You now understand asynchronous processing.
Use async for work that doesn't need immediate results.
Message queues decouple producers from consumers.
Pub/Sub delivers one message to many subscribers.
At-least-once with idempotent consumers is the common pattern.
Dead letter queues capture failed messages for investigation.
Kafka for streaming, RabbitMQ for tasks, SQS for simple queuing.
Next, you'll learn about storage systems for files and large objects.