ECS Terminology

ECS has quite a convoluted conceptual model with a lot of terminology. Since it's based on Docker, it extends that vocabulary of images and containers with the following:

• A container instance is an EC2 instance on which ECS will run Docker containers. It's just a plain EC2 instance with the ECS agent installed (included in the ECS AMI we configured earlier).
• A cluster is a collection of container instances abstracted into a pool of resources (mostly CPU and RAM).
• A task is one or more containers that ECS deploys onto a single container instance. Tasks represent copies of your code running somewhere on an EC2 instance.
• A task definition is like a template for tasks. It groups one or more container definitions, which specify configurations for Docker containers. When a task is created from a task definition, a container is created from each container definition.

• A service is used to manage long running tasks (like databases and web servers). It creates and maintains a given number of tasks, restarting them if they fail.

  You can also run "once off" tasks (i.e. non-restarting) without a service. This is useful for some use cases, such as batch jobs. However most tasks will be run by services, so we'll focus on those.

It's a lot to get your head around, so we'll walk through them in the following sections.