# Elastic Container Service (ECS)

## Essentials

* Container management service that supports Docker
* It allows you to easily create and manage a fleet of Docker containers on a cluster of EC2 instances.
  * Higher Instance Utilization
  * Consistency between all environments
  * Microservices
  * Continuous Deployment - automate builds
  * Load Balancing (ELB)
  * Autoscaling

### Components

* Docker Image
* Container Registry (ECR)
* Task Definitions
  * Docker Images
  * CPU, Memory
  * Launch Type - EC2 or Fargate (managed, serverless)
* ECS Cluster
  * EC2 Container Instances
    * [ECS Agents](/cloud-computing/aws/compute-services/elastic-container-service-ecs.md#ecs-agent)
      * Run [Tasks](/cloud-computing/aws/compute-services/elastic-container-service-ecs.md#ecs-tasks) from ECR

### ECS Agent

* Runs on each EC2 instance in the ECS cluster
* It communicates information about the instances to ECS, including:
  * Running tasks
  * Resource utilization
* The ECS agent is also responsible for starting/stopping tasks (when told to by ECS)

### ECS Tasks

* Actual representation of a Task Definition on an EC2 instance inside of your container cluster.
* ECS agent will start/stop these tasks based on instruction/schedule

#### Task Definition

* A JSON formatted text file that contains the "blueprint" for your application, including:
  * Which container/docker image to use
  * The repository (container registry) the image is located in
  * Which ports should be open on the container instance
  * What data volumes should be used with the container

#### Container/Docker Image

* Built from a Dockerfile
  * A plain text file (script) that specifies all of the components that are included in the container.
  * Instructions for what will be placed inside a given container.
* Contains all the downloaded software, code, runtime, system tools, and libraries (as outlined in the Dockerfile)
  * i.e. If the Dockerfile specifies PHP to be downloaded and installed, then the container/Docker Image will have PHP downloaded and installed.

### Elastic Container Service for Kubernetes (EKS)

* Manages availability and scalability of Kubernetes control plane nodes
  * Starts and stops containers
  * Schedule containers
  * Replaces unhealthy nodes


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