javascript-terraform

LocalStack Demo: Hot code swapping for Lambda functions using LocalStack’s code mounting in JavaScript

Prerequisites

• LocalStack
• Docker
• awslocal CLI
• Terraform
• tflocal CLI

Introduction to the sample

Other than the deployment of the sample, it is practically identical to our javascript hot reloading sample.

We will use terraform to deploy a hot reloaded lambda function, and then interact with it by invoking it and changing its source.

The source code of the created lambda function hotreloadlambda is located in the subfolder lambda_src.

Starting up

First, we need to make sure we start LocalStack with the right configuration. To use our new lambda provider, all you need to do is set PROVIDER_OVERRIDE_LAMBDA=v2, if you use a LocalStack version < 2.0.

If you want to use our old provider, please set LAMBDA_REMOTE_DOCKER to 0 (see the Configuration Documentation for more information):

LAMBDA_REMOTE_DOCKER=0 localstack start

Accordingly, if you are launching LocalStack via Docker or Docker Compose:

#docker-compose.yml

services:
  localstack:
    ...
    environment:
      ...
      - LAMBDA_REMOTE_DOCKER=0

Deploying

Now we can deploy our terraform stack by using tflocal. First, we initialize the terraform working directory using:

tflocal init

We can now check the plan of terraform for our deployment:

tflocal plan

Afterwards, we can deploy our stack on LocalStack:

tflocal apply

The terraform configuration will automatically deploy the lambda with hot reloading for the function code. The function code will be the contents of the lambda_src subdirectory.

Invoking the Lambda function

We can quickly make sure that it works by invoking it with a simple payload:

awslocal lambda invoke --function-name hotreloadlambda output.txt

The invocation itself returns:

{
	"Difference": 10,
	"Number1": 21,
	"Number2": 31,
	"Product": 651,
	"Quotient": 0.6774193548387096,
	"Sum": 52
}

Changing things up

Now, that we got everything up and running, the fun begins. Because the function code directory, in our case ./lambda_src is mounted into the executing container, any change that we save in this folder will affect the execution almost instantly.

For example, we can now make a minor change to the API and replace the number1 and number2 with new values, let's say 10 and 20. Without redeploying or updating the function, the result of the previous request will look like this:

{
	"Difference": 10,
	"Number1": 10,
	"Number2": 20,
	"Product": 200,
	"Quotient": 0.5,
	"Sum": 30
}