Dynamic Scaling

Target Tracking: you can choose a metric and whenever the metrics reported by CloudWatch match the conditions (below or above certain value) a new instance is launched.

AWS recommends scaling on metrics with a 1 min frequency.

Simple Scaling: an Alarm is attached to a scaling group and when the alarm is triggered Autoscaling will wait 300 seconds before allowing another scaling activity

Step scaling: in this case an alarm is attached to a scaling group. But the alarm settings and responses are defined in steps. For example, if the alarm is triggered when CPU is > 60 % and the alarm breach is at 70%, launch 2 instances, but if it's at 80% then launch directly 4 instances.

A very important difference to note here is that Target Scaling policy is the only one useful to mantain a metric at, or close to a specified target value** while Simple and Step make adjustments when a specific target value is reached.

Scheduled scaling

As the name suggest this is a scaling policy that you define at specific schedules / intervals. The scaling is not a reaction to something that happened, but an action that you take to prevent your instances to not perform in the way you'd like.

If you know that every day at lunch time you have a peak in usages of your application, you could set up a Scheduled Scaling for 11:45 to bring your instances to a certain number ( desired / min and max running instances ).

Predictive Scaling

Similarly to the scheduled scaling, but managed by AWS. Instances are scaled in advance of daily and weekly patterns in traffic flows.