Scale the size of your Auto Scaling group

Scaling is the ability to increase or decrease the compute capacity
of your application. Scaling starts with an event, or scaling action, which instructs an
Auto Scaling group to either launch or terminate Amazon EC2 instances.

Amazon EC2 Auto Scaling provides a number of ways to adjust scaling to best meet the needs of your
applications. As a result, it’s important that you have a good understanding of your
application. Keep the following considerations in mind:

• What role should Amazon EC2 Auto Scaling play in your application’s architecture? It’s common to
  think about automatic scaling primarily as a way to increase and decrease capacity,
  but it’s also useful for maintaining a steady number of servers.

• What cost constraints are important to you? Because Amazon EC2 Auto Scaling uses EC2 instances,
  you only pay for the resources that you use. Knowing your cost constraints helps you
  decide when to scale your applications, and by how much.

• What metrics are important to your application? Amazon CloudWatch supports a number of
  different metrics that you can use with your Auto Scaling group.

Scaling options

Amazon EC2 Auto Scaling provides several ways for you to scale your Auto Scaling group.

Maintain current instance levels at all times

You can configure your Auto Scaling group to maintain a specified number of running
instances at all times. To maintain the current instance levels, Amazon EC2 Auto Scaling performs a
periodic health check on running instances within an Auto Scaling group. When Amazon EC2 Auto Scaling finds
an unhealthy instance, it terminates that instance and launches a new one. For more
information, see Maintain a fixed number of instances in
your Auto Scaling group.

Scale manually

Manual scaling is the most basic way to scale your resources, where you specify
only the change in the maximum, minimum, or desired capacity of your Auto Scaling group.
Amazon EC2 Auto Scaling manages the process of creating or terminating instances to maintain the
updated capacity. 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 time and date. 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 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 in response to changing conditions, when you don’t know when those
conditions will change. You can set up Amazon EC2 Auto Scaling to respond for you. For more
information, see Dynamic scaling for Amazon EC2 Auto Scaling.

Use predictive scaling

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.