ssm keys doesnt work with variables - Value not found at "ssm" source

Are you certain it’s a bug?

• Yes, it looks like a bug

Is the issue caused by a plugin?

• It is not a plugin issue

Are you using the latest version?

• Yes, I’m using the latest version

Is there an existing issue for this?

• I have searched existing issues, it hasn’t been reported yet

Issue description

When trying to use variables its looks like you can’t use any variables

SubnetIds: ${ssm:/SLS/myapp-${opt:stage}/DBSubnets}

will result in

  - Cannot resolve variable at "resources.2.Resources.ElasticsearchDomain.Properties.VPCOptions.SubnetIds": Value not found at "ssm" source

and

custom:
 stage: ${opt:stage}
...
SubnetIds: ${ssm:/SLS/myapp-${self:custom.stage}/DBSubnets}

result with the same error.

Service configuration (serverless.yml) content

service: myservice
variablesResolutionMode: 20210326
custom:
    # Get stage from command line (serverless --stage local|staging|prod)
    stage: ${opt:stage}


provider:
    name: aws
    lambdaHashingVersion: 20201221
    runtime: nodejs14.x
    memorySize: 256
    stage: ${self:custom.stage}
    region: us-east-1

resources:
  Resources:
    ElasticsearchDomain:
      Type: AWS::OpenSearchService::Domain
      Properties:
        DomainName: tasks-${self:custom.stage}
        EngineVersion: 'OpenSearch_1.0'
        ClusterConfig:
          # DedicatedMasterEnabled: true
          InstanceCount: '1'
          # ZoneAwarenessEnabled: true
          InstanceType: 't3.small.search'
          # DedicatedMasterType: 't3.small.elasticsearch'
          # DedicatedMasterCount: '1'
        EBSOptions:
          EBSEnabled: true
          VolumeSize: '16'
          VolumeType: 'gp2'
        AdvancedOptions:
          rest.action.multi.allow_explicit_index: true
        VPCOptions:
          #SecurityGroupIds: ${ssm:/SLS/${AWS::StackName}/AppSecurityGroup}
          SubnetIds: ${ssm:/SLS/myapp-${self:custom.stage}/DBSubnets}

Command name and used flags

SLS_DEBUG=* npx sls package -s dev

Command output

Environment: darwin, node 15.14.0, framework 3.1.1 (local), plugin 6.0.0, SDK 4.3.1
Credentials: Local, "default" profile
Docs:        docs.serverless.com
Support:     forum.serverless.com
Bugs:        github.com/serverless/serverless/issues

Error:
Cannot resolve serverless.yml: Variables resolution errored with:
  - Cannot resolve variable at "resources.Resources.ElasticsearchDomain.Properties.VPCOptions.SubnetIds": Value not found at "ssm" source

Environment information

Serverless: Running "serverless" installed locally (in service node_modules)
Framework Core: 3.1.1 (local)
Plugin: 6.0.0
SDK: 4.3.1

[pgrzesik]

Thanks for sharing @NeilJ247 🙇 So there are two workarounds at this time:

• don’t use params and use ${sls:stage} to dynamically define SSM keys based on stage
• Use default null values for the params e.g.

production:
    DB_HOST: ${ssm:/Production/MyService/DB_HOST, null}

While the proposal to not resolve them is definitely reasonable, it would highly complicate the internal logic that resolves variables.

[pgrzesik]

Yes, in your specific case you would have to reengineer the SSM setup so setting to null might be the best choice for you.