Why does ec2.Vpc produce different output for same code when migrating from AWS CDK v1 to v2?

Question

I am working on migrating an AWS CDK project from v1 to v2. The following piece of code produce different output for

```
        self._vpc = vpc(
            self,
            "vpc",
            cidr="10.0.0.0/16",
            enable_dns_hostnames=True,
            enable_dns_support=True,
            max_azs=3,
        )
```

When I upgrade to AWS CDK v2, and run `cdk diff [stackname]` I get the following output :

```
[WARNING] aws-cdk-lib.aws_ec2.VpcProps#cidr is deprecated.
  Use ipAddresses instead
  This API will be removed in the next major release.
[WARNING] aws-cdk-lib.aws_ec2.VpcProps#cidr is deprecated.
  Use ipAddresses instead
  This API will be removed in the next major release.
Stack UprightPipelineStack/test/network-stack (test-network-stack)
Resources
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet1/NATGateway uprightvpcPublicSubnet1NATGateway86E25FDE 
 └─ [+] DependsOn
     └─ ["uprightvpcPublicSubnet1DefaultRoute900852B5","uprightvpcPublicSubnet1RouteTableAssociation7B05C537"]
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet2/NATGateway uprightvpcPublicSubnet2NATGateway117AA26A 
 └─ [+] DependsOn
     └─ ["uprightvpcPublicSubnet2DefaultRouteC847382C","uprightvpcPublicSubnet2RouteTableAssociation517ECEEF"]
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet3/NATGateway uprightvpcPublicSubnet3NATGateway27BAB1E3 
 └─ [+] DependsOn
     └─ ["uprightvpcPublicSubnet3DefaultRoute2A9BF176","uprightvpcPublicSubnet3RouteTableAssociation39653CF8"]
```

What does this change mean, and what can I do to have v2 produce the same infrastructure as v1?

Answer

I'll help you break down the output from the CDK.

**Deprecation Warning**

First, you have a deprecation warning appearing twice:

```
[WARNING] aws-cdk-lib.aws_ec2.VpcProps#cidr is deprecated.
  Use ipAddresses instead
  This API will be removed in the next major release.
```

This is to inform you that the [`VpcProps` interface ](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2.VpcProps.html) will no longer use the `cidr` property in the future and will instead use the `ipAddresses` property. On the [overview page for the EC2 module](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2-readme.html#specifying-a-cidr-directly), you can see the new way to input a CIDR value.

**CDK diff output**

The output from the `cdk diff` command can be challenging to understand sometimes, but is immensely helpful when performing upgrades or refactoring.

```
Stack UprightPipelineStack/test/network-stack (test-network-stack)
Resources
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet1/NATGateway uprightvpcPublicSubnet1NATGateway86E25FDE 
 └─ [+] DependsOn
     └─ ["uprightvpcPublicSubnet1DefaultRoute900852B5","uprightvpcPublicSubnet1RouteTableAssociation7B05C537"]
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet2/NATGateway uprightvpcPublicSubnet2NATGateway117AA26A 
 └─ [+] DependsOn
     └─ ["uprightvpcPublicSubnet2DefaultRouteC847382C","uprightvpcPublicSubnet2RouteTableAssociation517ECEEF"]
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet3/NATGateway uprightvpcPublicSubnet3NATGateway27BAB1E3 
 └─ [+] DependsOn
     └─ ["uprightvpcPublicSubnet3DefaultRoute2A9BF176","uprightvpcPublicSubnet3RouteTableAssociation39653CF8"]
```

I will go line-by-line for the first three lines of the diff to explain the output.

```
[~] AWS::EC2::NatGateway test/network-stack/upright_vpc/PublicSubnet1/NATGateway uprightvpcPublicSubnet1NATGateway86E25FDE 
```
- `[~]` means that the resource indicated here *might* change
- `AWS::EC2::NatGateway` indicates the resource is a NAT Gateway
- `test/network-stack/upright_vpc/PublicSubnet1/NATGateway` is the path to the resource in your CDK Stacks. The path might be a combination of file path and the hierarchy of constructs used to define your CDK stack
- `uprightvpcPublicSubnet1NATGateway86E25FDE` is the logical ID for the resource, it's the identifier in your CloudFormation stack

```
 └─ [+] DependsOn
```
This indicates that the preceding resource in the CloudFormation stack will have a `DependsOn` property added. If it were `[-]`, that would mean it was being removed.

```
     └─ ["uprightvpcPublicSubnet1DefaultRoute900852B5","uprightvpcPublicSubnet1RouteTableAssociation7B05C537"]
```
This identifies the logical IDs of the resources the NAT Gateway will be marked as depending upon. Based upon the IDs I am guessing it had to do with your route tables.

You can view additional information on the [docs page for the AWS CDK CLI toolkit](https://docs.aws.amazon.com/cdk/v2/guide/cli.html#cli-diff).

Based on the diff you shared, I doubt you will have any issues if you deploy your migrated coded. The [`DependsOn` property](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-dependson.html) is there to help CloudFormation deploy resources in the correct order.

Why does ec2.Vpc produce different output for same code when migrating from AWS CDK v1 to v2?

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