# SQS

**Introduction**

Amazon Simple Queue Service (SQS):

* A managed message queuing service to decouple and scale microservices, distributed systems, and serverless applications
* Use Case:
  * A write buffer for DynamoDB

**Feature**

* Message is 256 KB in size (use a pointer to S3 for larger messages)
* Message is kept in queued from 1minute \~ 14 days, in 4 days by default.
* Can be retrieved programmatically with SQS API
* SQS Types (can not modify dynamically):
  * Standard Queue (default)
    * Nearly-unlimited number of transactions per second
    * Delivery order loosely guarenteed
  * FIFO Queue
    * Limit to 300 messages/s without batching, 3,000 messages/s with batching.
      * If more messages, use **Kinesis Data Streams**.
    * Delivery order guarenteed
* Visibility time out mechanism
  * If a job finishes in time, the message is consumed
  * If a job doesn't finish in time, the message will become visible again.
  * Maximum of visibility time out is 12 hours 
  * [Changing the visibility timeout for a message](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-visibility-timeout.html)
* Polling
  * Short polling (default)
    * If no message left in the queue, return the response.
  * Long polling
    * For event source mapping (ex. attaching to Lambda Funciton)
    * Can specify batch size from 1\~10
    * If no message lef in the queue, it waits for the "Receive Message Wait Time" or a message comes to return the response.
    * Change "Receive Message Wait Time", can be 0\~20 seconds.
* DLQ usage
  * Set up on SQS level (not Lambda level because DLQ for Lambda is only for async invocations)
  * Or use a Lambda destination instead
* Tracking with application level must be implemented.