In an AWS CloudFormation template, how can I tag an EC2 instance with it's own Id without getting circular reference errors?

Question

I've specified an EC2 instance in my CloudFormation template, and I want to tag it with it's own InstanceId, like so:

"Resources": {
    "myInstance": {
        ...
        "Tags": [
            { "Key": "instance.id", "Value": { "Ref": "myInstance" } },
            ...
        ]
    }
}

But trying to create a stack from this template generates a AmazonCloudFormationException: "Circular dependency between resources: [myInstance]"

Running instances and tagging them using the EC2 API is very straightforward:

//this is C#, but that's not significant

var instance = ec2Client.RunInstances(...) ...;
var id = instance.InstanceId;
ec2Client.CreateTags(new CreateTagRequest
{
    Resources = { id },
    Tags = { new Tag { Key = "instance.id", Value = id } }
});

This approach arises naturally from the fact that instance tags cannot be created as part of the RunInstances operation, so all tags, not just the self-identifier, must be applied in a subsequent API operation.

So... can I accomplish the same thing using CloudFormation? Thanks very much!

(cross-posted to the AWS Forums)

Charles Engelke Charles Engelke · Accepted Answer · 2015-05-13T17:49:15

I don't think you can. CloudFormation only lets you tag a resource at creation time, and the value of the tag you want doesn't exist until the resource is created.

But I don't think this has to be a problem. The two use cases I can think of for tags are looking at an instance and seeing the tag values, or finding instances with certain tag values. You can see or find by instance-id without having a tag containing it.

In an AWS CloudFormation template, how can I tag an EC2 instance with it's own Id without getting circular reference errors?

2 Answers