Understanding ECS: What Are Clusters, Services, and Tasks?

Introduction

Lately, I’ve been studying for the AWS Certified Developer – Associate (DVA) exam.
As part of that, I came across Amazon ECS (Elastic Container Service), and I found myself confused by the terms Cluster, Service, and Task.

They’re often used in quick explanations, but I realized I didn’t really understand what each one meant or why they’re structured that way.

So I decided to dig deeper—and this post is a summary of what I’ve learned.
It’s partly for my own notes, but I hope it helps someone else too.

What’s the Difference Between a Cluster, Service, and Task?

These three are commonly explained like this:

• Task: A unit that runs containers
• Service: A mechanism to manage and maintain tasks
• Cluster: The infrastructure that runs services

While that sounds simple, I still had some questions:

• Isn’t a container the same as a task? What’s the difference?
• Why do we need this layered structure of Task → Service → Cluster?

Let’s look into each concept in more detail.

What Is a Task?

A Task in ECS is a group of one or more containers that run together, based on a Task Definition.

The Task Definition is like a blueprint that includes everything ECS needs to run your containers.

It specifies:

• Container images
• Environment variables and command-line arguments
• CPU and memory allocation
• Port mappings
• Logging configuration
• Required IAM roles
• Network settings (especially important for Fargate)

So to summarize:
A Task = A running instance of a Task Definition (which can include one or more containers).

What Is a Service?

A Service is what keeps your tasks running as intended.

If you want to run, say, two copies of a web app at all times, the service will launch and monitor tasks to ensure that at least two are always running.

Services handle things like:

• Choosing the launch type (Fargate or EC2)
• Automatically restarting tasks if they stop
• Auto-scaling tasks based on traffic
• Integrating with load balancers (ALB/NLB)

You can run tasks without a service for one-off jobs like batch processing or temporary workloads—but for long-running applications, services are key.

What Is a Cluster?

A Cluster is a logical grouping of infrastructure that runs your tasks and services.

It can include:

• Fargate capacity (fully managed by AWS)
• EC2 instances (managed by you)

The cluster provides the execution environment, and ECS decides where your tasks run inside it.
You can mix Fargate and EC2 tasks in the same cluster.

You also have flexibility in how you organize your clusters:

• Separate clusters per application
• Separate clusters for dev/staging/prod
• A single cluster shared by multiple applications (multi-tenant)

What’s the Typical Setup Order?

When using the AWS Console to configure ECS, here’s the order you’d usually follow:

1. Create a Cluster
   → Define where your containers will run (Fargate or EC2)

2. Create a Task Definition
   → Define how your containers should behave

3. Create a Service
   → Decide how the task should be deployed and maintained within the cluster

Think of it as:
Environment → Container Definition → Deployment and Scaling

Fargate vs EC2: What’s the Difference?

ECS supports two launch types: Fargate and EC2.

Feature	Fargate	EC2
Infrastructure Management	Fully serverless	You manage EC2 instances
Flexibility	Limited customization	Highly customizable
Billing	Per-task billing (CPU/Memory/Time)	Instance-based billing (always running)

Use Cases

Fargate is great for:

• Small to medium applications
• Short-lived or variable workloads
• Teams that want minimal infrastructure management

EC2 is better suited for:

• Large-scale applications
• Long-running workloads with predictable usage
• Situations where deep customization is needed

Final Thoughts

At first, I couldn’t wrap my head around the relationship between clusters, services, and tasks. But once I understood the roles and creation order, it finally started to make sense.

That said, the best way to truly grasp ECS is to build something with it.
I’m planning to use ECS in my personal projects whenever I get the chance—it’s a great way to solidify what I’ve learned.