AWS SAM is a framework for deploying severless applications. To deploy an application with SAM you have to create a template.yaml file which describes the entire stack that will be deployed to support the application.
If you’re like me and you love the DRY principle, you hate to see things like the following
Mappings:
EnvMapping:
dev:
VpcId: vpc-xxxxxxxxx
SubnetIds:
- subnet-xxxxxxxx
- subnet-xxxxxxxx
- subnet-xxxxxxxx
Foo: bar
performance:
VpcId: vpc-xxxxxxxxx
SubnetIds:
- subnet-xxxxxxxx
- subnet-xxxxxxxx
- subnet-xxxxxxxx
Bar: bing
feature:
VpcId: vpc-xxxxxxxxx
SubnetIds:
- subnet-xxxxxxxx
- subnet-xxxxxxxx
- subnet-xxxxxxxx
Baz: foo
prod:
VpcId: vpc-yyyyyyyy
SubnetIds:
- subnet-yyyyyyyy
- subnet-yyyyyyyy
- subnet-yyyyyyyy
This is obviously redundant. The VPC id and Subnet ids are being repeated and if you need to update your VPC, you would have to change the information for each environment.
The alternative is to create a VpcMapping and then use a value from the EnvMapping as a key. For example, see the solution below which removes the redundant VPC declarations and puts them into a map.
Mappings:
VpcMapping:
prod:
VpcId: vpc-yyyyyyyy
SubnetIds:
- subnet-yyyyyyyy
- subnet-yyyyyyyy
- subnet-yyyyyyyy
nonprod:
VpcId: vpc-xxxxxxxxx
SubnetIds:
- subnet-xxxxxxxx
- subnet-xxxxxxxx
- subnet-xxxxxxxx
EnvMapping:
dev:
Vpc: nonprod
Foo: bar
performance:
Vpc: nonprod
Bar: bing
feature:
Vpc: nonprod
Baz: foo
prod:
Vpc: prod
As you can see, the Vpc field serves as the VpcMapping key. Now, when you need to reference your VpcId or SubnetIds, you can use a nested !FindInMap function. See below
VpcId: !FindInMap
- VpcMapping
- !FindInMap
- EnvMapping
- !Ref Env
- Vpc
- VpcId
Or if you prefer that in a single line (retrieving the subnet ids this time)
SubnetIds: !FindInMap [VpcMapping, !FindInMap [EnvMapping, !Ref Env, Vpc], SubnetIds]
Hopefully this helps keep you DRY
