AWSTemplateFormatVersion: 2010-09-09
Description: 'CloudFormation template for creating CloudWatch Alarm and associated SNS topic to notify by email when threshold has been triggered.'

Parameters:
  SNSTopicName:
    Description: Please enter a SNS Topic Name For Alarm Notifications.
    Default: SNSTopic-ReservedInstanceExpiringSoon
    Type: String
    ConstraintDescription: Please enter a SNS Topic Name For Alarm Notifications.
  
  SNSNotificationEmail:
    Description: Please enter notifier email address.
    Type: String
    ConstraintDescription: Please enter notifier email address.
    Default: example@example.com
  
  CloudWatchAlarmName:
    Default: CWAlarm-ReservedInstanceExpiringSoon
    Description: Please enter name for CloudWatch Billing Alarm (no spaces and only underscores permitted).
    Type: String
  
  CloudWatchAlarmThreshold:
    Default: 30
    Description: Please enter a whole number (e.g. 30) for the CloudWatch alarm threshold.
    Type: String
    AllowedPattern: '[0-9][0-9]*'
    ConstraintDescription: Please enter a whole number (e.g. 30)
  
  ReservedInstanceId:
    Description: Please enter Reserved Instance Id for tracking days remaining (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).
    Default: 00000000-0000-0000-0000-000000000000
    Type: String
    ConstraintDescription: Please enter Reserved Instance Id for tracking days remaining.
  
  RIDaysRemainingLambdaFunction:
    Description: Please enter Lambda function ARN for Reserved Instance days remaining checking.
    Default: 'arn:aws:lambda:us-east-1:111122223333:function:IdentifySoonExpiringReservedInstances'
    ConstraintDescription: Please enter Lambda function ARN for Reserved Instance days remaining checking.
    Type: String

Resources:
  SNSTopic:
    Type: 'AWS::SNS::Topic'
    Properties:
      Subscription:
        - Endpoint: !Ref SNSNotificationEmail
          Protocol: email
      TopicName: !Ref SNSTopicName
  
  CloudWatchAlarm:
    Type: 'AWS::CloudWatch::Alarm'
    Properties:
      AlarmName: !Ref CloudWatchAlarmName
      AlarmDescription: !Join 
        - ''
        - - >-
            CloudWatch Alarm that triggers a notification via the SNS topic when
            the threshold is below:
          - !Ref CloudWatchAlarmThreshold
          - ' days.'
      MetricName: RIExpireCountdown
      Namespace: AWS/ReservedInstances
      Statistic: Maximum
      Period: '86400'
      EvaluationPeriods: '1'
      Threshold: !Ref CloudWatchAlarmThreshold
      ComparisonOperator: LessThanThreshold
      AlarmActions:
        - !Ref SNSTopic
      Dimensions:
        - Name: Reserved Instance Id
          Value: !Ref ReservedInstanceId
      InsufficientDataActions:
        - !Ref SNSTopic
  
  EventBridgeScheduleRule:
    Type: 'AWS::Events::Rule'
    Properties:
      Description: "EventBridge rule for invoking lambda function every day"
      ScheduleExpression: "cron(0 13 * * ? *)"
      State: "ENABLED"
      Targets:
        - 
          Arn: !Ref 'RIDaysRemainingLambdaFunction'
          Id: "TargetFunctionV1"
  
  PermissionForEventsToInvokeLambda:
    Type: 'AWS::Lambda::Permission'
    Properties:
      FunctionName: !Ref 'RIDaysRemainingLambdaFunction'
      Action: 'lambda:InvokeFunction'
      Principal: events.amazonaws.com
      SourceArn:
        Fn::GetAtt:
          - "EventBridgeScheduleRule"
          - "Arn"

Outputs:
  SNSTopicARN:
    Description: SNS Topic ARN
    Value: !Ref SNSTopic
    Export:
      Name: !Sub '${AWS::StackName}:SNSTopic-ARN'
  
  CloudWatchAlarmURL:
    Description: CloudWatch Alarm  URL
    Value: !Join 
      - ''
      - - 'https://console.aws.amazon.com/cloudwatch/home?region='
        - !Ref 'AWS::Region'
        - '#alarmsV2:alarm/'
        - !Ref CloudWatchAlarmName
    Export:
      Name: !Sub '${AWS::StackName}:CloudWatchAlarm-URL'
  
  CloudWatchAlarmARN:
    Description: CloudWatch Alarm ARN
    Value: !GetAtt CloudWatchAlarm.Arn
    Export:
      Name: !Sub '${AWS::StackName}:CloudWatchAlarm-ARN'