(aws-efs): EFS mount target will fail on subnet change

[kochie]

Describe the bug

When removing an EFS Mount point from a CDK deployment (say by defining fewer subnets than what are previously deployed), CDK will remove the correct mount point but will use the old unique Id for the deleted one if that subnet is in the middle of the subnet list. This will cause CloudFormation to fail as it tries to make a new Mount Point for the same EFS File System in the same AZ, which it can't do.

Expected Behavior

If I have a CDK construct

const fileSystem = new FileSystem(this, "EfsFileSystem", {
    vpc,
    vpcSubnets: {
        subnets: [subnetA, subnetB, subnetC]
    }
})

This will synth 3 Mount points, eg.

EfsFileSystemEfsMountTarget01:
  Properties:
    SubnetId: subnetA
EfsFileSystemEfsMountTarget02:
  Properties:
    SubnetId: subnetB
EfsFileSystemEfsMountTarget03:
  Properties:
    SubnetId: subnetC

When I remove one of the subnets, i.e. remove subnet B

const fileSystem = new FileSystem(this, "EfsFileSystem", {
    vpc,
    vpcSubnets: {
        subnets: [subnetA, subnetC]
    }
})

CDK synth will create 2 mount points with correct Ids

EfsFileSystemEfsMountTarget01:
  Properties:
    SubnetId: subnetA
EfsFileSystemEfsMountTarget03:
  Properties:
    SubnetId: subnetC

Current Behavior

If I have a CDK construct

const fileSystem = new FileSystem(this, "EfsFileSystem", {
    vpc,
    vpcSubnets: {
        subnets: [subnetA, subnetB, subnetC]
    }
})

This will synth 3 Mount points, eg.

EfsFileSystemEfsMountTarget01:
  Properties:
    SubnetId: subnetA
EfsFileSystemEfsMountTarget02:
  Properties:
    SubnetId: subnetB
EfsFileSystemEfsMountTarget03:
  Properties:
    SubnetId: subnetC

When I remove one of the subnets, i.e. remove subnet B

const fileSystem = new FileSystem(this, "EfsFileSystem", {
    vpc,
    vpcSubnets: {
        subnets: [subnetA, subnetC]
    }
})

CDK synth will create 2 mount points with an incorrect Id

EfsFileSystemEfsMountTarget01:
  Properties:
    SubnetId: subnetA
EfsFileSystemEfsMountTarget02:
  Properties:
    SubnetId: subnetC

Note the subnet names and unique Ids have changed for one of the mount targets.

Which is technically correct, but now CloudFormation will fail with this error:

mount target already exists in this AZ

This is because since the uniqueid of the mount target in subnetC has changed it's trying to create a new one.

Reproduction Steps

Create a CDK construct with an EFS attached to 3 subnets.

const fileSystem = new FileSystem(this, "EfsFileSystem", {
    vpc,
    vpcSubnets: {
        subnets: [subnetA, subnetB, subnetC]
    }
})

now delete subnetB.

Possible Solution

CDK needs to keep track of the subnets and mount target unique IDs somehow

Additional Information/Context

No response

CDK CLI Version

2.67.0

Framework Version

No response

Node.js Version

18.12.1

OS

Windows

Language

Typescript

Language Version

No response

Other information

No response

[pahud]

Agree. I think we probably should not name the MountTarget ID based on its index here:

aws-cdk/packages/aws-cdk-lib/aws-efs/lib/efs-file-system.ts

Line 438 in d71ae00

'EfsMountTarget' + (++mountTargetCount),

[kochie]

Perhaps based on the Subnet ID?

[pahud]

Perhaps based on the Subnet ID?

Yes that might be a good idea. Per our discussion on Slack, this might need a new feature flag for the breaking changes but we welcome and appreciate your PR if any.

[github-actions]

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