AWS Elastic Container Service (AWS ECS)

Today we talk about what is elastic container service by AWS – ECS. This is going to be a short, crisp but a complete understanding from end-to-end.

What is Elastic?

Elastic refers to the ability of flexibility of the service to easily auto-scale or adjust according to requirements of the user and load of the applications.

What is Container?

Imagine you write an web application which has html, css, javascript files. You also have a .png file which is the logo of your application. You also implement your own custom logs and define it in a log.xml file in the IDE. When you build your code, the IDE will recognise all the code pieces and files required to build and run your application. If your logo file is missing, you might get an error with build failing. So an application is not just one file but a collection of code files, configuration files and system files that are required to be compiled and built together and then run it. Now if you take all these files and run it in a box, that box is called a container. As the name suggests a container is an environment where the application code along with all of its related files (like libraries, system files, config files etc.) runs.

But how does the container know what files to run ? This is described by a container image.

What is Container Image (or simply image)?

A container image is a template file or a blue print required to define a container or what the container will run. It is just an arti-fact that defines an instruction of how to run a particular application in a container. Let’s take an example of Kafka. Kafka is an open-source message streaming application. In order to run this application locally, you need all the files related to Kafka that will run it successfully. You cannot download each file required for Kafka to run but rather you simply pull the container image of Kafka and launch a container using that image. Note that, there can be multiple sub-images to form a singular image file for an application. Images are typically built from a Dockerfile. A Dockerfile is a plaintext file that contains the instructions for building a container.

Image vs Container

What is AWS ECS?

It is a fully managed container orchestration service (or management service) that helps you easily deploy, manage, and scale containerised applications. It controls, manages, provisions and monitors your application that is running on containers.

Read more: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/Welcome.html

What is ECS Task Definition?

An ECS task definition is a simple JSON file that defines your application and the parameters like what operating system be running on the container, number of containers for your application etc. This file is like telling ECS that – “Hey, I have an image of my application and I want to give you the task to run the image in a container based on these rules.” See the example of a task definition below

{
  "family": "multi-container-task",
  "containerDefinitions": [ // you can define multiple container images in one task definition.
    {
      "name": "frontend",
      "image": "123456789012.dkr.ecr.us-west-2.amazonaws.com/my-app:latest",
      "cpu": 256,
      "memory": 512,
      "portMappings": [
        {
          "containerPort": 80,
          "hostPort": 80
        }
      ]
    },
    {
      "name": "backend",
      "image": "my-backend-image:latest",
      "cpu": 256,
      "memory": 512,
      "portMappings": [
        {
          "containerPort": 8080,
          "hostPort": 8080
        }
      ]
    }
  ] // Here 2 container images are defined - frontend and backend. Now ECS will spin up 2 
    // containers to run each individually.
}

What is ECS Task?

ECS Task is the actual instantiation of the task definition. ECS reads your task definition, creates the defined containers to run the tasks and starts running the containers. ECS Scheduler executes the following things in order –

1. Reads the task definition. Notices the number of images mentioned in the task definitions.
2. Pulls the images from the AWS ECR repository (registry) where the images are actually stored.
3. Spins new EC2 containers using the images.

What is ECS Cluster?

An ECS Cluster is a collection of tasks. Each cluster can have a number of tasks running inside it. It is just a logical grouping of the tasks.

See the detailed diagram below –

Read more about AWS – https://techshshila.in/category/aws/