AWS SQS (Simple Queue Service)
The world's oldest managed message queue — launched 2004, AWS's third service ever. Producer sends to a queue, a consumer pulls.
Two types: Standard (at-least-once, best-effort ordering, unlimited throughput) and FIFO (exactly-once deduplication, strict ordering, 3000 msg/s per queue with batching). Visibility timeout instead of ack — if the consumer doesn't delete within the timeout, the message becomes visible again. DLQ for poison pills.
The default glue in AWS architectures. Competitors: Cloud Tasks (GCP), Azure Storage Queues, RabbitMQ (self-hosted). Lambda integration via "SQS triggers" is the modern fan-out path.