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---
rfc pr: [#335](https://github.com/aws/aws-cdk-rfcs/pull/335)
tracking issue: https://github.com/aws/aws-cdk-rfcs/issues/1
---

CDK Accelerate

Shorten the development iteration speed -- the "edit-compile-test" loop -- for CDK applications.

Working Backwards

CHANGELOG

  • feat(cli): implement cdk deploy --accelerated and cdk deploy --watch

Help

cdk deploy --help

    -w --watch   Watch for file changes and deploy any updates. Implies
                 --accelerated=auto if --accelerated is not specified.

    --accelerated[=only|auto|no|ask]   Perform an accelerated deployment of the stack.
        (`--accelerated=only` will fail if there are non-accelerated updates). The
        default value is "only".

Examples:
    cdk deploy -w
    cdk deploy --accelerated
    cdk deploy --accelerated=only --watch

README

The cdk deploy --accelerated ("CDK accelerate") command accelerates the edit-compile-test loop for your CDK application during development by inspecting the assets and stack resources in your application, identifying those that can be updated in-place without a full CloudFormation stack update, and doing so using AWS service APIs directly. It can either do this as an interactive command or by running continuously and monitoring the input files.

For supported construct types (see list, below) the update command will identify assets or other stack resources that have changed since the last time the CDK application was deployed, and update them in-place according to their types. Assets will be uploaded to their storage medium. The CDK will then use AWS SDK commands to directly modify the stack resources in-place to synchronize them with your local code. The accelerate command can act on one or more stacks in the application.

Usage

The simplest use case for CDK accelerate is cdk deploy --watch. This mode of operation will identify the files that can affect the resources being watched, monitor them for changes, and perform the fastest supported form of deployment on the stacks that change, as they change.

$ cdk deploy --accelerated --watch ApplicationStack DataStack
Checking stack ApplicationStack for possible accelerated update
 * LambdaFunction[Appfunction]: No Changes
ApplicationStack can *not* be updated
Checking stack DataStack for possible accelerated update
 * LambdaFunction[StreamFunction]: No Changes

Watching stack inputs for changes:
 * LambdaFunction[Appfunction]: <spinner>
 * LambdaFunction[StreamFunction]: <spinner>

The watcher can only watch for file changes. For Lambda code, that means that directories will be watched for changes and zipped if they change. If the file asset is a zip file, then the update will fire whenever that changes. The existing docker asset builder will be used to watch for changes in local docker images.

In addition to the monitoring mode, it is possible to perform one-shot accelerated deployments on some or all of the stacks in the CDK application:

$ cdk deploy --accelerated ApplicationStack
Checking stack ApplicationStack for possible accelerated update
 * LambdaFunction[AppFunction]: File asset changed
ApplicationStack can be updated
1/1 stacks can be updated:
Updating LambdaFunction in ApplicationStack
...
Update complete!

If the update is to an attribute of the stack that cannot be updated, the command can offer to perform a full deployment if --accelerated=ask:

$ cdk deploy --accelerated=ask ApplicationStack
Checking stack ApplicationStack for possible accelerated update
 * LambdaFunction[Appfunction]: Function data changed
ApplicationStack has changes that can not be rapidly updated.
Perform a full deployment of ApplicationStack? [Y/n]: Y
Deploying ApplicationStack
...
Done!

When multiple stacks are provided, update only occurs if all pending changes are suitable for update. Otherwise, a full deployment is done:

$ cdk deploy --accelerated=auto ApplicationStack DataStack
Checking stack ApplicationStack for possible accelerated update
 * LambdaFunction[Appfunction]: Function data changed
ApplicationStack has changes that can not be rapidly updated.
Checking stack DataStack for possible accelerated update
 * LambdaFunction[StreamFunction]: File asset changed
DataStack can be updated
1/2 stacks can be accelerated, automatically performing a full deployment.
Deploying ApplicationStack
...
Deploying DataStack
...
Done!

In addition to running in a one shot mode, the cdk deploy --accelerated command also has a --watch command line option that enable it to monitor the assets on disk and perform an update when they change.

Resource Support

  • AWS Lambda Function
    • file and directory assets
  • StepFunction
    • Workflow definitions
  • AWS Fargate
    • image assets
  • ECS
    • image assets

Future Support Plans

  • API Gateway models

FAQ

What are we launching today?

The cdk deploy --watch and cdk deploy --accelerated command features, with support for rapid update of Lambda function code, images for ECS and Fargate task definitions, and AWS StepFunction workflows.

Why should I use this feature?

If you are developing a CDK application and want to publish your code changes without waiting for CloudFormation to perform a full, safe stack update, you should use this feature.

Does it work when my Lambda is in Go or Java?

