Instance Lifecycle

An ECS instance in an AS group goes through different statuses from its creation to its removal.

The instance status changes as shown in Figure 1 if you have not added a lifecycle hook to the AS group.

Figure 1 Instance lifecycle
Click to enlarge

When trigger condition 2 or 4 is met, the system autonomously puts instances into the next status.

**Table 1** Instance statuses
Status	Action	Description	Trigger Condition
Initial	-	The instance has not been added to the AS group.	The instance status changes to Adding when any of the following conditions occurs: You manually increase the expected number of instances of the AS group. The system automatically expands the AS group capacity. You manually add instances to the AS group.
Adding	Add the instance.	When trigger condition 1 is met, AS adds the instance to expand the AS group capacity.
Adding	(Optional) Associate the instance with a load balancing listener.	When trigger condition 1 is met, AS associates the created instance with the load balancing listener.
Enabled	-	The instance is added to the AS group and starts to process service traffic.	The instance status changes from Enabled to Removing when any of the following conditions is met: You manually decrease the expected number of instances of the AS group. The system automatically reduces the AS group capacity. A health check shows that an enabled instance is unhealthy, and the system removes it from the AS group. You manually remove instances from the AS group.
Removing	(Optional) Disassociate the instance from the load balancing listener.	When trigger condition 3 is met, the AS group starts to reduce resources and disassociate the instance from the load balancing listener.
Removing	Remove the instance.	After the instance is unbound from the load balancing listener, it is removed from the AS group.
Removed	-	The instance lifecycle in the AS group ends.	-

When an ECS instance is added to an AS group manually or through a scaling action, it goes through the Adding, Enabled, and Removing statuses. Then it is finally removed from the AS group.

If you have added a lifecycle hook to the AS group, the instance statuses change as shown in Figure 2. When a scale-out or scale-in event occurs in the AS group, the required instances are suspended by the lifecycle hook and remain in the wait status until the timeout period ends or you manually call back the instances. You can perform custom operations on the instances when they are in the wait status. For example, you can install or configure software on an instance before it is added to the AS group or download log files from an instance before it is removed. For details, see Managing Lifecycle Hooks.

Figure 2 Instance lifecycle
Click to enlarge

Under trigger condition 2, 4, 6, or 8, the system automatically changes the instance status.

**Table 2** Instance statuses
Status	Action	Description	Trigger Condition
Initial	-	The instance has not been added to the AS group.	The instance status changes to Adding when any of the following conditions occurs: You manually change the expected number of instances of the AS group. The system automatically expands the AS group capacity. You manually add instances to the AS group.
Adding	Add the instance.	When trigger condition 1 is met, AS adds the instance to expand the AS group capacity.
Pending addition	-	The lifecycle hook suspends the instance that is being added to the AS group and puts the instance into a wait status.	The instance status changes from Pending addition to Adding when either of the following conditions occurs: The default callback action is performed. You manually perform the callback action. For details, see Managing Lifecycle Hooks.
Adding	(Optional) Associate the instance with a load balancing listener.	When trigger condition 3 is met, AS associates the instance with the load balancing listener.
Enabled	-	The instance is added to the AS group and starts to process service traffic.	The instance status changes from Enabled to Removing when any of the following conditions occurs: You manually change the expected number of instances of the AS group. The system automatically reduces the AS group capacity. A health check shows that the instance is unhealthy after being enabled, and the system removes it from the AS group. You manually remove an instance from an AS group.
Removing	(Optional) Disassociate the instance from the load balancing listener.	When trigger condition 5 is met, the AS group starts to reduce resources and disassociate the instance from the load balancing listener.
Pending removal	-	The lifecycle hook suspends the instance that is being removed from the AS group and puts the instance into a wait status.	The instance status changes from Pending removal to Removing when either of the following conditions occurs: The default callback action is performed. You manually perform the callback action. For details, see Managing Lifecycle Hooks.
Removing	Remove the instance.	When trigger condition 7 is met, AS removes the instance from the AS group.
Removed	-	The instance lifecycle in the AS group ends.	-

Instances are added to an AS group manually or automatically. Then, they go through statues Adding, Pending addition, Adding, Enabled, Removing, Pending removal, and Removing and are finally removed from the AS group.

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