Creating a composite alarm

Composite alarms determine their states by monitoring the states of other alarms. You can use composite alarms to reduce alarm noise. For example, you can create a composite alarm where the underlying metric alarms go into ALARM when they meet specific conditions. You then can set up your composite alarm to go into ALARM and send you notifications when the underlying metric alarms go into ALARM by configuring the underlying metric alarms never to take actions. Currently, composite alarms can take the following actions:

• Notify SNS topics

• Create OpsItems in AWS Systems Manager Ops Center

• Create incidents in AWS Systems Manager Incident Manager

If you are not using CloudWatch cross-account observability, the composite alarm can monitor alarms in the current account.

Note

All of the underlying alarms in your composite alarm must be in the same account and the same Region as your composite alarm. However, if you set up a composite alarm in a CloudWatch cross-account observability monitoring account, the underlying alarms can watch metrics in different source accounts and in the monitoring account itself. For more information, see CloudWatch cross-account observability.

A single composite alarm can monitor 100 underlying alarms, and 150 composite alarms can monitor a single underlying alarm.

Rule expressions

All composite alarms contain rule expressions. Rule expressions tell composite alarms which other alarms to monitor and determine their states from. Rule expressions can refer to metric alarms and composite alarms. When you reference an alarm in a rule expression, you designate a function to the alarm that determines which of the following three states the alarm will be in:

• ALARM

  ALARM ("alarm-name or alarm-ARN") is TRUE if the alarm is in ALARM.

• OK

  OK ("alarm-name or alarm-ARN") is TRUE if the alarm is in ALARM.

• INSUFFICIENT_DATA

  INSUFFICIENT_DATA (“alarm-name or alarm-ARN") is TRUE if the named alarm is in INSUFFICIENT_DATA.

Note

TRUE always evaluates to TRUE, and FALSE always evaluates to FALSE.

Example expressions

The request parameter AlarmRule supports the use of the logical operators AND, OR, and NOT, so you can combine multiple functions into a single expressions. The following example expressions show how you can configure the underlying alarms in your composite alarm:

• ALARM(CPUUtilizationTooHigh) AND ALARM(DiskReadOpsTooHigh)

  The expression specifies that the composite alarm goes into ALARM only if CPUUtilizationTooHigh and DiskReadOpsTooHigh are in ALARM.

• ALARM(CPUUtilizationTooHigh) AND NOT ALARM(DeploymentInProgress)

  The expression specifies that the composite alarm goes into ALARM if CPUUtilizationTooHigh is in ALARM and DeploymentInProgress is not in ALARM. This is an example of a composite alarm that reduces alarm noise during a deployment window.

• (ALARM(CPUUtilizationTooHigh) OR ALARM(DiskReadOpsTooHigh)) AND OK(NetworkOutTooHigh)

  The expression specifies that the composite alarm goes into ALARM if (ALARM(CPUUtilizationTooHigh) or (DiskReadOpsTooHigh) is in ALARM and (NetworkOutTooHigh) is in OK. This is an example of a composite alarm that reduces alarm noise by not sending you notifications when either of the underlying alarms aren’t in ALARM while a network issue is occurring.

Creating a composite alarm

To create a composite alarm

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

2. In the navigation pane, choose Alarms, and then choose In alarm.

3. From the list of alarms, select the check box next to each of the existing alarms that you want to reference in your rule expression, and then choose Create composite alarm.

4. Under Specify composite alarm conditions, specify the rule expression for your new composite alarm.

   Note

   Automatically, the alarms that you selected from the list of alarms are listed in the Conditions box. By default, the ALARM function has been designated to each of your alarms, and each of your alarms is joined by the logical operator OR.

   You can use the following substeps to modify your rule expression:

   1. You can change the required state for each of your alarms from ALARM to OK or INSUFFICENT_DATA.

   2. You can change the logical operator in your rule expression from OR to AND or NOT, and you can add parentheses to group your functions.

   3. You can include other alarms in your rule expression or delete alarms from your rule expression.

   Example: Rule expression with conditions

   (ALARM("CPUUtilizationTooHigh") OR 
   ALARM("DiskReadOpsTooHigh")) AND 
   OK("NetworkOutTooHigh")

   In the example rule expression where the composite alarm goes into ALARM when ALARM ("CPUUtilizationTooHigh" or ALARM("DiskReadOpsTooHigh") is in ALARM at the same time as OK("NetworkOutTooHigh") is in OK.

5. When finished, choose Next.

6. Under Configure actions, you can choose from the following:

   For Notification

   • Select an exisiting SNS topic, Create a new SNS topic, or Use a topic ARN to define the SNS topic that will receive the notification.

   • Add notification, so your alarm can send multiple notifications for the same alarm state or different alarm states.

   • Remove to stop your alarm from sending notifications or taking actions.

   For Systems Manager action

   • Add Systems Manager action, so your alarm can perform an SSM action when it goes into ALARM.

