How an instance refresh works in an Auto Scaling group

The minimum healthy percentage is the percentage of the desired capacity to keep in service, healthy, and ready to use during an instance refresh so that the refresh can continue. For example, if the minimum healthy percentage is 90 percent and the maximum healthy percentage is 100 percent, then 10 percent of capacity will be replaced at a time. If the new instances don't pass their health checks, Amazon EC2 Auto Scaling terminates and replaces them. If the instance refresh can't launch any healthy instances, it will eventually fail, leaving the other 90 percent of the group untouched. If the new instances stay healthy and finish their warmup period, Amazon EC2 Auto Scaling can continue to replace other instances.

An instance refresh can replace instances one at a time, several at a time, or all at once. To replace one instance at a time, set both the minimum and maximum healthy percentage to 100 percent. This changes the behavior of an instance refresh to launch before terminating, which prevents the group's capacity from falling below 100 percent of its desired capacity. To replace all instances at once, set a minimum healthy percentage of 0 percent.

The maximum healthy percentage is the percentage of the desired capacity that your Auto Scaling group can increase to when replacing instances. The difference between the minimum and maximum cannot be greater than 100. A larger range increases the number of instances that can be replaced at the same time.

The instance warmup is the time period from when a new instance's state changes to InService to when it is considered to have finished initializing. During an instance refresh, if the instances pass their health checks, Amazon EC2 Auto Scaling does not immediately move on to replacing the next instance after determining that a newly launched instance is healthy. It waits for the warmup period before it moves on to replacing the next instance. This can be helpful when your application still needs some initialization time before it responds to requests.

The instance warmup works the same way as the default instance warmup. Therefore, the same scaling considerations apply. For more information, see Set the default instance warmup for an Auto Scaling group.

The desired configuration is the new configuration that you want Amazon EC2 Auto Scaling to deploy across your Auto Scaling group. For example, you can specify a new launch template and new instance types for your instances. During an instance refresh, Amazon EC2 Auto Scaling updates the Auto Scaling group to the desired configuration. If a scale-out event occurs during an instance refresh, Amazon EC2 Auto Scaling launches new instances with the desired configuration instead of the group's current settings. After the instance refresh succeeds, Amazon EC2 Auto Scaling updates the Auto Scaling group settings to reflect the new desired configuration that you specified as part of the instance refresh.

Skip matching tells Amazon EC2 Auto Scaling to ignore instances that already have your latest updates. This way, you don't replace more instances than you need to. This is helpful when you want to make sure your Auto Scaling group uses a particular version of your launch template and only replaces those instances that use a different version.

A checkpoint is a point in time where the instance refresh pauses for a specified amount of time. An instance refresh can contain multiple checkpoints. Amazon EC2 Auto Scaling emits events for each checkpoint. Therefore, you can add an EventBridge rule to send the events to a target, such as Amazon SNS, to be notified when a checkpoint is reached. After a checkpoint is reached, you have the opportunity to verify your deployment. If any problems are identified, you can cancel the instance refresh or roll it back. The ability to deploy updates in phases is a key benefit of checkpoints. If you don't use checkpoints, rolling replacements are performed continuously.