EC2 Auto Scaling Groups: Capacity Schedules and Policies
An ASG is a capacity controller, not a machine. Scaling it to zero terminates instances, which changes what a schedule means.
Terminate, not stop
An Auto Scaling group costs whatever its instances cost. AWS notes that EC2 Auto Scaling terminates, rather than stops, unneeded instances. When a scheduled action lowers desired capacity below current capacity, the group scales in and removes instances. The instances that come back in the morning are new ones launched from the template, so nothing on their local disks survives the night. That is fine for stateless fleets and wrong for anything that keeps state on the instance.
How ZopNight works with ASGs
- Capacity scheduling. ASG is on ZopNight’s Start/Stop supported list as capacity scaling. A stop lowers the group’s capacity, a start restores it, on a cron in your timezone with the usual overrides.
- Scaling policies. ZopNight’s autoscaler can manage target tracking and step scaling policies on an ASG and set its minimum, maximum and desired size. Smart defaults suggest those values from measured CPU statistics. Modes progress from monitor to recommend to autopilot, and a read-only account is limited to monitor.
- Existing policies are not overwritten. If the group already has policies, you choose to adopt them unchanged or replace them. Replace captures the full previous configuration so removing ZopNight’s policy restores it. A few shapes, such as predictive scaling, cannot be reconstructed, and Replace refuses them.
- Planned events. Event readiness pre-scales an ASG with AWS scheduled update actions and restores the original capacity when the event ends or is cancelled.
ZopNight’s ASG rules
- ASG Has No Scaling Policies flags groups that never scale on load.
- ASG Floor Lock Detected fires when the minimum size stops a group from scaling in during measured off-hours, and prices the saving from the measured idle fraction. The lever is a recurring schedule that lowers the floor, never a one-shot stop.
Before applying or removing a policy, the blast radius view treats the group as the root and shows the connected resources affected.
See the ASG fact sheet, ASG schedule mechanics, autoscaling and target tracking scaling.