Running a new task using Terraform without a service in AWS ECS

In Amazon Elastic Container Service (ECS), tasks are the fundamental unit of work that can be executed on a cluster. While services in ECS are used for long-running tasks, there are scenarios where you might need to run a task without associating it with a service. In this blog post, we’ll explore how to use Terraform to define and run a standalone ECS task without a service.

First thing first

Before proceeding, ensure you have the following:

• An AWS account with permissions to create ECS resources.
• Terraform installed on your local machine.
• Basic knowledge of ECS task definitions and Terraform.

Defining the ECS Task

To define an ECS task without a service, we’ll first create a task definition. This task definition will specify the container image, resource requirements, and any other configuration needed for the task.

resource "aws_ecs_task_definition" "task_definition" {
  family                   = "my-task"
  network_mode             = "awsvpc"
  cpu                      = 256
  memory                   = 512
  requires_compatibilities = ["FARGATE"]

  container_definitions = jsonencode([
    {
      name      = "my-container"
      image     = "nginx:latest"
      cpu       = 256
      memory    = 512
      essential = true
    }
  ])
}

In this example, we define a task named “my-task” using the nginx container image running on AWS Fargate.

Running the ECS Task

To run the ECS task, we’ll use a null resource with a local exec provisioner in Terraform. This provisioner will execute the aws ecs run-task command to start the task.

resource "null_resource" "migration_task_run" {
  depends_on = [
    aws_ecs_task_definition.task_definition    // to make sure that you run task only after creating the task definition
  ]

  provisioner "local-exec" {
    command = <<EOF
    aws ecs run-task \
      --cluster <<cluster_name>> \
      --task-definition <<task_definition_name>> \
      --count 1 --launch-type FARGATE \
      --network-configuration '{     // This is required if you have chosen awsvpc in network config for your task definition. Else, this can be ignored
      "awsvpcConfiguration": {
      "assignPublicIp":"DISABLED",
      "securityGroups": ["<<security_group>>"],
      "subnets": ["<<your subnets>>"]
      }
      }'
EOF
  }
}

Use cases:

Running a standalone ECS task without a service using Terraform provides a flexible way to execute short-lived tasks, such as batch processing or one-time operations. By defining the task with Terraform and using the aws ecs run-task command, you can easily manage and execute ECS tasks as part of your infrastructure automation workflows.

1. Automated Deployment Pipeline: Incorporate the execution of ECS tasks into your CI/CD pipeline. For example, after building and pushing a Docker image, use Terraform to update the ECS task definition and then automatically run the updated task.
2. Scheduled Tasks: Schedule tasks to run at specific times or intervals using a cron job or another scheduling mechanism. Terraform can be used to define and manage these scheduled tasks, ensuring they are executed as intended.
3. One-off Tasks: Run ad-hoc tasks that are not part of your regular ECS services. These tasks might be used for maintenance, data processing, or other temporary operations.
4. Data base migration: To run DB migration jobs as part of your CI/CD pipeline.
5. Environment Initialization: Use Terraform to initialize your ECS environment by running tasks that perform setup operations, such as database migrations, configuration loading, or resource provisioning.