Commits on Apr 7, 2021

1. refactor: Encapsulate overriding of logicalId in a single place …

   A resource's logicalId maps to the name of the created resource.
   
   For example, take this CloudFormation YAML for an ASG:
   
   ```yaml
   DataNodeAutoScalingGroup:
     Type: AWS::AutoScaling::AutoScalingGroup
     Properties:
       AvailabilityZones: !GetAZs ""
       LaunchConfigurationName: !Ref DataNodeLaunchConfig
       MinSize: 3
       MaxSize: 4
       Cooldown: 180
       HealthCheckType: EC2
       HealthCheckGracePeriod: 300
       VPCZoneIdentifier: !Ref PrivateVpcSubnets
       Tags:
       - { Key: Stack, Value: !Ref Stack, PropagateAtLaunch: true }
       - { Key: App, Value: "elk-es-data", PropagateAtLaunch: true }
       - { Key: Stage, Value: !Ref Stage, PropagateAtLaunch: true }
       - { Key: ElasticSearchCluster, Value: !Ref ClusterName, PropagateAtLaunch: true }
   ```
   
   This creates an ASG called `DataNodeAutoScalingGroup`. This pattern is widely used in YAML definitions of CloudFormation as YAML files are typically hand-written.
   
   CDK changes this as it auto-generates the logicalIds, as there one will gain more understanding about the resource from looking at the CDK template.
   
   In order to enable a no-op migration between a YAML stack a CDK stack, we allow the logicalId to be overridden.
   Our constructs seem to follow different rules regarding when to override the logicalId of a CloudFormation resource.
   
   For example, we have [this](https://github.com/guardian/cdk/blob/60cbf21698b3ddbb8cfd23556fd2af886e9551c7/src/constructs/rds/instance.ts#L38-L40):
   
   ```typescript
   if (props.overrideId || (scope.migratedFromCloudFormation && props.overrideId !== false))
   ```
   
   And [this](https://github.com/guardian/cdk/blob/60cbf21698b3ddbb8cfd23556fd2af886e9551c7/src/constructs/iam/policies/base-policy.ts#L15-L18):
   
   ```typescript
   if (props.overrideId) {
   ```
   
   This change aims to bring consistency and simplicity across our constructs by applying a universal rule that,
   `migratedFromCloudFormation` on the `GuStack` must be `true` and `existingLogicalId` to be set for the logicalId to be overridden.
   
   If these conditions are not met, we let CDK auto-generate the logicalId.
   
   It's worth noting that this doesn't implement the change on the constructs, it's simply to define the behaviour.
   This is mainly to keep the diff down - we'll migrate the constructs in follow-up PRs.
   
   Usage will roughly look like this:
   
   ```typescript
   const MyComponentProps extends GuMigratingResource { }
   
   class MyComponent extends SomeGuConstruct {
     constructor(scope: GuStack, id: string, props: MyComponentProps) {
       super(scope, id, props);
       GuMigratingResource.setLogicalId(this, scope, props);
     }
   }
   ```

   

   akash1810 committed Apr 7, 2021
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