Is there a way to add a trigger to a Lambda function in Cloudformation for s3 events, where the s3 bucket already exists? (i.e, is not created by said template)
I have tried to find an example of this online, but it appears that the only way to set this trigger in CF is by using the bucket notification configuration.
Cloudformation cannot do this directly. However, Cloudformation Custom Resources can call Lambda functions, and Lambda functions can do whatever you program them to do. You could write a Lambda function which creates or deletes some resource based on whatever logic you want.
See more: AWS Lambda-backed Custom Resources - AWS CloudFormation
