Along with the ability to Setup Elastic Load Balancing, which I showed you how to setup in my previous post. Amazon provides the ability to auto scale your instances. Auto Scaling groups allow you to set up groups of instances that will scale up and down depending on triggers you can create. For example you can set up a scaling group to always have 2 instances in it, and to scale up another server if the CPU utilization of the servers grows over a certain threshold. This is extremely helpful when you receive unexpected traffic and you are unable to react in time to add new instances. The beauty of Auto Scaling in conjunction with Elastic Load Balancing is that it will automatically assign the new instances to the load balancer you provide.
Creating an Auto Scaling Launch Config
The first step in setting up Auto Scaling is to create a launch config. The launch config is used to determine what ec2 image, and size (small, medium, etc) will be used to setup a new instance for your Auto Scaling group. To setup a launch config you will call the as-create-launch-config. For example to create a new launch config called auto-scaling-test that would launch the image ami-12345678 of size c1.medium you would run the following command:
as-create-launch-config auto-scaling-test --image-id ami-12345678 --instance-type c1.medium
Create an Auto Scaling Group
The next step to enabling Auto Scaling is to setup an Auto Scaling Group. An Auto Scaling group tells Amazon what zones you want your instances created in, the minimum and maximum number of instances to ever launch, and which launch config to utilize. To create an Auto Scaling group you will call the as-create-auto-scaling-group command. For example if you wanted to create a new group with a name of auto-scaling-test using the availability zones of us-east-1a with a minimum number of instances being 2 and a maximum of 4 using our newly created launch config you would run:
as-create-auto-scaling-group auto-scaling-test --availability-zones us-east-1a --launch-configuration auto-scaling-test --max-size 4 --min-size 2
When this command is executed 2 new instances will be created as per the directions of the launch config. the as-create-auto-scaling-group can also take be linked to a load balancer. Thus if we wanted to have this group setup with the load balancer we created in the previous article, you would run:
as-create-auto-scaling-group auto-scaling-test --availability-zones us-east-1a --launch-configuration auto-scaling-test --max-size 4 --min-size 2 --load-balancers test-balancer
After execution this would setup 2 new instances as per the instructions of the launch config, and register these instances with the load balancer test-balancer.
Creating Auto Scaling Triggers
Triggers are used by Auto Scaling to determine whether to launch or terminate instances within an Auto Scaling Group. To setup a trigger you will use the as-create-or-update-trigger command. Here is an example using the auto scaling group we created earlier:
as-create-or-update-trigger auto-scaling-test --auto-scaling-group auto-scaling-test --measure CPUUtilization --statistic Average --period 60 --breach-duration 120 --lower-threshold 30 --lower-breach-increment"=-1" --upper-threshold 60 --upper-breach-increment 2
Lets walk through what this command is doing. Basically what this command is saying is, create a new trigger called auto-scaling-test. This trigger should use the auto-scaling group called auto-scaling-test. It should measure the average CPU utilization of the current instances in the auto scaling group every 60 seconds. If the CPU utilization goes over 60% over the period of 120 seconds launch 2 new instances. Alternatively if the CPU utilization drops below 30% over the period of 120 seconds terminate 1 of the instances. Remember that the trigger will never terminate more instances than the minimum number of instances and it will not launch more instances than the maximum number of instances as defined in the Auto Scaling Group.
Shutting Down an Auto Scaling Group
Initially shutting down an Auto Scaling group can be a bit tricky as you cannot delete an Auto Scaling Group until all the instances are terminated or deregistered from the group. The best way to terminate an Auto Scaling group, it’s triggers and launch config is to do the following steps:
- Delete all triggers
- Update the Auto Scaling group to have a minimum and maximum number of instances of 0
- Wait for the instances registered with the Auto Scaling group to be terminated
- Delete the Auto Scaling group
- Delete the launch config
To do this with the examples we used above we would issue the following commands:
as-delete-trigger auto-scaling-test --auto-scaling-group auto-scaling-test
as-update-auto-scaling-group auto-scaling-test --min-size 0 -max-size 0
With those 4 commands you can easily delete your Auto Scaling group as well as any launch configs or triggers that are associated with it.
Auto Scaling provides an easy and efficient way to grow and shrink your hosting solution based on your current traffic. In events where you are hit with unexpected traffic Auto Scaling can provide a failsafe by automatically launching new instances and scaling up your solution to meet this new traffic. When the traffic subsides Auto Scaling will scale down your solution so that you are not wasting money by running more instances than you require.
One thing to note is that if you know before hand that you will be receiving a traffic spike at a specific time, it may be more benefitial to launch new instances manually before the spike. This will save your system from getting hammered before having the Auto Scaling launches new instances to cope with the additional load. If you rely on Auto Scaling alone in this scenario you could see many requests at the start of the heavy traffic timeout or fail as the minimum number of instances likely won’t be able to handle the traffic load.