Optional - Watch

CDKTN Watch

We can do better than calling cdktn deploy each time. cdktn watch is similar to cdktn deploy except that instead of being a one-shot operation, it monitors your code and assets for changes and attempts to perform a deployment automatically when a change is detected.

Once we set it up, we can use cdktn watch --auto-approve to detect changes that require full Terraform apply.

Modify your cdktf.json file

When the cdktn watch command runs, the files that it observes are determined by the "watchPattern" setting in the cdktf.json file. the watchPattern expects an array of glob patterns, e.g. ["main.ts", "../constructs/**/*.ts", "lib/*.ts"]. See the configure files to watch section of configuration file overview.

Your cdktf.json file should look similar to this:

{
  "language": "typescript",
  "app": "npx ts-node main.ts",
  // ...
  "watchPattern": [
    "./**/*.ts",
  ]
}

As you can see, the sample app comes with a suggested "watchPattern" setting. We do in fact want to observe our .js files in the lambda folder, so let's add "lambda/*.js" to the list:

{
  "language": "typescript",
  "app": "npx ts-node main.ts",
  // ...
  "watchPattern": [
    "./**/*.ts",
    "lambda/*.js",
  ]
}

Now you're all set to start watching!

Running cdktn watch

First, call cdktn watch:

cdktn watch --auto-approve

This will trigger an initial deployment and immediately begin observing the files we've specified in cdktf.json.

Let's change our lambda asset code in lambda/hello.js one more time:

exports.handler = async function(event) {
  console.log("request:", JSON.stringify(event, undefined, 2));
  return {
    statusCode: 200,
    headers: { "Content-Type": "text/plain" },
    body: `Good Night, CDKTN! You've hit ${event.path}\n`
  };
};

Once you save the changes to your Lambda code file, cdktn watch will recognize that your file has changed and trigger a new deployment.

Wrap Up

The rest of this tutorial will continue using cdktn deploy instead of cdktn watch. But if you want to, you can simply keep cdktn watch on. If you need to make a full deployment, cdktn watch will call cdktn deploy for you.

For a deeper dive on cdktn watch use cases, read Increasing Development Speed with CDKTN Watch.