[AWS CDK] CDK WATCH

Faster personal deployments

This section is not necessary to complete the workshop, but we recommend that you take the time to see how cdk deploy --hotswap and cdk watch can speed up your personal deployments.

It’s great that we have a working lambda! But what if we want to tweak the lambda code to get it just right? Let’s say that we have now decided that we want our lambda function to respond with "Good Morning, CDK!" instead of "Hello, CDK".

So far, it seems like the only tool we have at our disposal to update our stack is cdk deploy. But cdk deploy takes time; it has to deploy your CloudFormation stack and upload the lambda directory from your disk to the boostrap bucket. If we’re just changing our lambda code, we don’t actually need to update the CloudFormation stack, so that part of cdk deploy is wasted effort.

We really only need to update our lambda code. It would be great if we had some other mechanism for doing only that…

Timing cdk deploy

First, let’s time how long it takes to run cdk deploy. It will help us baseline how long a full CloudFormation deployment takes. To do this, we are going to change the code inside lambda/hello.js:

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

Then, we can run cdk deploy:

cdk deploy

The output will look something like this:

✨  Synthesis time: 6s

CdkWorkshopStack: deploying...
CdkWorkshopStack: creating CloudFormation changeset...



 ✅  CdkWorkshopStack

✨  Deployment time: 66.82s

Stack ARN:
arn:aws:cloudformation:REGION:ACCOUNT-ID:stack/CdkWorkshopStack/STACK-ID

✨  Total time: 72.82s

The exact time will vary but we should get a pretty good idea of how long a normal deployment takes!

Hotswap deployments

This command deliberately introduces drift in CloudFormation stacks in order to speed up deployments. For this reason, only use it for development purposes. Never use hotswap for your production deployments!

We can speed up that deployment time with cdk deploy --hotswap, which will assess whether a hotswap deployment can be performed instead of a CloudFormation deployment. If possible, the CDK CLI will use AWS service APIs to directly make the changes; otherwise it will fall back to performing a full CloudFormation deployment.

Here, we will use cdk deploy --hotswap to deploy a hotswappable change to your AWS Lambda asset code.

Timing cdk deploy --hotswap

Let’s change the lambda code in lambda/hello.js another time:

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

Now, let’s run cdk deploy --hotswap:

cdk deploy --hotswap

The output will look something like this:

✨  Synthesis time: 6.44s

⚠️ The --hotswap flag deliberately introduces CloudFormation drift to speed up deployments
⚠️ It should only be used for development - never use it for your production Stacks!

CdkWorkshopStack: deploying...
✨ hotswapping resources:
   ✨ Lambda Function 'CdkWorkshopStack-HelloHandler2E4FBA4D-tEZTcXqG8YYe'
✨ Lambda Function 'CdkWorkshopStack-HelloHandler2E4FBA4D-tEZTcXqG8YYe' hotswapped!

 ✅  CdkWorkshopStack

✨  Deployment time: 3.07s

Stack ARN:
arn:aws:cloudformation:REGION:ACCOUNT-ID:stack/CdkWorkshopStack/STACK-ID

✨  Total time: 9.51s

Wow, deploying a hotswapped change took 3 seconds, while a full deployment took 67 seconds! But take a look and read the warning message thoroughly - it’s important!

⚠️ The --hotswap flag deliberately introduces CloudFormation drift to speed up deployments
⚠️ It should only be used for development - never use it for your production Stacks!

Did the code actually change?

Wow that was fast. Did the code actually change? Let’s go to the AWS Lambda Console and double check!

1. Open the AWS Lambda Console (make sure you are in the correct region).

   

2. You should see our function:
3. Click on the function name to go to the console.
4. The code should be loaded onto the screen. Did your change show up?

5. 

CDK Watch

We can do better than calling cdk deploy or cdk deploy --hotswap each time. cdk watch is similar to cdk 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. By default, cdk watch will use the --hotswap flag, which inspects the changes and determines if those changes can be hotswapped. Calling cdk watch --no-hotswap will disable the hotswap behavior.

Once we set it up, we can use cdk watch to detect both hotswappable changes and changes that require full CloudFormation deployment.

Modify your cdk.json file

When the cdk watch command runs, the files that it observes are determined by the "watch" setting in the cdk.json file. It has two sub-keys, "include" and "exclude", each of which can be either a single string or an array of strings. Each entry is interpreted as a path relative to the location of the cdk.json file. Globs, both * and **, are allowed to be used.

Your cdk.json file should look similar to this:

{
  "app": "npx ts-node --prefer-ts-exts bin/cdk-workshop.ts",
  "watch": {
    "include": [
      "**"
    ],
    "exclude": [
      "README.md",
      "cdk*.json",
      "**/*.d.ts",
      "**/*.js",
      "tsconfig.json",
      "package*.json",
      "yarn.lock",
      "node_modules",
      "test"
    ]
  },
  "context": {
    // ...
  }
}

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

{
  "app": "npx ts-node --prefer-ts-exts bin/cdk-workshop.ts",
  "watch": {
    "include": [
      "**"
    ],
    "exclude": [
      "README.md",
      "cdk*.json",
      "**/*.d.ts",
      "tsconfig.json",
      "package*.json",
      "yarn.lock",
      "node_modules",
      "test"
    ]
  },
  "context": {
    // ...
  }
}

Now you’re all set to start watching!

Timing cdk watch

First, call cdk watch:

cdk watch

This will trigger an initial deployment and immediately begin observing the files we’ve specified in cdk.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, CDK! You've hit ${event.path}\n`
  };
};

Once you save the changes to your Lambda code file, cdk watch will recognize that your file has changed and trigger a new deployment. In this case, it will recognize that we can hotswap the lambda asset code, so it will bypass a CloudFormation deployment and deploy directly to the Lambda service instead.

How fast did deployment take?

Detected change to 'lambda/hello.js' (type: change). Triggering 'cdk deploy'

✨  Synthesis time: 5.57s

⚠️ The --hotswap flag deliberately introduces CloudFormation drift to speed up deployments
⚠️ It should only be used for development - never use it for your production Stacks!

CdkWorkshopStack: deploying...
✨ hotswapping resources:
   ✨ Lambda Function 'CdkWorkshopStack-HelloHandler2E4FBA4D-tEZTcXqG8YYe'
✨ Lambda Function 'CdkWorkshopStack-HelloHandler2E4FBA4D-tEZTcXqG8YYe' hotswapped!

 ✅  CdkWorkshopStack

✨  Deployment time: 2.54s

Stack ARN:
arn:aws:cloudformation:REGION:ACCOUNT-ID:stack/CdkWorkshopStack/STACK-ID

✨  Total time: 8.11s

Wrap Up

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

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