Issues with !Sub and !GetAtt on EC2 Metadata

Serverless Framework
1

Hi, I’m having issues to obtain some RDS Instance attributes from my EC2 Metadata configSet using !Sub and !GetAtt functions combined.

So, at the resources key in my serverless.yml file I have (among other things) an EC2 instance (OASInstance) and an RDS instance (OASRDSInstance). I’m trying to build a connection string for my DB hosted on RDS, and for that, I’m using !Sub and !GetAtt functions inside a file definition on the Metadata block at my EC2 instance definition.

I’ve tried this in several ways, using short and long syntax format, but I make sure that this one works on a CloudFormation template:

- Fn::Sub:
  - "${RDSEndpointURL}:${RDSEndpointPort}:ORCL"
  - {RDSEndpointURL: !GetAtt OASRDSInstance.Endpoint.Address, RDSEndpointPort: !GetAtt OASRDSInstance.Endpoint.Port}

To give you more context, those lines are in a content block for a path like this one:

image
image1185×35 7.66 KB

The error I’m getting when I try to deploy this template is this one:

Serverless Error ---------------------------------------   
Invalid variable reference syntax for variable RDSEndpointURL. You can only reference env vars, options, & files. You can check our docs for more info.

However, the same code block works perfectly if I use it in the same place on a CloudFormation template.

My guess is that serverless is confusing ${RDSEndpointURL} and ${RDSEndpointPort} with local template variables, and doesn’t realize that those are just for the !Sub context.

What should I do?

Thanks

2

Well, I found a workaround.

I replace !Sub with !Join.

This is what I had:

- Fn::Sub:
  - "${RDSEndpointURL}:${RDSEndpointPort}:ORCL"
  - {RDSEndpointURL: !GetAtt OASRDSInstance.Endpoint.Address, RDSEndpointPort: !GetAtt OASRDSInstance.Endpoint.Port}

And this is the replacement:

- !Join 
  - ''
  - - !GetAtt OASRDSInstance.Endpoint.Address
    - ':'
    - !GetAtt OASRDSInstance.Endpoint.Port
    - ':'
    - ${self:custom.RDSDbName}

However, I would like to know if there is a better way.