HealthCheck

An object representing a container health check. Health check parameters that are specified in a container definition override any Docker health checks that exist in the container image (such as those specified in a parent image or from the image's Dockerfile). This configuration maps to the HEALTHCHECK parameter of docker run.

The Amazon ECS container agent only monitors and reports on the health checks specified in the task definition. Amazon ECS does not monitor Docker health checks that are embedded in a container image and not specified in the container definition. Health check parameters that are specified in a container definition override any Docker health checks that exist in the container image.

You can view the health status of both individual containers and a task with the DescribeTasks API operation or when viewing the task details in the console.

The health check is designed to make sure that your containers survive agent restarts, upgrades, or temporary unavailability.

Amazon ECS performs health checks on containers with the default that launched the container instance or the task.

The following describes the possible healthStatus values for a container:

• HEALTHY-The container health check has passed successfully.

• UNHEALTHY-The container health check has failed.

• UNKNOWN-The container health check is being evaluated, there's no container health check defined, or Amazon ECS doesn't have the health status of the container.

The following describes the possible healthStatus values based on the container health checker status of essential containers in the task with the following priority order (high to low):

• UNHEALTHY-One or more essential containers have failed their health check.

• UNKNOWN-Any essential container running within the task is in an UNKNOWN state and no other essential containers have an UNHEALTHY state.

• HEALTHY-All essential containers within the task have passed their health checks.

Consider the following task health example with 2 containers.

• If Container1 is UNHEALTHY and Container2 is UNKNOWN, the task health is UNHEALTHY.

• If Container1 is UNHEALTHY and Container2 is HEALTHY, the task health is UNHEALTHY.

• If Container1 is HEALTHY and Container2 is UNKNOWN, the task health is UNKNOWN.

• If Container1 is HEALTHY and Container2 is HEALTHY, the task health is HEALTHY.

Consider the following task health example with 3 containers.

• If Container1 is UNHEALTHY and Container2 is UNKNOWN, and Container3 is UNKNOWN, the task health is UNHEALTHY.

• If Container1 is UNHEALTHY and Container2 is UNKNOWN, and Container3 is HEALTHY, the task health is UNHEALTHY.

• If Container1 is UNHEALTHY and Container2 is HEALTHY, and Container3 is HEALTHY, the task health is UNHEALTHY.

• If Container1 is HEALTHY and Container2 is UNKNOWN, and Container3 is HEALTHY, the task health is UNKNOWN.

• If Container1 is HEALTHY and Container2 is UNKNOWN, and Container3 is UNKNOWN, the task health is UNKNOWN.

• If Container1 is HEALTHY and Container2 is HEALTHY, and Container3 is HEALTHY, the task health is HEALTHY.

If a task is run manually, and not as part of a service, the task will continue its lifecycle regardless of its health status. For tasks that are part of a service, if the task reports as unhealthy then the task will be stopped and the service scheduler will replace it.

When a container health check fails for a task that is part of a service, the following process occurs:

• The task is marked as UNHEALTHY.

• The unhealthy task will be stopped, and during the stopping process, it will go through the following states:

• DEACTIVATING - In this state, Amazon ECS performs additional steps before stopping the task. For example, for tasks that are part of services configured to use Elastic Load Balancing target groups, target groups will be deregistered in this state.

• STOPPING - The task is in the process of being stopped.

• DEPROVISIONING - Resources associated with the task are being cleaned up.

• STOPPED - The task has been completely stopped.

• After the old task stops, a new task will be launched to ensure service operation, and the new task will go through the following lifecycle:

• PROVISIONING - Resources required for the task are being provisioned.

• PENDING - The task is waiting to be placed on a container instance.

• ACTIVATING - In this state, Amazon ECS pulls container images, creates containers, configures task networking, registers load balancer target groups, and configures service discovery status.

• RUNNING - The task is running and performing its work.

For more detailed information about task lifecycle states, see Task lifecycle in the Amazon Elastic Container Service Developer Guide.

The following are notes about container health check support:

• If the Amazon ECS container agent becomes disconnected from the Amazon ECS service, this won't cause a container to transition to an UNHEALTHY status. This is by design, to ensure that containers remain running during agent restarts or temporary unavailability. The health check status is the "last heard from" response from the Amazon ECS agent, so if the container was considered HEALTHY prior to the disconnect, that status will remain until the agent reconnects and another health check occurs. There are no assumptions made about the status of the container health checks.

• Container health checks require version 1.17.0 or greater of the Amazon ECS container agent. For more information, see Updating the Amazon ECS container agent.

• Container health checks are supported for Fargate tasks if you're using platform version 1.1.0 or greater. For more information, see Fargate platform versions.

• Container health checks aren't supported for tasks that are part of a service that's configured to use a Classic Load Balancer.

For an example of how to specify a task definition with multiple containers where container dependency is specified, see Container dependency in the Amazon Elastic Container Service Developer Guide.