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SQS: Simple Queue Service

SQS is a service that manages and operates message oriented middleware. It enables you to decouple and scale microservices, distributed systems, and serverless applications.

What's a queue? - A form of asynchronouse service to service communication used in multiple application architectures. - Messages are stored on teh queue until they are processed and deleted

Standard Queue

- Oldest offering from AWS (over 10 years old)
- Fully managed service
- Scales from 1 message per second to 10,000 per second
- Default retention of messages: 4 days
    - A maximum of 14 days
- There is no limit to how many messages can be in the queue
- Low latency (10 ms on publish and recieve)
- Horizontal scaling in terms of number of consumers
- Can have duplicate messages
- Can have messages out of order (best effort ordering)
- Limitation of 256kb per message sent

Delay Queue

- Delay a message so a consumer doesn't see it immediately
    - up to 15 minute delay
- Default is 0 seconds making messages available right away
- You can set a default at queue level
- YOu can override the default using the **DelaySeconds** parameter

Producing Messages

- Define the body
- Add message attributes (metadata - optional)
- Optionally can provide Delay Delivery
- You recieve back:
    - Message identifier
    - MD5 hash of the body

Consuming Messages

- Poll SQS for messages
    - receive up to 10 messages at a time
- Next step is to process the message within the visibility timeout
- Finally, delete the message using the message ID & receipt handle

Visibility timeout

- When a consumer polls a message from a queue, the message is *invisible* to the other consumers for a defined period called **Visibility Timeout**
    - Can be set between 0 seconds to 12 hours
    - Default 30 seconds
    - If it takes 15 minutes or greater and consumer fails to process the message, you must wait before processing the message again
    - IF set to 30 seconds or lower and consuemr needs time to process the message, another consumer will receive the message and will be processed more than once
- **ChangeMessageVisibilty**
    - An API to change the visibility while processing a message
- **DeleteMessage**
    - An API to tell SQS the message was successfully processed

Dead Letter Queue

- To be continued