Welcome to session five, auto scaling, scaling rules. Once you have an active scaling group with an active scaling configuration, the next thing to do is to create scaling rules. Scaling rules are created from the scaling rules tab from within a scaling group. Each scaling group can have up to 60 scaling rules. The scaling rule can be executed manually or automatically and it defines specific scaling actions, that is how many instances should be added to or removed from the scaling group.

Four different rule types are available. The first one, a simple scaling rule, which specifies the scaling activity for the scaling group. The operation to add or remove instances can be by count or percentage. If the scaling activity is triggered by an event trigger task, the scaling activity must wait for the cool-down period to expire.

Next, is a step scaling rule. With a step scaling rule, the number of instances in the scaling group is increased or decreased based on a set of metric alerts that are either automatically collected by Cloud Monitor or metrics that are custom reported to Cloud Monitor. Each configuration change is based on the size of the alert breach that is generated.

A tracking scaling rule keeps the total number of instances close to the target value of the scaling group by automatically calculating the number of instances for scaling, based on the metrics selected and the target value that is entered as a percentage. And with a predictive scaling rule, you can apply a prediction for the scaling groups by configuring parameters, such as the performance metrics and the target value as a percentage that the prediction needs to work with.

If as a result of executed a scaling rule, the number of ECS instances in a scaling group is less than the minimum size value or greater than the maximum size value of instances allowed for the group, auto-scaling will automatically adjust the number of instances to be added or removed, so that the minimum or maximum instance values cannot be exceeded.

For example, let's say a scaling group is created and it's set to a maximum instance count of three ECS instances, and a minimum count of two and let's say that currently there are two ECS instances in the group and a scaling rule is added to add three more instances, well in this case, auto-scaling only adds one instance. This automatic adjustment stops the maximum instance count being exceeded and likewise, if the group now contains three instances and we have a scaling rule that is going to remove five instances, there are only three instances in the groups, so we can't possibly remove five and if we were to remove all three of those instances, then the size of the group would be zero, which is less than, in this scenario, the minimum size of two.

So auto-scaling will only subtract or remove a single ECS instance, which will bring the size of the group down from three to two, which is the minimum group size. Modifying and deleting the scaling rule. You can modify existing scaling rules from the scaling rules tab in the console, by clicking on the Edit link. It depends on what kind of rule was created as to what can be changed, but generally, you can edit the name and the type of operation that will be carried out. However, you cannot change the rule type.

If you no longer require a rule, you can simply select the Delete link. It is worth noting that if you delete a scaling group, any rules associated to the group will be deleted as well. That concludes this session.

In the next session, session six, I will cover manual scaling operations. I look forward to seeing you there.