部署期间，所需任务计入ECS服务

Question

Let's say, I have a ECS service running. It is configured to run at minimum 2 tasks, desired count is 2 and maximum tasks count is 10. Minimum healthy percent is set at 100, max percent set to 150.

The ECS service is managed using CloudFormation template.

If I update my ECS service CloudFormation template by changing the docker image tag I want to use and apply those changes to the stack - a new deployment is started.

Assume, that before the deployment, the application was under a high load and tasks count was increased to the maximum of 10 running tasks by auto-scaling.

What I expect from the ECS service deployment in such conditions is the following:

1. 10 old tasks are running.
2. 5 new tasks are spawned.
3. 5 old tasks are set to draining state.
4. 5 old tasks are stopped.
5. 5 new tasks are spawned.
6. Remaining 5 old tasks are set to draining state.
7. 5 old tasks stopped.

What I get is:

1. 8 old tasks are set to draining state.
2. Desired count of new deployment set to 10 by auto-scaling.
3. 5 new tasks are spawned.
4. 8 old tasks are stopped.
5. 5 new tasks are spawned.
6. 2 old tasks set to draining state.
7. 2 old tasks stopped.

The issue is, that ECS does not take into the account the number of old tasks running before the deployment. It tries to deploy with the desired count of 2, which is set originally in the CloudFormation template. This leads to unexpected termination of 8 tasks before the auto-scaling kicks in.

Is there a way to achieve something like the "expected" scenario during the new version deployment if the service is updated over CloudFormation? I know, I could manually change the desired count in the template before the deployment to the number of tasks, that is currently running. Maybe there is a better way?

Answer 1

If your desired count is 10 at a particular moment and you set minimum health percentage = 150 and the maximum percentage is 200. Then It will spawn 5 more task and initiate the draining of 5 old tasks.

Also, you don't have to change the desired count and take care of the task which is in draining state. It will be drained once the connection will be closed.

Answer 2

It appears, that it is possible to not provide the desired count . Then, then the new deployment uses the old desired count.