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cdk-watch 👀

Run your CDK Stack's Lambda functions as if they were in a development environment.

  • As simple as cdkw "MyApp/MyApi/**"
  • Your code will be watched and built with the same esbuild config as when deploying your stack
  • Simply switch-out your existing NodejsFunction with the WatchableNodejsFunction construct
  • Opt-in to real-time logs, so no more digging through CloudWatch to find your Lambda's logs
  • Load your node modules as a separate lambda layer (allows for faster build/upload times)
  • Written in TypeScript
  • No extra infrastructure required, unless you are opting-in to real-time logs


Getting Started

1. CDK Updates

Simply switch to use the WatchableNodejsFunction rather than the NodejsFunction.

- import {NodejsFunction} from '@aws-cdk/aws-lambda-nodejs';
+ import {WatchableNodejsFunction} from 'cdk-watch';

// ...

- const lambda = new NodejsFunction(this, 'Lambda', {
+ const lambda = new WatchableNodejsFunction(this, 'Lambda', {
  entry: path.resolve(__dirname, '../src/my-lambda.ts'),
  handler: 'handler',
});

2. Run Your Stack

# Run all Lambda functions
$ yarn cdkw "**"

# Run just your API Lambdas, for example
$ yarn cdkw "MyStack/API/**"

# Run without logs
$ yarn cdkw "MyStack/API/**" --no-logs

# Pass context to synth
$ yarn cdkw "MyStack/API/**" -c foo=bar -c hello=world

# If you are using npm
$ npm run cdkw "**"

Skip to the command reference.


Real-time Logs

cdk-watch provides real-time logs over web-sockets to make the development feedback-loop faster when debugging your lambdas. This is an additional feature that requires opt-in, and you have two options for achieving this.

  1. Turn on for all Lambdas: To turn on real-time logging by default for all watchable lambdas in your stack you can set the context variable cdk-watch:forceRealTimeLoggingEnabled to true.
  2. To set an individual Lambda to support real-time logging you can pass a prop to the WatchableNodejsFunction: watchOptions.realTimeLoggingEnabled=true.

How does real-time logging work?

When deploying your stack the WatchableNodejsFunction will include the necessary infrastructure to support WebSockets via API Gateway. It'll also assign a Lambda Layer Extension to wrap your lambda, the wrapper will patch the console and forward all logs to all API Gateway connections. If you have multiple lambdas in your stack it'll only create the require infrastructure once, and reuse it for all lambdas that need it.

Node Modules Layer

CDK-Watch allows you to install your node-modules as a stand alone layer. This means that when you deploy, cdk-watch will install your modules in a separate asset and install them as the lambda's layer. This is great for dev-performance as the upload bundle will be much smaller. You can configure this using the bundling.nodeModulesLayer property:

bundling: {
  // Install only "knex" as a standalone layer
  nodeModulesLayer: {include: ['knex']}
}

OR:

bundling: {
  // Install every module found in your package.json except "knex"
  nodeModulesLayer: {exclude: ['knex']}
}

How, what & why?

Why would you want to do this?

  • Deploying via CDK is slow, it takes 1 minute to release an update to a zero-dependency, 1 line Lambda function
  • AWS provides a tonne of great services, therefore running your code and CDK Stack on AWS when developing makes sense rather than attempting to fully replicate the environment locally, or missing out on using some of it's services
  • Provided you keep your Lambda's small, an update via cdk-watch will take a couple of seconds

How does it work?

  1. cdkw will run cdk synth
  2. At synthesis the WatchableNodejsFunction construct will write a manifest file including Lambda Logical IDs, their Stack names and their esbuild config
  3. The CLI will now read the manifest and make API calls to fetch the Physical IDs of the lambdas
  4. The CLI will start polling the logs of each Lambda
  5. The CLI will now start esbuild in watch mode for each Lambda
  6. When the code successfully rebuilds it Zips and uploads the output to Lambda directly by calling updateFunctionCode

How is this different from Serverless-Stack?

SST runs your Lambda functions locally by mocking the deployed lambda with your local environment. It comes with a number of performance benefits due to the fact that it doesn't need to upload your function code every time it change. That said, cdk-watch does not come with a slow feedback loop, you can expect 1s-5s from the moment you save a file to the moment its available on AWS. The upside of cdk-watch is we deploy no extra infrastructure to facilitate the development environment so there is less discrepancy between prod and pre-prod environments.


Command Reference

For more info on each command add a --help flag after any one of them.

  • Watch: cdkw "**" watch your Lambda where the CDK construct path matches the glob
  • List: cdkw ls "**" lists all your available WatchableNodejsFunction Constructs, a convenience command to help you get find paths you need
  • Once: cdkw once "**" updates your lambda's once, rather than in watch
  • Logs: cdkw logs "**" just tails logs for each of the matching lambdas

✈️ / planes.studio