What is Amazon SQS and SNS?

Amazon SQS (Simple Queue Service) is a fully managed message queue for decoupling and scaling microservices, distributed systems, and serverless applications. Messages are stored until consumers process them. Queue types: Standard queue — maximum throughput (unlimited TPS), at-least-once delivery, best-effort ordering. FIFO queue — exactly-once processing, first-in-first-out order, up to 3,000 TPS with batching. Key features: message size up to 256KB (larger via SQS Extended Client with S3); retention period 1 minute to 14 days (default 4 days); visibility timeout (message hidden while being processed, becomes visible again if not deleted — prevents double processing); dead-letter queue (DLQ — failed messages after N receive attempts); long polling (reduces empty receives, max 20s wait); delay queues (postpone delivery up to 15 min). Amazon SNS (Simple Notification Service) is a fully managed pub/sub messaging service. Publishers send messages to topics; all subscribers receive a copy. Subscribers: SQS, Lambda, HTTP/HTTPS, Email, SMS, mobile push (FCM, APNs). Fan-out pattern: SNS topic → multiple SQS queues — decouple event processing. FIFO topics: strict ordering, exactly-once delivery (paired with SQS FIFO). Message filtering: subscribers receive only messages matching their filter policy — reduces unnecessary processing. SNS + SQS fan-out: one S3 upload event → SNS → multiple SQS queues → different Lambda processors (thumbnail, metadata extraction, virus scan) — fully decoupled.