Reliable Background Jobs: Queues, Retries, and Idempotency

1. The Fallacy of At-Most-Once Processing

Network dropouts, worker memory exhaustion, and external API timeouts make guaranteed single execution mathematically impossible. Production queue architectures must be built around "At-Least-Once Delivery" coupled with strict application-level idempotency.

// Idempotent Job Execution in Laravel
public function handle(PaymentGateway $gateway): void
{
    // Acquire a distributed advisory lock using the idempotency key
    $lock = Cache::lock("job:settlement:{$this->orderId}", 30);

    if (! $lock->get()) {
        $this->release(5); // Another worker is actively processing; back off
        return;
    }

    try {
        DB::transaction(function () use ($gateway) {
            $order = Order::lockForUpdate()->findOrFail($this->orderId);
            
            if ($order->isSettled()) {
                return; // Already executed by previous attempt; idempotent no-op
            }

            $gateway->capture($order);
            $order->markAsSettled();
        });
    } finally {
        $lock->release();
    }
}

2. Exponential Backoff with Random Jitter

When an external service experiences an outage, retrying immediately causes a "thundering herd" effect that keeps the downstream service overwhelmed. Applying exponential backoff with randomized jitter spreads out incoming retries across an expanding time window.

3. Dead-Letter Queues (DLQ) and Poison Pill Triage

Tasks that fail repeatedly past maximum retry thresholds must not block the queue. They are automatically evacuated to a Dead-Letter Queue with full failure context, allowing engineers to inspect the offending payload without losing customer data.