🔗 Creating AWS Resources with Localstack and Docker Compose

To test AWS resources locally, you can simulate then in Docker with the localstack/localstack image.

When spawning a server (which I call my "app" from now on), like a java/kotlin backend which polls AWS SQS and publishes to AWS SNS, you want the SQS queue and SNS topic to exist before java -jar executes.

🔗 Setting Up docker-compose.yml

You can create the AWS resources by creating another one-off Docker container with the AWS CLI. To make sure your app does not start before the AWS CLI has created the resources, we alter docker-compose.yml's depends_on to include a condition. The condition states that localstack must have started, and that our AWS CLI create-resources task exited successfully. Only then, will the java app start up.

Our app also reads properties to tweak its AWS Java SDK Clients, redirecting their AWS Endpoint to new URL.

Note that we use localstack (not localhost) inside docker-compose. The region is us-east-1, unless you tell localstack to use a different region. The AWS credentials must be set, but their value can be anything.

It looks like this:

version: "3.8"

services:
  # This uses my app.jar Java server
  app:
    image: my-java-service:latest
    ports:
      - "8080:8080" # http://localhost:8080/health
    links:
      - localstack:localstack
    environment:
      # Make sure your code reads the environment variables and tweaks its AWS SDK.
      # This process is not automatic.
      - aws.localstack.enabled=true
      - aws.region=us-east-1
      - aws.localstack.sqs.endpointOverride=http://localstack:4566
      - aws.localstack.sns.endpointOverride=http://localstack:4566
      - myQueue.sqs.url=http://localstack:4566/000000000000/my-cool-queue
      - myTopic.sns.arn=arn:aws:sns:us-east-1:000000000000:my-cool-topic
    depends_on:
      # The conditions are important!
      localstack:
        condition: service_healthy
      create-resources:
        condition: service_completed_successfully
  
  localstack:
    container_name: localstack_main
    image: localstack/localstack:1.4.0
    ports:
      - "127.0.0.1:4566:4566"            # LocalStack Gateway
      - "127.0.0.1:4510-4559:4510-4559"  # external services port range
    environment:
      - DOCKER_HOST=unix:///var/run/docker.sock
      - DEBUG=0
    volumes:
      - "./volume:/var/lib/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
  
  # Our one-off task with AWS-CLI
  create-resources:
    restart: "no"
    image: amazon/aws-cli:2.11.0
    depends_on:
      localstack:
        condition: service_healthy
    environment:
      - AWS_DEFAULT_REGION=us-east-1
      - AWS_ACCESS_KEY_ID=x
      - AWS_SECRET_ACCESS_KEY=x
    entrypoint: [ "sh", "-c" ]
    command: [ "aws --endpoint-url=http://localstack:4566 sns create-topic --name my-cool-topic &&
     aws --endpoint-url=http://localstack:4566 sqs create-queue --queue-name my-cool-queue" ]

The localstack service in docker-compose.yml was copied from https://github.com/localstack/localstack/blob/master/docker-compose.yml and slightly trimmed down.

Tip: You can do this with a database too:

app:
  depends_on:
    db:
      condition: service_started

🔗 Inside the Server

The kotlin code that creates the SQS client looks like this:

private fun createSqsClient(config: Config): SqsClient =
    SqsClient.builder()
        .region(config.awsConfig.sqsRegion)
        .apply {
          if (config.awsConfig.awsUseLocalstack) {
            this.endpointOverride(URI(config.awsConfig.sqsEndpointOverride!!))
            this.credentialsProvider(
                StaticCredentialsProvider.create(
                    AwsBasicCredentials.create("x", "x"),
                ),
            )
          }
        }
        .build()

and its config:

data class AwsConfig(
    val myQueueUrl: String,
    val myTopicArn: String,
    val awsUseLocalstack: Boolean,
    val snsEndpointOverride: String?,
    val sqsEndpointOverride: String?,
    val sqsRegion: Region = Region.US_EAST_1,
    val snsRegion: Region = Region.US_EAST_1,
)