Using step functions to trigger a lambda once another has finished executing

Question

I have two lambdas. Lambda A is triggered once an SQS is populated with messages. I want Lambda B to execute once Lambda A is done executing. How can I do this? Lambda A will have multiple invocations running at the same time, does that make a difference?

Answer 1

Steps functions are created for this purpose. You can transfer data from one lambda to another, so you will have the context of what is being worked on by the previous lambda will be sent to the next lambda.

Answer 2

To do this, you'd need to trigger the Step Function directly from a lambda. As far as I know, you can't trigger Step Functions directly from SQS.

So, either

1. If Lambda A is triggered by something else than SQS, trigger your Step Function instead. It will then run Lambda A and Lambda B depending on how you set it up.

2. If Lambda is triggered by SQS, you make a third lambda (Lambda 0), triggered by SQS, whose sole purpose is to in turn trigger your Step Function (which will run Lambda A then Lambda B) or you directly trigger Lambda B from Lambda A (in which case, Step Functions are pointless and you should rather go with SQS / SNS).

A note on this; one thing Step Functions cannot do out of the box is, for example execute Lambda B only once all Lambda A invocations are done. It will act on a per execution basis.