# Tutorial: Creating an Amazon CloudWatch alarm for Multi-AZ DB cluster replica lag for Amazon RDS
Tutorial: Creating a CloudWatch alarm for DB cluster replica lag

You can create an Amazon CloudWatch alarm that sends an Amazon SNS message when replica lag for a Multi-AZ DB cluster has exceeded a threshold. An alarm watches the `ReplicaLag` metric over a time period that you specify. The action is a notification sent to an Amazon SNS topic or Amazon EC2 Auto Scaling policy.

**To set a CloudWatch alarm for Multi-AZ DB cluster replica lag**

1. Sign in to the AWS Management Console and open the CloudWatch console at [https://console.aws.amazon.com/cloudwatch/](https://console.aws.amazon.com/cloudwatch/).

1. In the navigation pane, choose **Alarms**, **All alarms**.

1. Choose **Create alarm**.

1. On the **Specify metric and conditions** page, choose **Select metric**.

1. In the search box, enter the name of your Multi-AZ DB cluster and press Enter.

   The following image shows the **Select metric** page with a Multi-AZ DB cluster named `rds-cluster` entered.  
![\[Select metric page\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-select-metric.png)

1. Choose **RDS**, **Per-Database Metrics**.

1. In the search box, enter **ReplicaLag** and press Enter, then select each DB instance in the DB cluster.

   The following image shows the **Select metric** page with the DB instances selected for the **ReplicaLag** metric.  
![\[Select metric page with DB instances selected for the ReplicaLag metric\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-metric-replica-lag.png)

   This alarm considers the replica lag for all three of the DB instances in the Multi-AZ DB cluster. The alarm responds when any DB instance exceeds the threshold. It uses a math expression that returns the maximum value of the three metrics. Start by sorting by metric name, and then choose all three **ReplicaLag** metrics.

1. From **Add math**, choose **All functions**, **MAX**.  
![\[The Add math setting\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-select-metric-math.png)

1. Choose the **Graphed metrics** tab, and edit the details for **Expression1** to **MAX([m1,m2,m3])**.

1. For all three **ReplicaLag** metrics, change the **Period** to **1 minute**.

1. Clear selection from all metrics except for **Expression1**.

   The **Select metric** page should look similar to the following image.  
![\[The Select metric page with the metric selected\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-select-metric-expression1.png)

1. Choose **Select metric**.

1. On the **Specify metric and conditions** page, change the label to a meaningful name, such as **ClusterReplicaLag**, and enter a number of seconds in **Define the threshold value**. For this tutorial, enter **1200** seconds (20 minutes). You can adjust this value for your workload requirements.

   The **Specify metric and conditions** page should look similar to the following image.  
![\[The Specify metric and conditions page\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-specify-metric-conditions.png)

1. Choose **Next**, and the **Configure actions** page appears.

1. Keep **In alarm** selected, choose **Create new topic**, and enter the topic name and a valid email address.  
![\[The Configure actions page\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-configure-actions.png)

1. Choose **Create topic**, and then choose **Next**.

1. On the **Add name and description** page, enter the **Alarm name** and **Alarm description**, and then choose **Next**.  
![\[The Add name and description page\]](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/images/multi-az-db-cluster-cw-tutorial-add-name-and-description.png)

1. Preview the alarm that you're about to create on the **Preview and create** page, and then choose **Create alarm**.