Hot Reloading your Typescript Lambda with LocalStack & Webpack

This is a simple example of how to hot reload your Typescript Lambda with Webpack with the help of LocalStack's Hot Reloading feature.

Pre-requisites

• LocalStack
• Docker
• Node.js
• AWS CLI
• awslocal
• yarn
• jq

Start LocalStack

Start your LocalStack Docker container with the following command:

localstack start

Install dependencies

Install the dependencies with the following command:

yarn install

Build the Lambda

Build the Lambda with the following command:

npm run build

Note that the build script is using Nodemon to watch for changes in the src directory and rebuild the Lambda. This would be useful when you are developing & testing your Lambda, while making changes to the code in real-time.

With every change, you will see the following output:

yarn run build
yarn run v1.22.19
$ yarn run clean && yarn run webpack --watch
$ rimraf dist/*
$ cross-env TS_NODE_PROJECT="tsconfig.webpack.json" webpack --mode production --watch
asset api.js 1.4 KiB [emitted] [minimized] (name: api)
./src/api.ts 1.42 KiB [built] [code generated]
./src/util.ts 314 bytes [built] [code generated]
webpack 5.75.0 compiled successfully in 602 ms
[nodemon] 3.0.1
[nodemon] to restart at any time, enter `rs`
[nodemon] watching path(s): dist/api.js
[nodemon] watching extensions: js,mjs,cjs,json
[nodemon] starting `node dist/api.js`
[nodemon] clean exit - waiting for changes before restart
asset api.js 1.4 KiB [emitted] [minimized] (name: api)
./src/api.ts 1.42 KiB [built] [code generated]
./src/util.ts 314 bytes [built] [code generated]
webpack 5.75.0 compiled successfully in 455 ms
[nodemon] restarting due to changes...
[nodemon] starting `node dist/api.js`

Deploy the Lambda

Deploy the Lambda with the following command:

awslocal lambda create-function \
    --function-name localstack-example \
    --runtime nodejs18.x \
    --role arn:aws:iam::000000000000:role/lambda-ex \
    --code S3Bucket="hot-reload",S3Key="$(PWD)/dist" \
    --handler api.default

Additionally, you can create a Lambda Function URL with the following command:

function_url=$(awslocal lambda create-function-url-config --function-name localstack-example --auth-type NONE | jq -r '.FunctionUrl')

Test the Lambda

You can test the Lambda by sending POST requests to the Lambda Function URL:

curl -X POST \
  -H "Content-Type: application/json" \
  -d '{"name": "John", "age": 30}' \
  "$function_url"

The following output would be displayed:

{"payload":{"name":"John","age":30}}                                                                         

You can additionally test the Lambda with the following command:

curl -X GET "$function_url"

This will return the following output:

{"error":"Only JSON payload is accepted"}

Hot Reload the Lambda

Go to the src directory and make changes to the api.ts file. For example, change the following line:

return errorResponse("Only JSON payloads are accepted", 406);

Make the errorResponse function return "Only JSON payload is accepted" instead of "Only JSON payloads are accepted". Save the file and run the last curl command again:

curl -X GET "$function_url"

This will return the following output:

{"error":"Only JSON payload is accepted"}

You can perform further changes to the api.ts file and test the Lambda in real-time.

How does it work?

The Lambda Hot Reloading feature in LocalStack allows you to hot reload your Lambda code in real-time. In this sample, this is achieved using the following:

• The build script in the package.json file uses Nodemon to watch for changes in the src directory and rebuild the Lambda. This is enabled using the nodemon-webpack-plugin plugin, which has been pre-configured in the webpack.config.js file.
• The S3Bucket and S3Key parameters in the awslocal lambda create-function command are used to deploy the Lambda code from the dist directory. This is done by specifying the dist directory as the S3Key parameter. Everytime the Lambda is updated, Nodemon triggers another build and the dist directory is updated with the latest code changes. LocalStack then automatically updates the Lambda code with the latest changes from the dist directory.

Notes

This sample application is inherited from a public repository.