Serverless.yml custom stack with cloudformation output variable - amazon-web-services

Question

I am newbie for serverless so pardon me if this is very basic. I am having an issue in which i am creating AMAZON COGNITO POOL and i want to use this userPoolId into my custom stack block to connect it with appsync. Below is my serverless.yml
custom:
accountId: 123xxxxxxxx
appSync:
apiId: 123xyzxxxxxxx # only required for update-appsync
authenticationType: AMAZON_COGNITO_USER_POOLS
userPoolConfig:
awsRegion: ap-southeast-1
defaultAction: ALLOW
userPoolId: (here it only takes string but i want to reference)
resources:
Resources:
# Cognito - User pool
CognitoUserPool:
Type: AWS::Cognito::UserPool
Properties:
UserPoolName: abc_xyz_pool
# Cognito - Client
CognitoUserPoolClient:
Type: AWS::Cognito::UserPoolClient
Properties:
ClientName: abc_xyz_pool
GenerateSecret: false
UserPoolId:
Ref: CognitoUserPool
# Cognito - Identity
CognitoIdentityPool:
Type: AWS::Cognito::IdentityPool
Properties:
IdentityPoolName: sc_identity_pool
AllowUnauthenticatedIdentities: false
CognitoIdentityProviders:
- ClientId:
Ref: CognitoUserPoolClient
ProviderName:
Fn::GetAtt: [CognitoUserPool, ProviderName]
I can reference inside the Resources block but i cannot reference it inside the custom block

Answer 1

The custom block in serverless.yml gets evaluated before resources are created and therefore cannot reference those outputs. Even within CFN there are limitations to where and how you reference them.
You can however reference outputs from other CloudFormation stacks.
You should split your serverless project into two projects, the first establishing the user pool, and the second consuming that infrastructure.
In your first project you have your user pool resources, and export the ID for future use in other stacks like so:
Resources:
Outputs:
MyUserPoolId:
Value:
Ref: CognitoUserPool # Key name of user pool resource
Export:
Name: MyUserPoolId
In your second project that needs the pool ID you would import it:
custom:
appSync:
userPoolConfig:
userPoolId:
Fn::ImportValue: MyUserPoolId
Your first project will need to be deployed for the second project to import the exported value.
You can also use ENV variables but that still requires you to establish your user pool first.