Fargate Spot Tasks Not Restarting After Interruption: Is Auto‑Rescheduling Enabled?

Answer to the Core Question

When a task that is part of an ECS service running on Fargate Spot receives a 2‑minute interruption notice, the ECS service scheduler automatically stops the task and launches a replacement to keep the service’s desired count unchanged. No manual action is required.

Why It Happens

• The scheduler monitors the STOPPED state of tasks. When the STOPPED reason is Spot interruption, it triggers a new RUNNING task that uses the same task definition.
• EventBridge (formerly CloudWatch Events) emits an ECS Task State Change event for the original task (reason: Spot interruption) followed by a state change for the new task.

When It Does Not Happen

• Standalone tasks (not part of a service) are not automatically replaced.
• If the service’s desiredCount is zero or the service is disabled, the scheduler will not launch a replacement.
• Non‑Spot launch types do not trigger interruption handling.

Practical Verification Steps

1. Create an ECS service:

   aws ecs create-service \
     --cluster my-cluster \
     --service-name spot-demo \
     --task-definition my-task:1 \
     --desired-count 1 \
     --launch-type FARGATE \
     --capacity-provider-strategy "{"capacityProvider":"FARGATE_SPOT","weight":1}"
   

2. Enable the Spot interruption notice in the task definition (set SpotInterruptionNoticeEnabled to true).
3. In EventBridge, add a rule for ECS Task State Change events and route them to a CloudWatch Log group.
4. Trigger an interruption (manually via the console or wait for AWS to do it). Observe the following log sequence:

   {"detail":{"lastStatus":"STOPPED","stopCode":"SpotInterruption"}}
   {... new task RUNNING ...}
   

5. Confirm in the ECS console that the old task has been removed and a new one is running.

IAM & Permissions

• Ensure the ECS service role (or the task role if you use aws:ecs:RunTask) has ecs:RunTask and ecs:CreateTask permissions.
• Typical minimal policy:

  {
    "Version": "2012-10-17",
    "Statement": [
      {"Effect": "Allow", "Action": ["ecs:RunTask", "ecs:CreateTask"], "Resource": "*"}
    ]
  }
  

• EventBridge rules do not require special permissions beyond the default service role.

Missing Diagnostic Detail

If you are observing that the task is not restarting, confirm that the task was launched as part of a service and that the service’s desiredCount is greater than zero. If either condition is not met, the scheduler will not auto‑reschedule.