Amazon EC2 Auto Scaling benefits

Adding Amazon EC2 Auto Scaling to your application architecture is one way to maximize the benefits
of the AWS Cloud. When you use Amazon EC2 Auto Scaling, your applications gain the following
benefits:

• Better fault tolerance. Amazon EC2 Auto Scaling can detect when an instance is unhealthy,
  terminate it, and launch an instance to replace it. You can also configure
  Amazon EC2 Auto Scaling to use multiple Availability Zones. If one Availability Zone becomes
  unavailable, Amazon EC2 Auto Scaling can launch instances in another one to compensate.

• Better availability. Amazon EC2 Auto Scaling helps ensure that your application always has
  the right amount of capacity to handle the current traffic demand.

• Better cost management. Amazon EC2 Auto Scaling can dynamically increase and decrease
  capacity as needed. Because you pay for the EC2 instances you use, you save
  money by launching instances when they are needed and terminating them when they
  aren’t.

Example: Cover variable
demand

To demonstrate some of the benefits of Amazon EC2 Auto Scaling, consider a basic web application
running on AWS. This application allows employees to search for conference rooms
that they might want to use for meetings. During the beginning and end of the week,
usage of this application is minimal. During the middle of the week, more employees
are scheduling meetings, so the demand on the application increases
significantly.

The following graph shows how much of the application’s capacity is used over the
course of a week.

Traditionally, there are two ways to plan for these changes in capacity. The first
option is to add enough servers so that the application always has enough capacity
to meet demand. The downside of this option, however, is that there are days in
which the application doesn’t need this much capacity. The extra capacity remains
unused and, in essence, raises the cost of keeping the application running.

The second option is to have enough capacity to handle the average demand on the
application. This option is less expensive, because you aren’t purchasing equipment
that you’ll only use occasionally. However, you risk creating a poor customer
experience when the demand on the application exceeds its capacity.

By adding Amazon EC2 Auto Scaling to this application, you have a third option available. You can
add new instances to the application only when necessary, and terminate them when
they’re no longer needed. Because Amazon EC2 Auto Scaling uses EC2 instances, you only have to pay
for the instances you use, when you use them. You now have a cost-effective
architecture that provides the best customer experience while minimizing
expenses.

Example: Web app architecture

In a common web app scenario, you run multiple copies of your app simultaneously
to cover the volume of your customer traffic. These multiple copies of your
application are hosted on identical EC2 instances (cloud servers), each handling
customer requests.

Amazon EC2 Auto Scaling manages the launch and termination of these EC2 instances on your behalf.
You define a set of criteria (such as an Amazon CloudWatch alarm) that determines when the
Auto Scaling group launches or terminates EC2 instances. Adding Auto Scaling groups to your network
architecture helps make your application more highly available and fault
tolerant.

You can create as many Auto Scaling groups as you need. For example, you can create an
Auto Scaling group for each tier.

To distribute traffic between the instances in your Auto Scaling groups, you can introduce
a load balancer into your architecture. For more information, see Elastic Load Balancing.

Example: Distribute instances across
Availability Zones

Availability Zones are isolated locations in a given AWS Region. Each Region has
multiple Availability Zones designed to provide high availability for the Region.
Availability Zones are independent, and therefore you increase application
availability when you design your application to use multiple zones. For more
information, see Resilience in Amazon EC2 Auto Scaling.

An Availability Zone is identified by the AWS Region code followed by a letter
identifier (for example, us-east-1a). If you create your VPC and
subnets rather than using the default VPC, you can define one or more subnets in
each Availability Zone. Each subnet must reside entirely within one Availability
Zone and cannot span zones. For more information, see How Amazon VPC works in the
Amazon VPC User Guide.

When you create an Auto Scaling group, you must choose the VPC and subnets where you will
deploy the Auto Scaling group. Amazon EC2 Auto Scaling creates your instances in your chosen subnets. Each
instance is thus associated with a specific Availability Zone chosen by Amazon EC2 Auto Scaling.
When instances launch, Amazon EC2 Auto Scaling tries to evenly distribute them between the zones
for high availability and reliability.

The following image shows an overview of multi-tier architecture deployed across
three Availability Zones.

Instance distribution

Amazon EC2 Auto Scaling automatically tries to maintain equivalent numbers of instances in
each enabled Availability Zone. Amazon EC2 Auto Scaling does this by attempting to launch new
instances in the Availability Zone with the fewest instances. If there are
multiple subnets chosen for the Availability Zone, Amazon EC2 Auto Scaling selects a subnet
from the Availability Zone at random. If the attempt fails, however, Amazon EC2 Auto Scaling
attempts to launch the instances in another Availability Zone until it
succeeds.

In circumstances where an Availability Zone becomes unhealthy or unavailable,
the distribution of instances might become unevenly distributed across the
Availability Zones. When the Availability Zone recovers, Amazon EC2 Auto Scaling automatically
rebalances the Auto Scaling group. It does this by launching instances in the enabled
Availability Zones with the fewest instances and terminating instances
elsewhere.

Rebalancing
activities

Rebalancing activities fall into two categories:
Availability Zone rebalancing and capacity rebalancing.

Availability Zone
rebalancing

After certain actions occur, your Auto Scaling group can become unbalanced between
Availability Zones. Amazon EC2 Auto Scaling compensates by rebalancing the Availability Zones.
The following actions can lead to rebalancing activity:

• You change the Availability Zones associated with your Auto Scaling group.

• You explicitly terminate or detach instances or place instances in
  standby, and then the group becomes unbalanced.

• An Availability Zone that previously had insufficient capacity
  recovers and now has additional capacity.

• An Availability Zone that previously had a Spot price above your
  maximum price now has a Spot price below your maximum price.

When rebalancing, Amazon EC2 Auto Scaling launches new instances before terminating the
earlier ones. This way, rebalancing does not compromise the performance or
availability of your application.

Because Amazon EC2 Auto Scaling attempts to launch new instances before terminating the
earlier ones, being at or near the specified maximum capacity could impede or
completely halt rebalancing activities.

To avoid this problem, the system can temporarily exceed the specified maximum
capacity of a group during a rebalancing activity. It can do so by a margin of
10 percent or one instance, whichever is greater. The margin is extended only if
the group is at or near maximum capacity and needs rebalancing. This can happen
because of user-requested rezoning, or to compensate for zone availability
issues. The extension lasts only as long as needed to rebalance the group
(typically a few minutes).

Capacity
Rebalancing

You can turn on Capacity Rebalancing for your Auto Scaling groups
when using Spot Instances. This lets Amazon EC2 Auto Scaling attempt to launch a Spot Instance
whenever Amazon EC2 reports that a Spot Instance is at an elevated risk of
interruption. After launching a new instance, it then terminates an earlier
instance. For more information, see Use Capacity Rebalancing to handle
Amazon EC2 Spot interruptions.