To create an alarm based on a math expression

Open the CloudWatch console at https://console.aws.amazon.com/cloudwatch/ .

In the navigation pane, choose Alarms, Create Alarm.

Choose Select Metric and do one of the following: Choose the service namespace that contains a specific metric. Continue choosing options as they appear to narrow the choices. When a list of metrics appears, select the check box next to the right metric.

In the search box, enter the name of a metric, dimension, or resource ID and press Enter. Choose one of the results and continue until a list of metrics appears. Select the check box next to the right metric. (Optional) To add another metric to use in the math expression, under All metrics, choose All, find the specific metric, and then select the check box next to it. You can add up to 10 metrics.

Choose Graphed metrics. For each metric added, do the following: Under Statistic, choose one of the statistics or predefined percentiles, or specify a custom percentile (for example, p95.45 ). Under Period, choose the evaluation period for the alarm. When evaluating the alarm, each period is aggregated into one data point. You can also choose whether the y-axis legend appears on the left or right while you're creating the alarm. This preference is used only while you're creating the alarm.

Choose Add a math expression. A new row appears for the expression.

On the new row, under the Details column, enter the math expression and press Enter. For information about the functions and syntax that you can use, see Metric Math Syntax and Functions. To use a metric or the result of another expression as part of the formula for this expression, use the value shown in the Id column: for example, m1+m2 or e1-MIN(e1). You can change the value of Id. It can include numbers, letters, and underscores, and it must start with a lowercase letter. Changing the value of Id to a more meaningful name can also make the alarm graph easier to understand.

(Optional) Add more math expressions, using both metrics and the results of other math expressions in the formulas of the new math expressions.

When you have the expression to use for the alarm, clear the check boxes to the left of every other expression and every metric on the page. Only the check box next to the expression to use for the alarm should be selected. The expression that you choose for the alarm must produce a single time series and show only one line on the graph. Then choose Select metric. The Specify metric and conditions page appears, showing a graph and other information about the math expression that you have selected.

For Whenever expression is, specify whether the expression must be greater than, less than, or equal to the threshold. Under than..., specify the threshold value.

Choose Additional configuration. For Datapoints to alarm, specify how many evaluation periods (data points) must be in the ALARM state to trigger the alarm. If the two values here match, you create an alarm that goes to ALARM state if that many consecutive periods are breaching. To create an M out of N alarm, specify a lower number for the first value than you specify for the second value. For more information, see Evaluating an Alarm.

For Missing data treatment, choose how to have the alarm behave when some data points are missing. For more information, see Configuring How CloudWatch Alarms Treat Missing Data.

Choose Next.

Under Notification, select an SNS topic to notify when the alarm is in ALARM state, OK state, or INSUFFICIENT_DATA state. To have the alarm send multiple notifications for the same alarm state or for different alarm states, choose Add notification. To have the alarm not send notifications, choose Remove.

To have the alarm perform Auto Scaling or EC2 actions, choose the appropriate button and choose the alarm state and action to perform.

When finished, choose Next.

Enter a name and description for the alarm. The name must contain only ASCII characters. Then choose Next.