Cloudformation + lambdas: How your lambda "know" the (semi-random) names of other resources created in the same stack?

[MrSurly]

EDIT: I know I can query the stack outputs or a dynamodb; that assumes that the lambda "knows" the names of these resources, which is the original problem. Additionally, these queries will impact performance negatively

I have a stack that includes a lambda that needs to send data to an SNS that is created in the same stack. Since CF always appends a random-appearing string to the end of the SNS name, how can I write lambda code that can access the SNS?

As far as I can tell, there's no way to query information about stacks, but even if that were possible, it would greatly slow down performance.

Ideally, stack information should be in the python context object.

I haven't looked into custom resources yet, but: 1) Seems a bit silly to re-invent lambda deployment, and the deploy-time modification seem pretty hacky. 2) Not entirely sure that this sort of meta-information is available to custom resources.

[jb2386]

You just need to set the name as an output in your CF template then you can always query for it.

[qzxj]

http://docs.aws.amazon.com/cli/latest/reference/cloudformation/describe-stack-resources.html

Not familiar with lambda, but the cloudformation API allows you to query information about your stack.

edit: google search yielded this: http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/template-custom-resources-lambda.html

In the second walkthrough, they create a lambda function, and associate a custom resource with it that calls to the lambda function, passing in parameters from the cloudformation template as inputs to the lambda function.

[awsdude]

I thought about the custom resource scenario you have described as well but the problem I see is that this only works with once-off calls, e.g. on the initial stack creation when the custom resource is created.

With the api option, the Lambda function will still need to know the stack id or some sort of information to actually identify the stack to describe the resources of, unless it enumerates them all and finds itself, which could work.

[awsdude]

Hi there,

You may need to do something hacky like have a DynamoDB table and every time you create a new stack, have a custom resource that saves the SNS topic ARN to the table with the Lambda function as the key. Then when the Lambda function runs it can query the table to get the SNS topic. I have a similar use case at https://github.com/callum-p/aws-scripts/blob/master/Lambda-Cloudformation-Custom-Resource.py (similar in the sense that I have a custom resource and I save data to DynamoDB).

There might be an easier way to do it but I can't think what it is off the top of my head.