CODEWITHSUNDEEP
amazon-web-servicesamazon-ec2autoscaling
X.Otano
X.Otano

Reputation: 2169

What "desired instances" is needed for? AWS Amazon Webservices AutoScaling group

i am facing a strange situation on Amazon AWS. I don´t understand for what the desired Instances number is used for? I have an autoscaling group that contains scale up and scale down actions configured.

I have a custom PHP file that run actions Scale up and Scale down depending on some external factors. I want to know which number I have to write in desired instances to not affect my autoscaling actions.

For example:

  1. I set desired to 2
  2. I have 2 instances running
  3. I run Scale Down action
  4. Instances is 1
  5. Autoscaling group will automatically start another instance, so my scale down is not useful because I ended by having 2 running

What can I do?

Many thanks!

Upvotes: 31

Views: 24887

Answers (6)

Salindaw
Salindaw

Reputation: 1626

A Simple Summary

@Start

Instances= Desired Instances

If no scaling policies or scheduled actions

Instances= Desired Instances

Else: ( There are scaling policies)

Min <= Instances <= Max

An Auto Scaling group will start by launching as many instances as are specified for desired capacity. If there are no scaling policies or scheduled actions attached to the Auto Scaling group, Amazon EC2 Auto Scaling maintains the desired amount of instances, performing periodic health checks on the instances in the group. Unhealthy instances will be terminated and replaced with new ones. Hence if there is a scaling policy attached then the number of instances will vary between a minimum and maximum instances.

Upvotes: 2

display name
display name

Reputation: 328

For a blue/green deployment with Terraform, you may consider using your max desired of 10 initially. Post-deploy, you can follow-up and set the desired_capacity value in Terraform back down to some value and let whatever triggers you have setup for the ASG to let it scale down naturally. (remember, 'desired_capacity' is a target of instance that "should" be in the ASG where 'min_size' is the minimum amount allowed in the ASG)

https://www.terraform.io/docs/providers/aws/r/autoscaling_group.html

resource "aws_autoscaling_group" "example" {
  availability_zones = 
  ["${data.aws_availability_zones.available.names}"]
  max_size = 20
  min_size = 2
  desired_capacity = 10
  launch_configuration = "${aws_launch_configuration.example.name}"
}

Upvotes: 1

stobiewankenobi
stobiewankenobi

Reputation: 704

These answers are all good, but I would like to add one more situation.

Say you are doing a blue/green deployment and you have bursty traffic. Your min is 1, because 50% of the time you only have 1 instance. However say you're doing CI/CD and deployments happen all the time all day long. A dev could push code to production right in the middle of a large scaling event where the instances have scaled up to 10. Now if you use a tool like terraform to deploy autoscaling groups etc... it will reset the autoscaling group back down to 1 and you run the risk of having an interruption of services.

For this reason, during a deployment, we have a terraform override value that we programmatically hand to terraform on invocation. Prior to invoking terraform, we use the aws cli to figure out the current desired capacity that autoscaling has scaled the instances to, and we pass that value onto terraform so the new autoscaling group comes up with the same number of hosts as the previous one.

Upvotes: 6

Aadarsh Sharma
Aadarsh Sharma

Reputation: 1

You have set desired to 2.Your auto scaling group(asg) can scale above the desired capacity but cannot scale below it.By specifying desired capacity you bound the asg to anyhow maintain this capacity.If your instances are less that your desired capacity,your asg will scale up but it will not scale down below the desired capacity. For eg, your desired capacity is 5, and your are running 10 instances.At this stage asg can scale down upto 5 but not below it.

Upvotes: -1

Apokai
Apokai

Reputation: 359

I think that desired value is meant to set manually a number of instances at a specific time. I think that 99% of the time, you let your desired == minimum values and let you ASG policies do the scaling.

Upvotes: 6

Peter
Peter

Reputation: 981

The ASG will always try to maintain the Desired Capacity. If you scale up or down, and the new number of instances is different than the Desired Capacity, the ASG will add or remove an instance to go back to the desired capacity.

If you use scaling policies, and the policy condition is met, the ASG will change the Desired Capacity to match the result of your scaling policy. E.g., you have a Desired Capacity of 2, and a policy that says to scale up if the CPU utilization goes over a threshold.

If that policy is fulfilled, then the Desired Capacity will increase to 3, and so on.

So manually scaling up and down will result in your ASG restoring the number of instances to the Desired Capacity.

If you want to manually scale up and down, you could set your Max and Min to a wide value, and move Desired Capacity within it.

So you could do Max=10, Min=1, Desired=3. Then you could scale up or down just by changing the Desired Capacity. (This is how we use Auto Scaling, and I think why I gave you a bad answer before.)

If you want to terminate an instance and change the Desired Capacity at the same time, the CLI can do that.

See Terminate instance in ASG, and the CLI ASG documentation more generally.

Upvotes: 29

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