maximumPercent

If a service is using the rolling update (ECS) deployment type, the maximumPercent 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 desiredCount (rounded down to the nearest integer). This parameter enables you to define the deployment batch size. For example, if your service is using the REPLICA service scheduler and has a desiredCount of four tasks and a maximumPercent 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 maximumPercent value for a service using the REPLICA service scheduler is 200%.

The Amazon ECS scheduler uses this parameter to replace unhealthy tasks by starting replacement tasks first and then stopping the unhealthy tasks, as long as cluster resources for starting replacement tasks are available. For more information about how the scheduler replaces unhealthy tasks, see Amazon ECS services.

If a service is using either the blue/green (CODE_DEPLOY) or EXTERNAL deployment types, and tasks in the service use the EC2 launch type, the maximum percent value is set to the default value. The maximum percent value is used to define the upper limit on the number of the tasks in the service that remain in the RUNNING state while the container instances are in the DRAINING state.

You can't specify a custom maximumPercent value for a service that uses either the blue/green (CODE_DEPLOY) or EXTERNAL deployment types and has tasks that use the EC2 launch type.

If the service uses either the blue/green (CODE_DEPLOY) or EXTERNAL deployment types, and the tasks in the service use the Fargate launch type, the maximum percent value is not used. The value is still returned when describing your service.