I have a T2 Micro instance on AWS Beanstalk with Autoscaling set up. The autoscaling policy uses the Network Out parameter and currently I have it set at 6mb. However, this results in a lot of instances being created and terminated (as the Net Out goes above 6mb). My question is what is an appropriate auto-scaling Net Out policy for a Micro Instance. I understand that a Micro instance should support a Network bandwidth of about 70 Mbit so perhaps the Net Out auto scale can safely be set to about 20 Mbit?
1 Answers
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Determining a scale-out trigger for an Auto Scaling group is always difficult.
It needs to be something that identifies that the instance is "busy", to know when to add/remove instances. This varies greatly depending upon the application.
The specific issue with T2 instances is that they have CPU credits. If these credits are exhausted, then there is an artificial maximum level of CPU available. Thus, T2 instances should never have a scaling policy based on CPU.
In your case, you are using networking as the scaling trigger. This is good if network usage is an indication of the instance being "busy", resulting in a bottleneck. If, on the other hand, networking is not the bottleneck then this is not a good scaling trigger.
Traditionally, busy computers are either limited in CPU, Network or Disk access. You will need to study a "busy" instance to discover which of these dimensions is the best indicator that the instance is "busy" such that it cannot handle any additional load.
Alternatively, you might want the application to generate its own metrics, such as the number of messages being simultaneously processed. These can be pushed to Amazon CloudWatch as a custom metric, which can then be used for scaling in/out.
You can even get fancy and use information from a database to trigger scaling events: AWS Autoscaling Based On Database Query Custom Metrics - powerupcloud
