AWS CDK App Life Cycle

Hi Folks,

This blog is snippet from AWS CDK documentation, where I am sharing the AWS CDK App life cycle workflow. Please refer documentation link for the details.

Documentation : https://docs.aws.amazon.com/cdk/v2/guide/apps.html#lifecycle

App lifecycle

The following diagram shows the phases that the AWS CDK goes through when you call the cdk deploy. This command deploys the resources that your app defines.

An AWS CDK app goes through the following phases in its lifecycle.

1. Construction (or Initialization)

Your code instantiates all of the defined constructs and then links them together. In this stage, all of the constructs (app, stacks, and their child constructs) are instantiated and the constructor chain is executed. Most of your app code is executed in this stage.

2. Preparation

All constructs that have implemented the prepare method participate in a final round of modifications, to set up their final state. The preparation phase happens automatically. As a user, you don't see any feedback from this phase. It's rare to need to use the "prepare" hook, and generally not recommended. Be very careful when mutating the construct tree during this phase, because the order of operations could impact behavior.

3. Validation

All constructs that have implemented the validate method can validate themselves to ensure that they're in a state that will correctly deploy. You will get notified of any validation failures that happen during this phase. Generally, we recommend performing validation as soon as possible (usually as soon as you get some input) and throwing exceptions as early as possible. Performing validation early improves diagnosability as stack traces will be more accurate, and ensures that your code can continue to execute safely.

4. Synthesis

This is the final stage of the execution of your AWS CDK app. It’s triggered by a call to app.synth(), and it traverses the construct tree and invokes the synthesize method on all constructs. Constructs that implement synthesize can participate in synthesis and emit deployment artifacts to the resulting cloud assembly. These artifacts include AWS CloudFormation templates, AWS Lambda application bundles, file and Docker image assets, and other deployment artifacts. Cloud assemblies describes the output of this phase. In most cases, you won't need to implement the synthesize method.

5. Deployment

In this phase, the AWS CDK Toolkit takes the deployment artifacts cloud assembly produced by the synthesis phase and deploys it to an AWS environment. It uploads assets to Amazon S3 and Amazon ECR, or wherever they need to go. Then, it starts an AWS CloudFormation deployment to deploy the application and create the resources.

By the time the AWS CloudFormation deployment phase (step 5) starts, your AWS CDK app has already finished and exited. This has the following implications:

• The AWS CDK app can’t respond to events that happen during deployment, such as a resource being created or the whole deployment finishing. To run code during the deployment phase, you must inject it into the AWS CloudFormation template as a custom resource. For more information about adding a custom resource to your app, see the AWS CloudFormation module, or the custom-resource example.
• The AWS CDK app might have to work with values that can’t be known at the time it runs. For example, if the AWS CDK app defines an Amazon S3 bucket with an automatically generated name, and you retrieve the bucket.bucketName (Python: bucket_name) attribute, that value is not the name of the deployed bucket. Instead, you get a Token value. To determine whether a particular value is available, call cdk.isUnresolved(value) (Python: is_unresolved).