Updating a service

You can update an existing service to change some of the service configuration parameters, such as the number of tasks that are maintained by a service, which task definition is used by the tasks, or if your tasks are using the Fargate launch type, you can change the platform version your service uses. A service using a Linux platform version can't be updated to use a Windows platform version and vice versa. If you have an application that needs more capacity, you can scale up your service. If you have unused capacity to scale down, you can reduce the number of desired tasks in your service and free up resources.

If you want to use an updated container image for your tasks, you can create a new task definition revision with that image and deploy it to your service by using the force new deployment option in the console.

The service scheduler uses the minimum healthy percent and maximum percent parameters (in the deployment configuration for the service) to determine the deployment strategy.

If a service is using the rolling update (ECS) deployment type, the minimum healthy percent represents a lower limit on the number of tasks in a service 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). The parameter also applies while any container instances are in the DRAINING state if the service contains tasks using the EC2 launch type. 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 they are reported as healthy by the load balancer. The default value for minimum healthy percent is 100%.

If a service is using the rolling update (ECS) deployment type, the maximum percent parameter represents an upper limit on the number of tasks in a service 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). The parameter also applies while any container instances are in the DRAINING state if the service contains tasks using the EC2 launch type. 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. That's provided that the cluster resources required to do this are available. The default value for the maximum percent is 200%.

If a service is using the blue/green (CODE_DEPLOY) deployment type and tasks that use the EC2 launch type, the minimum healthy percent and maximum percent values are set to the default values. They are only used to define the lower and 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. If the tasks in the service use the Fargate launch type, the minimum healthy percent and maximum percent values are not used. They are currently visible when describing your service.

When the service scheduler replaces a task during an update, the service first removes the task from the load balancer (if used) and waits for the connections to drain. Then, the equivalent of docker stop is issued to the containers running in the task. This results in a SIGTERM signal and a 30-second timeout, after which SIGKILL is sent and the containers are forcibly stopped. If the container handles the SIGTERM signal gracefully and exits within 30 seconds from receiving it, no SIGKILL signal is sent. The service scheduler starts and stops tasks as defined by your minimum healthy percent and maximum percent settings.