I'd like to make CloudFormation look in Parameter Store for a particular parameter - and if not found, look for a different one.
The actual use case is that stacks are deployed for different branches and we'd like to have top-level parameters for all branches that can optionally be overridden by creating a branch-specific parameter. Something like this:
dev-param = 120 <-- top-level, applies if branch-specific parameter doesn't exist
dev-param.mybranch = 60 <-- branch-specific parameter
Have tried a couple of ways but got an error when deploying the stack for both - see below.
When attempting to use dynamic references:
Parameters: [ssm:dev-param.mybranch] cannot be found.
When attempting to use CloudFormation SSM Parameter Types.
Template format error: Every Default member must be a string.
For the latter, the Default: field specifies the Parameter Store key name. This needs to be generated dynamically from other CloudFormation parameters, e.g. there is a parameter for the environment type so development key names begin with dev- and production keys begin with prod-.
Is there another way to achieve this?
You can't do that without a custom resource or a macro.