You can use an Amazon ECS service to run and maintain a specified number of instances of a task definition simultaneously in an Amazon ECS cluster. If one of your tasks fails or stops, the Amazon ECS service scheduler launches another instance of your task definition to replace it. This helps maintain your desired number of tasks in the service.
You can also optionally run your service behind a load balancer. The load balancer distributes traffic across the tasks that are associated with the service.
We recommend that you use the service scheduler for long running stateless services and applications. The service scheduler ensures that the scheduling strategy that you specify is followed and reschedules tasks when a task fails. For example, if the underlying infrastructure fails, the service scheduler reschedules a task. You can use task placement strategies and constraints to customize how the scheduler places and terminates tasks. If a task in a service stops, the scheduler launches a new task to replace it. This process continues until your service reaches your desired number of tasks based on the scheduling strategy that the service uses. The scheduling strategy of the service is also referred to as the service type.
The service scheduler also replaces tasks determined to be unhealthy after a container health check or a load balancer target group health check fails. This replacement depends on the maximumPercent and desiredCount service definition parameters. If a task is marked unhealthy, the service scheduler will first start a replacement task. Then, the following happens.
If the replacement task has a health status of HEALTHY, the service scheduler stops the unhealthy task
If the replacement task has a health status of UNHEALTHY, the scheduler will stop either the unhealthy replacement task or the existing unhealthy task to get the total task count to equal desiredCount.
If the maximumPercent parameter limits the scheduler from starting a replacement task first, the scheduler will stop an unhealthy task one at a time at random to free up capacity, and then start a replacement task. The start and stop process continues until all unhealthy tasks are replaced with healthy tasks. Once all unhealthy tasks have been replaced and only healthy tasks are running, if the total task count exceeds the desiredCount, healthy tasks are stopped at random until the total task count equals desiredCount. For more information about maximumPercent and desiredCount, see Service definition parameters.
The service scheduler includes logic that throttles how often tasks are restarted if tasks repeatedly fail to start. If a task is stopped without having entered a RUNNING state, the service scheduler starts to slow down the launch attempts and sends out a service event message. This behavior prevents unnecessary resources from being used for failed tasks before you can resolve the issue. After the service is updated, the service scheduler resumes normal scheduling behavior. For more information, see Amazon ECS service throttle logic and Viewing Amazon ECS service event messages.
There are two compute options that distribute your tasks.
A capacity provider strategy causes Amazon ECS to distribute your tasks in one or across multiple capacity providers.
For the capacity provider strategy, the console selects a compute option by default. The following describes the order that the console uses to select a default:
If your cluster has a default capacity provider strategy defined, it is selected.
If your cluster doesn't have a default capacity provider strategy defined but you have the Fargate capacity providers added to the cluster, a custom capacity provider strategy that uses the FARGATE capacity provider is selected.
If your cluster doesn't have a default capacity provider strategy defined but you have one or more Auto Scaling group capacity providers added to the cluster, the Use custom (Advanced) option is selected and you need to manually define the strategy.
If your cluster doesn't have a default capacity provider strategy defined and no capacity providers added to the cluster, the Fargate launch type is selected.
A launch type causes Amazon ECS to launch our tasks directly on either Fargate or on the EC2 instances registered to your clusters.
By default the service starts in the subnets in your cluster VPC.
Service auto scaling is the ability to increase or decrease the desired number of tasks in your Amazon ECS service automatically. Amazon ECS leverages the Application Auto Scaling service to provide this functionality.
For more information, see Automatically scale your Amazon ECS service.
Service load balancingAmazon ECS services hosted on AWS Fargate support the Application Load Balancers, Network Load Balancers, and Gateway Load Balancers. Use the following table to learn about what type of load balancer to use.
Load Balancer type Use in these casesApplication Load Balancer
Route HTTP/HTTPS (or layer 7) traffic.Application Load Balancers offer several features that make them attractive for use with Amazon ECS services:
Each service can serve traffic from multiple load balancers and expose multiple load balanced ports by specifying multiple target groups.
They are supported by tasks hosted on both Fargate and EC2 instances.
Application Load Balancers allow containers to use dynamic host port mapping (so that multiple tasks from the same service are allowed per container instance).
Application Load Balancers support path-based routing and priority rules (so that multiple services can use the same listener port on a single Application Load Balancer).
Use virtual appliances, such as firewalls, intrusion detection and prevention systems, and deep packet inspection systems.
For more information, see Use load balancing to distribute Amazon ECS service traffic.
Interconecting servicesIf you need an application to connect to other applications that run as Amazon ECS services, Amazon ECS provides the following ways to do this without a load balancer:
Service Connect - Allows for service-to-service communications with automatic discovery using short names and standard ports.
Service discovery - Service discovery uses Route 53 to create a namespace for your service, which allows it to be discoverable through DNS.
Amazon VPC Lattice - VPC Lattice is a fully managed application networking service to connect, secure, and monitor your services across multiple accounts and VPCs. There is a cost associated with it.
For more information, see Interconnect Amazon ECS services.
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