Auto increment EC2 instance Name

Question

I have seen many questions on Stackoverflow asking if there is a way to auto-increment instance names. For example:

foo1
foo2
fooN

I am trying to see if there is a way of doing this in Powershell. I am using AutoLaunchConfiguration/ASG to spin up the instances. Is there a programmatic way of auto-incrementing instance ways? Every resource I have seen so far says it's possible but no on provides a means of doing this. Here are some of the resources I am looking at:

• Method to provide incremental name to ec2 instances created using auto scaling group in aws
• How to create variable number of EC2 instance resources in Cloudformation template?

Does anyone have a working example of doing this using AutoLaunchConfiguration and ASG? If so, can you please provide a link/article that can provide me with this information?

Answer 1

The first thing is to decide what is meant by "auto-incrementing".

For example, if these instances already exist:

• foo1
• foo2

then obviously the next instance would be foo3.

But let's say that an instance has previously been terminated because load reduced, and the currently-running instances are:

• foo1
• foo3

Then the question is whether the next instance should be foo2 or foo4.

If the answer is foo4, then think of the situation where foo4 is later terminated and another instance is launched. Should it be foo4 or foo5 (since there was previously a foo4).

It really boils down to determining why you want an auto-incremented name.

The reasons are typically:

• The need for a unique name
• The need for a human-friendly name
• The need for a hint as to the order in which instances where launched

If the need is for a unique name, then the Instance ID can fulfil this need perfectly, without needing an additional name.

Let's assume you want to go with a human-friendly name and you are okay to re-use names that were previously used (eg if foo1 and foo3 exists, then use foo2 next). In such a case, you would need something that logically inspects the existing instances and finds the first unused number.

If, on the other hand, you never wanted to re-use names, then you would need some place to store the 'current' number so that it can be incremented for the next instance.

Then, finally, comes the question of how to assign the auto-incremented name (which is your original question, but the above is also very important to understand first). My recommendation would be:

• Add some code to each instance that is triggered through the User Data so that it will run when the instance is launched (eg PowerShell)
• The code will inspect the existing instances with describe-instances or the place where the count is kept, then assign itself the next number
• The code can then call create-tags on itself, adding a Name tag with the appropriate name