Choose your scaling method
Amazon EC2 Auto Scaling provides several ways for you to scale your Auto Scaling group.
Maintain a fixed number of instances
The default for an Auto Scaling group is to not have any attached scaling policies or scheduled actions, which causes it to maintain a fixed size. After you create your Auto Scaling group, it starts by launching enough instances to meet its desired capacity. If there are no scaling conditions attached to the group, it continues to maintain its desired capacity even if an instance becomes unhealthy. Amazon EC2 Auto Scaling monitors the health of each instance in your Auto Scaling group. When it finds that an instance has become unhealthy, it replaces it with a new instance. You can read a more in-depth description of this process in Health checks for instances in an Auto Scaling group.
Scale manually
Manual scaling is the most basic way to scale your Auto Scaling group. You can either update the desired capacity of the Auto Scaling group or terminate instances in the Auto Scaling group. For more information, see Manual scaling for Amazon EC2 Auto Scaling.
Scale based on a schedule
Scaling by schedule means that scaling actions are performed automatically as a function of date and time. This is useful when you know exactly when to increase or decrease the number of instances in your group, simply because the need arises on a predictable schedule. For more information, see Scheduled scaling for Amazon EC2 Auto Scaling.
Scale dynamically based on demand
A more advanced way to scale your resources, using dynamic scaling, lets you define a scaling policy that dynamically resizes your Auto Scaling group to meet changes in demand. For example, let's say that you have a web application that currently runs on two instances and you want the CPU utilization of the Auto Scaling group to stay at around 50 percent when the load on the application changes. This method is useful for scaling as traffic changes occur, when you don't know when the traffic will change. You can configure scaling policies to respond for you. There are multiple policy types (or a combination of them) that you can use to scale in response to traffic changes. For more information, see Dynamic scaling for Amazon EC2 Auto Scaling.
Scale proactively
You can also combine predictive scaling and dynamic scaling (proactive and reactive approaches, respectively) to scale your EC2 capacity faster. Use predictive scaling to increase the number of EC2 instances in your Auto Scaling group in advance of daily and weekly patterns in traffic flows. For more information, see Predictive scaling for Amazon EC2 Auto Scaling.
