StatusCode, body)
    }
    return body, nil
}

// RETRY with exponential backoff for idempotent requests
func callWithRetry(ctx context.Context, url string, maxRetries int) ([]byte, error) {
    var lastErr error
    for attempt := 0; attempt < maxRetries; attempt++ {
        if attempt > 0 {
            delay := time.Duration(attempt*attempt) * 100 * time.Millisecond
            select {
            case <-time.After(delay):
            case <-ctx.Done(): return nil, ctx.Err()
            }
        }
        data, err := callAPI(ctx, url)
        if err == nil { return data, nil }
        lastErr = err
    }
    return nil, fmt.Errorf("after %d retries: %w", maxRetries, lastErr)
}

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