Identifiers

The AWS CDK deals with many types of identifiers and names. To use the AWS CDK effectively and avoid errors, you need to understand the types of identifiers.

Identifiers must be unique within the scope in which they are created; they do not need to be globally unique in your AWS CDK application.

If you attempt to create an identifier with the same value within the same scope, the AWS CDK throws an exception.

Construct IDs

The most common identifier, id, is the identifier passed as the second argument when instantiating a construct object. This identifier, like all identifiers, need only be unique within the scope in which it is created, which is the first argument when instantiating a construct object.

Let's look at an example where we have two constructs with the identifier MyBucket in our app. However, since they are defined in different scopes, the first in the scope of the stack with the identifier Stack1, and the second in the scope of a stack with the identifier Stack2, that doesn't cause any sort of conflict, and they can coexist in the same app without any issues.

import { App, Construct, Stack, StackProps } from '@aws-cdk/core';
import * as s3 from '@aws-cdk/aws-s3';

class MyStack extends Stack {
  constructor(scope: Construct, id: string, props: StackProps = {}) {
    super(scope, id, props);

    new s3.Bucket(this, 'MyBucket');
  }
}

const app = new App();
new MyStack(app, 'Stack1');
new MyStack(app, 'Stack2');

Paths

The constructs in an AWS CDK application form a hierarchy rooted in the App class. We refer to the collection of IDs from a given construct, its parent construct, its grandparent, and so on to the root of the construct tree, as a path.

The AWS CDK typically displays paths in your templates as a string, with the IDs from the levels separated by slashes, starting at the node just below the root App instance, which is usually a stack. For example, the paths of the two Amazon S3 bucket resources in the previous code example are Stack1/MyBucket and Stack2/MyBucket.

You can access the path of any construct programmatically, as shown in the following example, which gets the path of myConstruct (or my_construct, as Python developers would write it). Since IDs must be unique within the scope they are created, their paths are always unique within a AWS CDK application.

const path: string = myConstruct.node.path;

Unique IDs

Since AWS CloudFormation requires that all logical IDs in a template are unique, the AWS CDK must be able to generate a unique identifier for each construct in an application. Resources have paths that are globally unique (the names of all scopes from the stack to a specific resource) so the AWS CDK generates the necessary unique identifiers by concatenating the elements of the path and adding an 8-digit hash. (The hash is necessary to distinguish distinct paths, such as A/B/C and A/BC, that would result in the same AWS CloudFormation identifier, since AWS CloudFormation identifiers are alphanumeric and cannot contain slashes or other separator characters.) The AWS CDK calls this string the unique ID of the construct.

In general, your AWS CDK app should not need to know about unique IDs. You can, however, access the unique ID of any construct programmatically, as shown in the following example.

const uid: string = Names.uniqueId(myConstruct);

The address is another kind of unique identifier that uniquely distinguishes CDK resources. Derived from the SHA-1 hash of the path, it is not human-readable, but its constant, relatively short length (always 42 hexadecimal characters) makes it useful in situations where the "traditional" unique ID might be too long. Some constructs may use the address in the synthesized AWS CloudFormation template instead of the unique ID. Again, your app generally should not need to know about its constructs' addresses, but you can retrieve a construct's address as follows.

const addr: string = myConstruct.node.addr;

Logical IDs

Unique IDs serve as the logical identifiers, which are sometimes called logical names, of resources in the generated AWS CloudFormation templates for those constructs that represent AWS resources.

For example, the Amazon S3 bucket in the previous example that is created within Stack2 results in an AWS::S3::Bucket resource with the logical ID Stack2MyBucket4DD88B4F in the resulting AWS CloudFormation template.

Logical ID stability

Avoid changing the logical ID of a resource after it has been created. Since AWS CloudFormation identifies resources by their logical ID, if you change the logical ID of a resource, AWS CloudFormation creates a new resource with the new logical ID, then deletes the existing one. Depending on the type of resource, this may cause service interruption or data loss, or both.