What is the minimum healthy percent and maximum percent in Amazon ECS

Question

I already have the experience in Docker and EC2. But I'm new to ECS. Can someone help me to understand what these two parameters actually does, their difference and usage.

Official Docs says:

The minimum healthy percent represents a lower limit on the number of your service's tasks that must remain in the RUNNING state during a deployment, as a percentage of the desired number of tasks (rounded up to the nearest integer). This parameter enables you to deploy without using additional cluster capacity. For example, if your service has a desired number of four tasks and a minimum healthy percent of 50%, the scheduler may stop two existing tasks to free up cluster capacity before starting two new tasks. Tasks for services that do not use a load balancer are considered healthy if they are in the RUNNING state; tasks for services that do use a load balancer are considered healthy if they are in the RUNNING state and the container instance it is hosted on is reported as healthy by the load balancer. The default value for minimum healthy percent is 50% in the console and 100% for the AWS CLI, the AWS SDKs, and the APIs.

The maximum percent parameter represents an upper limit on the number of your service's tasks that are allowed in the RUNNING or PENDING state during a deployment, as a percentage of the desired number of tasks (rounded down to the nearest integer). This parameter enables you to define the deployment batch size. For example, if your service has a desired number of four tasks and a maximum percent value of 200%, the scheduler may start four new tasks before stopping the four older tasks (provided that the cluster resources required to do this are available). The default value for maximum percent is 200%.

I still didn't get a clear idea about these two parameters.

1. With minimum healthy percent, if my service has a desired number of 4 tasks and a minimum healthy percent of 25%, how many existing/new tasks will the scheduler start/stop?
2. With maximum percent, if my service has a desired number of 4 tasks and a maximum percent value of 50%, how many existing/new tasks will the scheduler start/stop?
3. If my service has only one task running, how to set these parameters to get existing task stopped and new task running.

Answer 1

1. With desired tasks at 4 and minimum health at 25% then during deployment ECS is allowed to stop all tasks except 1 before it starts new tasks
2. Maximum should be 100+ so that configuration does not make sense. But if you have 4 desired tasks and maximum at 150% then ECS is allowed to start 2 new tasks before it stops other tasks.
3. • If you want to make sure there is never more than 1 task running during redeployment then you need to set desired to 1, minimum to 0% and maximum to 100%.
   • If you want to make sure there is always at least 1 task running you need to set desired to 1, minimum to 100% and maximum to 200%.

Other example. If you have desired at 4, minimum at 50% and maximum of 150%. Then ECS can decide what it will do during deployment.

• If the cluster does not have more resources it can decide to stop 2 tasks, start 2 new tasks in the new version, wait for the new task to be healthy. Then stop the two remaining tasks and start two new ones.
• If the cluster has more resources then ECS can decide to start two new tasks before stopping of existing tasks.

Or you could look at it this way. During redeployment ECS needs to run between MinimumPercent/100*desired and MaximumPercent/100*desired tasks. In this case between 2-6 tasks.