When an event-triggered task scales your group, Auto Scaling needs time to let new instances start serving traffic before another scaling decision makes sense. A cooldown period enforces that wait: after a simple scaling rule finishes, Auto Scaling ignores all new scaling requests from event-triggered tasks until the cooldown expires.
Cooldown periods apply only to event-triggered tasks. Scaling triggered by scheduled tasks, manual execution, or changes to the expected, minimum, or maximum instance count is not affected and proceeds immediately.
If your scaling strategy is still being designed, consider target tracking or step scaling policies instead of simple scaling rules. Those policy types respond proportionally to metric changes and do not rely on cooldown periods to regulate frequency.
How it works
When a simple scaling rule completes — for example, adding three instances — the cooldown timer starts the moment the last instance is added to or removed from the scaling group. During the cooldown, Auto Scaling rejects any new scaling requests from event-triggered tasks.
If a scaling operation is triggered but no instances are actually added or removed (for example, the group is already at its maximum capacity), the cooldown timer does not start.
Two parameters control cooldown duration:
| Parameter | Scope | Fallback |
|---|---|---|
| Default Cooldown Time (Seconds) | Scaling group | None |
| Cooldown Time | Individual simple scaling rule | Falls back to Default cooldown (seconds) if not set |
When both are set, Cooldown Time takes priority for that specific rule.
Considerations
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Scope: Cooldowns apply only to event-triggered tasks of the system monitoring type. Other scaling trigger types are not affected.
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Start time: The timer starts when the last instance finishes being added to or removed from the scaling group, not when the scaling operation is initiated.
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No-op scaling: If a scaling operation completes without changing any instances, the cooldown does not start.
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Disable/re-enable resets the cooldown: If you disable and then re-enable a scaling group, any active cooldown is cleared. The next event-triggered scaling operation runs without restriction from the previous cooldown.
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Manual execution bypasses the cooldown: Manually running a scaling rule ignores any active cooldown and executes immediately.
When to use rule-level vs. group-level cooldown
The group-level Default cooldown (seconds) applies to all simple scaling rules that do not have their own Cooldown Time set.
Set a rule-level Cooldown Time when a specific rule needs a different cooldown from the group default. A common pattern is to give scale-in rules a shorter cooldown than scale-out rules. Terminating instances completes faster than launching them, so a shorter scale-in cooldown lets the group shrink more quickly in response to sustained low load — reducing cost without sacrificing stability.
Configure the cooldown period
Set the default cooldown for a scaling group
Modify an existing scaling group
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Log on to the Auto Scaling console.
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Log on to the Auto Scaling console.
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In the left-side navigation pane, click Scaling Groups.
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In the top navigation bar, select the region where Auto Scaling is activated.
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Find the scaling group you want to update and click Modify in the Actions column.
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Update the Default cooldown (seconds) value.
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Click OK.
Set during scaling group creation
On the Create by Form tab, set Default cooldown (seconds) to the value that fits your workload.
For instructions on creating a scaling group, see Create an ECS scaling group or Create a scaling group of the Elastic Container Instance type.
Set the cooldown for a simple scaling rule
Modify an existing simple scaling rule
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Log on to the Auto Scaling console.
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Log on to the Auto Scaling console.
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In the left-side navigation pane, click Scaling Groups.
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In the top navigation bar, select the region where Auto Scaling is activated.
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Find the scaling group that contains the rule and click Details in the Actions column.
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Go to Scaling Rules and Tasks > Edit Scaling Rule, find the rule, and click Modify in the Actions column.
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Update the Cooldown Time value.
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Click OK.
Set during simple scaling rule creation
When creating a simple scaling rule, set Cooldown Time to override the group default for that rule.
For instructions on creating scaling rules, see Configure scaling rules.
API reference
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Modify the default cooldown period of a scaling group: ModifyScalingGroup
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Modify the cooldown period of a simple scaling rule: ModifyScalingRule