Why instances launch separately

If a task uses the distinctInstance placement constraint, ECS places each task on a different container instance. This means tasks won’t share the same underlying instance, even if an existing node has enough resources available.

You can define which container instances Amazon ECS uses for tasks. distinctInstance can be defined in any of these actions:

• CreateService
• UpdateService
• RunTask

How placement constraints work

Placement constraints let you control which container instances ECS can use for tasks.

When distinctInstance is applied, ECS ensures that no two tasks from the same group run on the same instance. This behavior is expected and continues until you remove or update the constraint in your service or task definition.