AWS Cognito User Pool triggers support

This is a Feature Proposal

Description

I’d like to be able to create functions that hook into a User Pool for various use-cases such as preSignup, customMessage etc. (which are defined here).

Currently I deploy the functions then manually attach them via AWS Console (although this can probably be automated using the AWS SDK).

I am aware that Lambda event sources are different from Lambda triggers (at least conceptually), but in practice they both seem to be only fired when an event occurs. Therefore, it would be lovely if I could define them in serverless.yml with something as follows:

functions:
  preSignup:
    handler: handler.preSignup
    events:
      - cognito-user-pool:
          pool: <POOL-ID>
          trigger: PreSignUp

Similar or dependent issues:

• #3400

[horike37]

Thank you for the greate proposal @hassankhan 😍 That could be super helpful and really useful! I often use Cognito UserPool too, and face the same problem 😅

As you say, those triggers are a little different from Lambda event sources, so it may be better to change the syntax to distinguish events from Lambda triggers.

For example

functions:
  preSignup:
    handler: handler.preSignup
    aws-triggers:
      - cognitoUserPool:
          pool: <POOL-ID>
          target: preSignUp

What do you think? @pmuens @eahefnawy

[pmuens]

Awesome!!! 💯 Thank you very much @hassankhan for working on this one! 🥇 And thank you @horike37 for helping out! 💪

Already added the PR to the review list! 🎉