To an extent, yes, but there are caveats. In order to update the lambda code you will need to run your build process to create the jar or executable that you are uploading as lambda code.

Does this feature work with aws-lambda-go for compiled Lambdas?

Yes, if your lambda is defined by an aws-lambda-go, aws-lambda-python, or aws-lambda-nodejs construct, the accelerate and watch capabilities will work within those source trees to automatically deploy changes to your code as you make it.

What about "other resource type"?

Based on customer response, we will consider expanding the set of AWS resources and asset types implemented in the CDK directly. The plugin mechanism for resource refreshing will allow other resource types to be supported by either community or vendor addition.

Internal FAQ

Why are we doing this?

The overhead of developing a Lambda application using the CDK is significant, since each code change currently requires a CloudFormation stack update to apply new code, or manually introducing drift to the application by inspecting the stack and manipulating the resources directly.

The CDK accelerate tool will allow the CDK to handle the resource changes rather than this manual process, introducing some implicit safety and reducing the manual labor of the update.

Why should we not do this?

This solution has a risk of introducing a deployment tool that users might use to shortcircuit the safe CloudFormation deployment process. If a user runs CDK update on their production stack, it can perform updates without adequate stack update safety checks. Releasing a public tool with instant update capability into the CDK may not be the right way to make this functionality public. To mitigate this, accelerate only runs on explicitly selected stacks, and does not support the --all flag.

What changes are required to enable this change?

  1. An implementation of the cdk deploy --accelerated command in the CDK CLI that can examine the Application stacks for updates that can be applied and apply them.
  2. The CDK CLI must be able to query a CDK resource for the set of filesystem resources it must monitor for the --watch operation, and run a filesystem monitor for that.

Is this a breaking change?

No, it is not.

What are the drawbacks of this solution?

  • Updating like this still entails a fair amount of preprocesing time, since for complex projects (Golang and Java lambdas, for example) there remains a compilation step.
  • The update model requires the AWS account used to publish the stack to also have sufficient permissions to update the underlying resources, rather than simply requiring the account to have CloudFormation access.
  • Runtimes that require compilation or assembly -- Java lambdas, docker images -- do not benefit from the --watch support as naturally, and require some manual steps.
  • updating Lambda functions that are using aliases and provisioned concurrency can take several minutes to switch over between the two versions.

What alternative solutions did you consider?

We considered SAM accelerate for a similar purpose, but SAM covers a relatively small set of possible application structures in AWS, while CDK is intended to address the whole set of them.

We also considered introspecting the CDK model ourselves, but concluded that there was little value in reinventing the wheel, so to speak, when CDK already had all of the information we'd need to deliver this.

For identifying stacks that are subject to accelerated deployment, we are considering defining a full "Personal Development Stack" model, possibly based off of information in the CDK App context.

What is the high level implementation plan?

We will start from the prototype CDK update command that identifies the Lambda resources and then publishes them using the CDK CLI, and extend that to implement support for ECR images associated with ECS and Fargate tasks, API Gateway definitions, and Step Function workflows. Those will be implemented directly in the CLI code as part of the launch for the feature. The CLI implementation will be designed to conform to an interface that provides:

  • Watchable filesystem resources
  • A way of updating the watchable resource list
  • A method for determining whether the construct can be updated in place
  • A method for updating the construct in place

If changes to the CDK constructs are necessary to implement the accelerated development process, we will make those changes as well. In the longer term we must lay the groundwork for moving the logic defining the update process into the Construct library, which implies a design for passing these values by way of the Cloud Assembly.

We will implement a filesystem watcher for the CDK CLI that works on one or more directory trees, watching for changes. It will base its watch list on the set of files indicated by the CLI, and update them when those responses change.

Additionally, a --watch flag and a file watcher will be added to support monitoring the inputs of stack resources for changes.

Are there any open issues that need to be addressed later?

  • This RFC can be extended to add support for further pluggable asset and update targets. The accelerate capabilities are attached to the CDK constructs, not the CDK CLI, so any CDK construct that can perform accelerated deployment can implement that capability in whatever manner is appropriate.
  • This RFC will be enhanced significantly when the CDK asset model is enriched to support asset construction directly.
  • Direct support for monitoring container repositories for changes (possibly via polling) instead of only supporting local rebuild.

How do we keep production from being affected by CDK accelerate?

CDK accelerate uses the AWS IAM access controls already in place in your account to maintain the safety of your production deployments. The interface to CDK accelerate requires the developer to specify the stacks that they will be deploying explicitly, so by default it is affecting only the developer's stacks, and when a production stack is defined it is up to the AWS account administrator to ensure that the interactive developer's roles do not have modification access to the accelerated resources.