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AWS SQS
SQS ("Simple Queue Service") is a service that allows you to send, store and receive messages between apps and software components built in AWS. It helps with decoupling and scaling.
Amazon SQS is a distributed queue system that enables web service applications to quickly and reliably queue messages that one component in the application generates to be consumed by another component where a queue is a temporary repository for messages that are awaiting processing.
As the name indicates, its operating mode is that of a queue data structure offering first-in, first-out and other queue implementations.
Example use case
A request is made to an API Gateway endpoint with a body. The body is then parsed and inserted into a database.
The benefit of adding SQS as a buffer between the request and the updating of the database:
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It can better handle spikes in activity, buffering requests to the database until it is ready to handle them. This prevents the messages getting lost if the server is overloaded
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There is a retry mechanism built into SQS. If the database write fails, the message stays in the queue allowing for retries
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It facilitates decoupling. Without SQS as the middleman the responsibilites of the lambda would be compounded - it would receive requests and update the DB, plus any additional processes such sending a message to SNS. In the solution we have two lambdas co-ordinating actions in a decouped manner via SQS.
Dead letters
As SQS allows for multiple retries we could end up in a situation where a malformed message is continually processed in a loop. To avoid this you can set a maxiumum retry limit and, when this is exceeded, shift the problematic message to the dead letter queue (DLQ) and remove it from the main queue.
If the DLQ reaches a certain threshold this can trigger additional handling such as raising an Alert in CloudWatch or other monitoring tool.
Note that a DLQ is not a distinct entity within the SDK, it is just another SQS queue that is designated to store failures.
See AWS SDK Syntax.