I'm trying to understand whether I need SQS in my workflow if someone can help explain. In my app, when an action is taken, it submits info to SNS topic which invokes LAMBDA to do some processing. This is working great as it is.
When I do research online, it seems that people are using SQS in this stack as well where SNS would put info on SQS and then SQS would then invoke LAMBDA.
I guess what I'm trying to understand is the need for SQS in this. What value doe that add or in other words, what am I losing by invoking my LAMBDA directly from SNS?
Best Answer
Primary advantage of having a SQS in between SNS and Lambda is Reprocessing. Assume that the Lambda fails to process certain event for some reason (e.g. timeout or lack of memory footprint), you can increase the timeout (to max 5 minutes) or memory (to max of 1.5GB) and restart your polling and you can reprocess the older events.
This would not be possible in case of SNS to Lambda, wherein if Lambda fails the event is lost. And even if you configure DLQ you would still have to make provisions for reading that separately and processing the message
So if your events are critical and you don't want to miss out on them, then go for SNS - SQS - Lambda
The other advantage of having SQS is cost saving on Lambda invocations (Thanks @codesinthedark for bringing this up). You can have much better scaling and less cost, as it allows you to process messages in batches. So one lambda can be executed for a batch of 10 messages while in case of direct SNS each message would trigger a lambda invocation.