Cloudwatch Get InstanceId - amazon-web-services

Question

How do I get the InstanceId of all instances of Cloudwatch Alarm, I am trying to create a cloudwatch alarm to send email if the disk reach 90% usage.
Resources:
EC2DiskHealth:
Type: AWS::CloudWatch::Alarm
Properties:
AlarmDescription: { "Fn::Join" : ["", [{ "Ref" : "AWSEBEnvironmentName" }, ": Disk Usage" ]]}
Namespace: System/Linux
MetricName: DiskSpaceAvailable
Dimensions:
- Name: InstanceId
Value : { "Ref" : "instance-id" }
- Name: Filesystem
Value: /dev/xvda1
- Name: MountPath
Value: /
Statistic: Average
Period: 60
EvaluationPeriods: 5
Threshold:
Fn::GetOptionSetting:
OptionName: ELBHealth
DefaultValue: "90"
ComparisonOperator: GreaterThanThreshold
AlarmActions:
- arn:aws:sns:awsregion:sns
InsufficientDataActions:
- arn:aws:sns:awsregion:sns
OKActions:
- arn:aws:sns:awsregion:sns
Output:
I should be able to get the instance-id in order for the alarm to work.
Dimensions:
- Name: InstanceId
Value : { "Ref" : "instance-id" }
Error:
Service:AmazonCloudFormation, Message:Template format error: Unresolved resource dependencies [instance-id] in the Resources block of the template

Answer 1

It appears that your situation is:
You have some existing Amazon EC2 instances
You are running some script/code on the instances that send a metric called DiskSpaceAvailable at regular intervals to Amazon CloudWatch
You wish to create a CloudFormation template
The template should create an Alarm for every EC2 instance when DiskSpaceAvailable exceeds a certain metric
This is not possible.
An Amazon CloudWatch template can create resources and can refer to resources, but it cannot go out and discover resources, nor perform loops over discovered resources.
A template could, for example, create an instance and then add an alarm specifically for that instance. However, it won't auto-discover resources.
You can write an AWS Lambda-backed Custom Resource that can do whatever you wish (you'd have to write the code), but your code would need to create the alarms rather than CloudFormation.
Bottom line: Your use-case is best done via your own code (Lambda or just straight code) rather than using CloudFormation.

Answer 2

Your question is to send an email when cloudwatch detect that the disk instance is over 90% used.
It is the basics of cloudwatch task : create the email notification in the cloudwatch alarm itself, set the emails and save.
more details here (it is an example related to CPU, but it is the same principle) :
https://docs.aws.amazon.com/fr_fr/AmazonCloudWatch/latest/monitoring/US_AlarmAtThresholdEC2.html
If you want your instance-id from the instance itself, its instance-id is available through its metadata :
curl http://169.254.169.254/latest/meta-data/instance-id