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Container instance draining
There are times when you might need to remove a container instance from a cluster; for example, to perform system updates, update the Docker daemon, or scale down the cluster size. Container instance draining enables you to remove a container instance from a cluster without impacting tasks in your cluster.
When you set a container instance to DRAINING, Amazon ECS prevents new tasks
from being scheduled for placement on the container instance. Service tasks on the
draining container instance that are in the PENDING state are stopped
immediately. If there are container instances in the cluster that are available,
replacement service tasks are started on them.
Service tasks on the container instance that are in the RUNNING state are
stopped and replaced according to the service's deployment configuration parameters,
minimumHealthyPercent and maximumPercent.
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If
minimumHealthyPercentis below 100%, the scheduler can ignoredesiredCounttemporarily during task replacement. For example,desiredCountis four tasks, a minimum of 50% allows the scheduler to stop two existing tasks before starting two new tasks. If the minimum is 100%, the service scheduler can't remove existing tasks until the replacement tasks are considered healthy. If tasks for services that do not use a load balancer are in theRUNNINGstate, they are considered healthy. Tasks for services that use a load balancer are considered healthy if they are in theRUNNINGstate and the container instance they are hosted on is reported as healthy by the load balancer. -
The
maximumPercentparameter represents an upper limit on the number of running tasks during task replacement, which enables you to define the replacement batch size. For example, ifdesiredCountof four tasks, a maximum of 200% starts four new tasks before stopping the four tasks to be drained (provided that the cluster resources required to do this are available). If the maximum is 100%, then replacement tasks can't start until the draining tasks have stopped.
For more information, see Service definition parameters.
Any PENDING or RUNNING tasks that do not belong to a service
are unaffected; you must wait for them to finish or stop them manually.
A container instance has completed draining when there are no more
RUNNING tasks (although the state remains as DRAINING).
You can verify this using the ListTasks operation with the containerInstance
parameter.
When you change the status of a container instance from DRAINING to
ACTIVE, the Amazon ECS scheduler can schedule tasks on the instance
again.
Draining instances
You can use the UpdateContainerInstancesState API action or the update-container-instances-state command to change the status of a
container instance to DRAINING.
The following procedure demonstrates how to set your instance to
DRAINING using the AWS Management Console.
To set your instance to DRAINING using the console
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Open the Amazon ECS console at https://console.aws.amazon.com/ecs/
. -
In the navigation pane, choose Clusters and select the cluster.
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Choose ECS Instances and select the check box for the container instances.
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Choose Actions, Drain instances.
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After the instances are processed, choose Done.
