What’s an Auto Scaling Group?

Scale for your users, make yourself available.

• In real-life, the load on your websites and application can change.

• In the cloud, you can create and get rid of servers very quickly.

• The goal of an Auto Scaling Group (ASG) is to:

  • Scale out (add EC2 instances) to match an increased load.

  • Scale in (remove EC2 instances) to match a decreased load.

  • Ensure we have a minimum and a maximum number of EC2 instances running.

  • Automatically register new instances to a load balancer

  • Re-create an EC2 instance in case a previous one is terminated (ex: if unhealthy)

• ASG are free (you only pay for the underlying EC2 instances)

Auto Scaling Group Attributes

• A Launch Template (older “Launch Configurations” are deprecated)

  • AMI + Instance Type

  • EC2 User Data

  • EBS Volumes

  • Security Groups

  • SSH Key Pair

  • IAM Roles for your EC2 Instances

  • Network + Subnets Information

  • Load Balancer Information

• Min Size / Max Size / Initial Capacity

• Scaling Policies

CloudWatch Alarms & Scaling

• It is possible to scale an ASG based on CloudWatch alarms.

• An alarm monitors a metric (such as Average CPU, or a custom metric)

• Metrics such as Average CPU are computed for the overall ASG instances.

• Based on the alarm:

  • We can create scale-out policies (increase the number of instances)

  • We can create scale-in policies (decrease the number of instances)

Auto Scaling Groups – Dynamic Scaling Policies

• Target Tracking Scaling

  • Most simple and easy to set-up

  • Example: I want the average ASG CPU to stay at around 40%

• Simple / Step Scaling

  • When a CloudWatch alarm is triggered (example CPU > 70%), then add 2 units

  • When a CloudWatch alarm is triggered (example CPU < 30%), then remove 1

• Scheduled Actions

  • Anticipate a scaling based on known usage patterns

  • Example: increase the min capacity to 10 at 5 pm on Fridays

• Predictive scaling:

  • continuously forecast load and schedule scaling ahead

Predictive Scaling

Good metrics to scale on

• CPUUtilization: Average CPU utilization across your instances

• RequestCountPerTarget: to make sure the number of requests per EC2 instances is stable

• Average Network In / Out (if you’re application is network bound)

• Any custom metric (that you push using CloudWatch)

Auto Scaling Groups - Scaling Cooldowns

• After a scaling activity happens, you are in the cooldown period (default 300 seconds)

• During the cooldown period, the ASG will not launch or terminate additional instances (to allow for metrics to stabilize)

• Advice: Use a ready-to-use AMI to reduce configuration time in order to be serving request fasters and reduce the cooldown period