Cognito User Migration Trigger - Exception during user migration - Exception Location - amazon-web-services

Question

We're using a lambda function to respond to the 'User Migration' trigger in AWS Cognito. When something like a syntax error occurs, you can see it in cloud watch logs. However, "Exception during user migration" errors seen on the login page are no where to be found in the cloud watch logs.
Where are we supposed to look for these? I can't find any anything in the documentation and assumed it would have gone to cloud watch.
I can't test it in the lambda interface because one of the parameters being passed into the lambda function will have a function nested within the object and I can't create a test JSON setup that has that. There's also no test trigger for user migration that is pre-built.
Any ideas as to why I can't see this in cloud watch or where the exceptions would be shown would be greatly appreciated.

Answer 1

Unfortunately Cogntio doesn't expose any logs (or metrics, for that matter!).
The closest you can get is to view the lambda's logs in CloudWatch. If you log your response, and watch your lambda's error metric then you should mostly be able to debug issues internal to the lambda.
This does leave a few edge cases:
You won't see anything if the lambda can't be invoked (this would only happen under heavy concurrent loads either on that single lambda, or on all lambdas across your account)
If you return a bad response the lambda will succeed but the trigger action will fail and Cognito will give you a fairly generic message. At this point you're at the mercy of AWS' documentation to work out what's wrong (which can be a bit hit and miss- although StackOverflow always helps!).
You can find an example payload for the lambda in the trigger documentation:
{
"userName": "THE USERNAME",
"request": {
"password": "THE PASSWORD"
},
"response": {
// it is your responsibility to fill this bit in and return the completed object back:
"userAttributes": {
"string": "string",
...
},
"finalUserStatus": "string",
"messageAction": "string",
"desiredDeliveryMediums": [ "string", ... ],
"forceAliasCreation": boolean
}
}
n.b. As an aside, which you might know, but Lambda payloads always have to be in JSON, which does not store functions. So you should always be able to derive a test payload to use in the console.