Health Check

Health Check Methods

A health check removes unhealthy instances from an AS group. Then, AS adds new instances to the AS group so that the number of instances is the same as the expected number. There are two types of AS group health checks.

ECS health check: checks the ECS instance status. If an ECS instance is stopped or deleted, it is considered unhealthy. health check is the default health check method for an AS group. The AS group periodically uses the check result to determine the status of each ECS instance in the AS group. If an ECS instance is unhealthy, AS removes the ECS instance from the AS group.
ELB health check: determines the ECS instance status using a load balancing listener. If the AS group uses load balancers, the health check method can also be ELB health check.
If you add multiple load balancers to an AS group, an ECS instance is considered healthy only when all load balancers detect that the ECS instance is healthy. If any load balancer detects that an ECS instance is unhealthy, the ECS instance will be removed from the AS group.

In both the ECS health check and ELB health check methods, AS removes unhealthy ECS instances from the AS group. Whether a removed ECS instance will be deleted depends on how the instance was added to the AS group.

**Table 1** Instance removal and deletion rules
Instance Type	Description	Billing Mode	Removed If Unhealthy	Deleted When Removed
Automatically added instances	Instances automatically created and added to an AS group in a scaling action	Pay-per-use By default, this type of instances is billed on a pay-per-use basis.	Yes	Yes
Automatically added instances		Yearly/Monthly The billing mode of an automatically added instance can be manually changed from pay-per-use to yearly/monthly.	Yes	No
Manually added instances	Instances manually created and added to an AS group	Pay-per-use	Yes	No
Manually added instances	Instances manually created and added to an AS group	Yearly/Monthly	Yes	No

Constraints

Even when an AS group is disabled, AS still checks the health of instances in the AS group, but does not remove unhealthy instances.
AS does not check the health of instances in standby state.

Parent topic: AS Group and Instance Monitoring

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