Configure SQS Dead letter Queue to raise a cloud watch alarm on receiving a message

Question

I was working with Dead letter Queue in Amazon SQS. I want that whenever a new message is received by the queue it should raise a CloudWatch alarm. The problem is I configured an alarm on the metric: number_of_messages_sent of the queue but this metric don't work as expected in case of Dead letter Queues as mentioned in the Amazon SQS Dead-Letter Queues - Amazon Simple Queue Service documentation.

Now some suggestions on this were use number_of_messages_visible but I am not sure how to configure this in an alarm. So if i set that the value of this metric>0 then this is not same as getting a new message in the queue. If an old message is there then the metric value will always be >0. I can do some kind of mathematical expression to get the delta in this metric for some defined period (let's say a minute) but I am looking for some better solution.

Answer 1

I struggled with the same problem and the answer for me was to use NumberOfMessagesSent instead. Then I could set my criteria for new messages that came in during my configured period of time. Here is what worked for me in CloudFormation.

Note that individual alarms do not occur if the alarm stays in an alarm state from constant failure. You can setup another alarm to catch those. ie: Alarm when 100 errors occur in 1 hour using the same method.

Updated: Because the metrics for NumberOfMessagesReceived and NumberOfMessagesSent are dependent on how the message is queued, I have devised a new solutions for our needs using the metric ApproximateNumberOfMessagesDelayed after adding a delay to the dlq settings. If you are adding the messages to the queue manually then NumberOfMessagesReceived will work. Otherwise use ApproximateNumberOfMessagesDelayed after setting up a delay.

MyDeadLetterQueue:
    Type: AWS::SQS::Queue
    Properties:
      MessageRetentionPeriod: 1209600  # 14 days
      DelaySeconds: 60 #for alarms

DLQthresholdAlarm:
 Type: AWS::CloudWatch::Alarm
    Properties:
      AlarmDescription: "Alarm dlq messages when we have 1 or more failed messages in 10 minutes"
      Namespace: "AWS/SQS"
      MetricName: "ApproximateNumberOfMessagesDelayed"
      Dimensions:
        - Name: "QueueName"
          Value:
            Fn::GetAtt:
              - "MyDeadLetterQueue"
              - "QueueName"
      Statistic: "Sum"
      Period: 300  
      DatapointsToAlarm: 1 
      EvaluationPeriods: 2       
      Threshold: 1
      ComparisonOperator: "GreaterThanOrEqualToThreshold"
      AlarmActions:
        - !Ref MyAlarmTopic