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So now let's talk about

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what is an auto scaling group.

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So when we deploy a website or an application,

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the load can change over time because we may have

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more users visiting our websites over time.

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And we've seen that in the cloud, in AWS,

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it's possible for us to create

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and also get rid of servers very quickly

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with the EC2 instance creation API call.

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So, if you wanna automate this,

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we can create an auto-scaling group.

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So the goal of the ASG is to scale out,

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that means add EC2 instances and you need to remember this,

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scale out to match an increased load

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or to scale in that means removing EC2 instances

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to match a decreased load.

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And so therefore the size of our ASG

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is going to vary over time.

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Overall, we can also define parameters

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to ensure we'll have a minimum

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and a maximum number of EC2 instances running at any time

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in our ASG.

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ASG also have the superpower

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that if you are pairing it with a load balancer,

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then any EC2 instances as part of the ASG

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will be linked to the load balancer.

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Another superpower is that

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if one instance is deemed unhealthy, it is terminated

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and a new EC2 instance is created to replace it.

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So, auto scaling groups are free

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and you're only going to pay

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for whatever resources are created underneath

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such as your EC2 instances.

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So let's see how an ESG works in AWS.

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So we set a minimum capacity,

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which is how many instances you want at minimum in your ASG.

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For example, two.

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Then you set a desired capacity,

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which is how many instances you want in your ASG,

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for example, four and then you set a maximum capacity

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which is how many instances

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at a maximum do I want in my ASG.

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And then that means that if you move the desire capacity

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to higher number,

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but that's still less than a maximum capacity,

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then you can scale out as needed.

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That means scale out means adding EC2 instances

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and so therefore your ASG can grow bigger

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and bigger and bigger.

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In this instance, the maximum capacity is seven.

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As I said, the ASG also works with a load balancer.

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So if we have four instances registered in our ASG,

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then the ELB is going to distribute traffic

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to all these instances right away

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and so your users can access a load balanced website.

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But the ELB also has the ability

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to check for the health of your EC2 instances

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using the health check

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and that health check can be passed on to the ASG.

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That means that the ASG can terminate EC2 instances

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if they are deemed unhealthy by the load balancer,

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which is very handy.

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Also, if you scale out, that means if you add EC2 instances,

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then of course the ELB is going to send traffic

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to them as well and spread the load.

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Therefore, it's a really great combination

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to use a load balancer and an auto scaling group.

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Now, in terms of attributes to create your ASG,

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you need to create a launch template.

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There used to be a thing called launch configurations

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but it's deprecated, but the idea is the same.

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A launch template contains information

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on how to launch EC2 instances within your ASG.

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So you have information about the AMI

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and the instance type, the EC2 user data,

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the EBS volumes, security groups, SSH key pair,

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IAM roles for your EC2 instances,

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network and subnet information

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as well as load balancer information and more if you want.

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And so all these parameters look a lot like

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the ones we specified when we created an EC2 instance.

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On top of it, your ASG has a min size,

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a max size and an initial capacity we need to define

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as well as scaling policies.

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Talking about scaling policies,

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let's see how CloudWatch alarm integrates with auto scaling.

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So you don't know what CloudWatch is yet of course,

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but let me just tell you a little bit about it right now.

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So it's possible to scale in and out,

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an ASG based on CloudWatch alarms.

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So for example, we have an ASG right here

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with three EC2 instances

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and the alarm is going to be triggered

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and therefore we're going to get a scale out activity.

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So what would trigger an alarm?

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Well, it's a metric you can return, for example,

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for the average CPU or any custom metric you want.

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So for example, if the average CPU

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as a whole for your ASG is too high,

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then you need to add EC2 instances

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and therefore the alarm is going to be triggered

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and is going to trigger a scaling activity

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in your auto scaling group

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and this is why it's called an auto scaling group

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because paired with alarms,

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there is an automatic aspect of scaling behind the scenes.

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So based on the alarm, we can create scale out policies.

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That means increasing the number of instances

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or we can create scale in policies

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to decrease the number of instances.

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And all these things together are what composes ASG.

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I hope you liked it and I will see you in the next lecture.