How Amazon ECS places tasks on container instances

You can use task placement to configure Amazon ECS to place your tasks on container instances that meet certain criteria, for example an Availability Zone or instance type.

The following are task placement components:

• Task placement strategy - The algorithm for selecting container instances for task placement or tasks for termination. For example, Amazon ECS can select container instances at random, or it can select container instances such that tasks are distributed evenly across a group of instances.

• Task group - A group of related tasks, for example database tasks.

• Task placement constraint - These are rules that must be met in order to place a task on a container instance. If the constraint is not met, the task is not placed and remains in the PENDING state. For example, you can use a constraint to place tasks only on a particular instance type.

Amazon ECS has different algorithms for the launch types.

EC2 launch type

For tasks that use the EC2 launch type, Amazon ECS must determine where to place the task based on the requirements specified in the task definition, such as CPU and memory. Similarly, when you scale down the task count, Amazon ECS must determine which tasks to terminate. You can apply task placement strategies and constraints to customize how Amazon ECS places and terminates tasks.

The default task placement strategies depend on whether you run tasks manually (standalone tasks) or within a service. For tasks running as part of an Amazon ECS service, the task placement strategy is spread using the attribute:ecs.availability-zone. There isn't a default task placement constraint for tasks not in services. For more information, see Schedule your containers on Amazon ECS.

Note

Task placement strategies are a best effort. Amazon ECS still attempts to place tasks even when the most optimal placement option is unavailable. However, task placement constraints are binding, and they can prevent task placement.

You can use task placement strategies and constraints together. For example, you can use a task placement strategy and a task placement constraint to distribute tasks across Availability Zones and bin pack tasks based on memory within each Availability Zone, but only for G2 instances.

When Amazon ECS places tasks, it uses the following process to select container instances:

1. Identify the container instances that satisfy the CPU, GPU, memory, and port requirements in the task definition.

2. Identify the container instances that satisfy the task placement constraints.

3. Identify the container instances that satisfy the task placement strategies.

4. Select the container instances for task placement.

Fargate launch type

Task placement strategies and constraints aren't supported for tasks using the Fargate launch type. Fargate will try its best to spread tasks across accessible Availability Zones. If the capacity provider includes both Fargate and Fargate Spot, the spread behavior is independent for each capacity provider.