Container Instances

With the EC2 launch type you run a fleet of EC2 hosts that ECS schedules tasks onto.

A container instance is an EC2 instance that has joined an ECS cluster. ECS packs tasks onto these instances based on their available CPU, memory, and ports.

What you manage

The EC2 instance type, count, and AMI (use the ECS-optimized AMI).
Patching, scaling, and the underlying Docker daemon.

In return you get full control of the host and can achieve high task density at lower cost.

Example

# List the EC2 hosts registered to a cluster
aws ecs list-container-instances --cluster prod

# Describe their remaining CPU/memory
aws ecs describe-container-instances --cluster prod --container-instances <id>

When to use it

An ML team registers P3 GPU EC2 instances as ECS container instances so their GPU-accelerated inference containers can run on the EC2 launch type.
A cost-optimised team buys Reserved EC2 instances, registers them as container instances, and lets ECS pack multiple tasks onto each host.
An ops engineer lists all container instances in a cluster to find one that is failing health checks before draining and replacing it.

More examples

Launch EC2 Instance into ECS Cluster

Launches a t3.medium EC2 instance with the ECS agent pre-installed (ECS-optimised AMI) and points it at the target cluster via user data.

aws ec2 run-instances \
  --image-id ami-0c55b159cbfafe1f0 \
  --instance-type t3.medium \
  --iam-instance-profile Name=ecsInstanceRole \
  --user-data '#!/bin/bash
echo ECS_CLUSTER=my-ec2-cluster >> /etc/ecs/ecs.config' \
  --count 1

List Container Instances in a Cluster

Lists the ARNs of all active container instances registered in the cluster, useful for auditing fleet size.

aws ecs list-container-instances \
  --cluster my-ec2-cluster \
  --status ACTIVE \
  --output table

Describe Container Instance Resources

Shows the remaining CPU units and memory on a specific container instance to check available capacity for new tasks.

aws ecs describe-container-instances \
  --cluster my-ec2-cluster \
  --container-instances arn:aws:ecs:us-east-1:123456789012:container-instance/abc123 \
  --query 'containerInstances[0].{ec2Id:ec2InstanceId,cpu:remainingResources[?name==`CPU`].integerValue|[0],memory:remainingResources[?name==`MEMORY`].integerValue|[0]}'

Discussion

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