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If the demand on your application is rapidly increasing, a simple scaling policy may not add enough instances quickly enough. Using a step scaling policy, you can instead add instances based on how much the aggregate metric exceeds the threshold.

To illustrate, suppose your group starts out with four instances. You want to add more instances to the group as the average CPU utilization of the group increases. When the utilization hits 50 percent, you want to add two more instances. When it goes above 60 percent, you want to add four more instances.

You'd first create a CloudWatch Alarm to monitor the average CPU utilization and set the alarm threshold to 50 percent, since this is the utilization level at which you want to start increasing the desired capacity.

You must then specify at least one step adjustment. Each step adjustment consists of the following:

 A lower bound

 An upper bound

 The adjustment type

 The amount by which to increase the desired capacity

The upper and lower bounds define a range that the metric has to fall within for the step adjustment to execute. Suppose that for the first step you set a lower bound of 50 and an upper bound of 60, with a ChangeInCapacity adjustment of 2. When the alarm triggers, Auto Scaling will consider the metric value of the group's average CPU utilization. Suppose it's 55 percent. Because 55 is between 50 and 60, Auto Scaling will execute the action specified in this step, which is to add two instances to the desired capacity.

Suppose now that you create another step with a lower bound of 60 and an upper bound of infinity. You also set a ChangeInCapacity adjustment of 4. If the average CPU utilization increases to 62 percent, Auto Scaling will note that 60 <= 62 < infinity and will execute the action for this step, adding four instances to the desired capacity.

You might be wondering what would happen if the utilization were 60 percent. Step ranges can't overlap. A metric of 60 percent would fall within the lower bound of the second step.

With a step scaling policy, you can optionally specify a warm‐up time, which is how long Auto Scaling will wait until considering the metrics of newly added instances. The default warm‐up time is 300 seconds. Note that there are no cooldown periods in step scaling policies.