   To learn more about Systems Manager actions, see Configuring CloudWatch to create OpsItems from alarms in the AWS Systems Manager User Guide and Incident creation in the Incident Manager User Guide. To create an alarm that performs an SSM Incident Manager action, you must have the correct permissions. For more information, see Identity-based policy examples for AWS Systems Manager Incident Manager in the Incident Manager User Guide.

7. When finished, choose Next.

8. Under Add name and description, enter an alarm name and optional description for your new composite alarm. Your alarm name must contain only ASCII characters.

9. When finished, choose Next.

10. Under Preview and create, confirm your information, and then choose Create composite alarm.

    Note

    You can create a cycle of composite alarms, where one composite alarm and another composite alarm depend on each other. If you find yourself in this scenario, your composite alarms stop being evaluated, and you can't delete your composite alarms because they're dependent on each other. The easiest way to break the cycle of dependecy between your composite alarms is to change the function AlarmRule in one of your composite alarms to False.

Composite alarm action suppression

With composite alarm action suppression, you define alarms as suppressor alarms. Suppressor alarms prevent composite alarms from taking actions. For example, you can specify a suppressor alarm that represents the status of a supporting resource. If the supporting resource is down, the suppressor alarm prevents the composite alarm from sending notifications. Composite alarm action suppression helps you reduce alarm noise, so you spend less time managing your alarms and more time focusing on your operations.

You specify suppressor alarms when you configure composite alarms. Any alarm can function as a suppressor alarm. When a suppressor alarm changes states from OK to ALARM, its composite alarm stops taking actions. When a suppressor alarm changes states from ALARM to OK, its composite alarm resumes taking actions.

WaitPeriod and ExtensionPeriod

When you specify a suppressor alarm, you set the parameters WaitPeriod and ExtensionPeriod. These parameters prevent composite alarms from taking actions unexpectedly while suppressor alarms change states. Use WaitPeriod to compensate for any delays that can occur when a suppressor alarm changes from OK to ALARM. For example, if a suppressor alarm changes from OK to ALARM within 60 seconds, set WaitPeriod to 60 seconds.

In the image, the composite alarm changes from OK to ALARM at t2. A WaitPeriod starts at t2 and ends at t8. This gives the suppressor alarm time to change states from OK to ALARM at t4 before it suppresses the composite alarm's actions when the WaitPeriod expires at t8.

Use ExtensionPeriod to compensate for any delays that can occur when a composite alarm changes to OK following a suppressor alarm changing to OK. For example, if a composite alarm changes to OK within 60 seconds of a suppressor alarm changing to OK, set ExtensionPeriod to 60 seconds.

In the image, the suppressor alarm changes from ALARM to OK at t2. An ExtensionPeriod starts at t2 and ends at t8. This gives the composite alarm time to change from ALARM to OK before the ExtensionPeriod expires at t8.

Composite alarms don't take actions when WaitPeriod and ExtensionPeriod become active. Composite alarms take actions that are based on their currents states when ExtensionPeriod and WaitPeriod become inactive. We recommended that you set the value for each parameter to 60 seconds, as CloudWatch evaluates metric alarms every minute. You can set the parameters to any integer in seconds.

The following examples describe in more detail how WaitPeriod and ExtensionPeriod prevent composite alarms from taking actions unexpectedly.

Note

In the following examples, WaitPeriod is configured as 2 time units, and ExtensionPeriod is configured as 3 time units.

Examples

Example 1: Actions are not suppressed after WaitPeriod

In the image, the composite alarm changes states from OK to ALARM at t2. A WaitPeriod starts at t2 and ends at t4, so it can prevent the composite alarm from taking actions. After the WaitPeriod expires at t4, the composite alarm takes its actions because the suppressor alarm is still in OK.

Example 2: Actions are suppressed by alarm before WaitPeriod expires

In the image, the composite alarm changes states from OK to ALARM at t2. A WaitPeriod starts at t2 and ends at t4. This gives the suppressor alarm time to change states from OK to ALARM at t3. Because the suppressor alarm changes states from OK to ALARM at t3, the WaitPeriod that started at t2 is discarded, and the suppressor alarm now stops the composite alarm from taking actions.

Example 3: State transition when actions are suppressed by WaitPeriod

In the image, the composite alarm changes states from OK to ALARM at t2. A WaitPeriod starts at t2 and ends at t4. This gives the suppressor alarm time to change states. The composite alarm changes back to OK at t3, so the WaitPeriod that started at t2 is discarded. A new WaitPeriod starts at t3 and ends at t5. After the new WaitPeriod expires at t5, the composite alarm takes its actions.

Example 4: State transition when actions are suppressed by alarm

In the image, the composite alarm changes states from OK to ALARM at t2. The suppressor alarm is already in ALARM. The suppressor alarm stops the composite alarm from taking actions.

Example 5: Actions are not suppressed after ExtensionPeriod

In the image, the composite alarm changes states from OK to ALARM at t2. A WaitPeriod starts at t2 and ends at t4. This gives the suppressor alarm time to change states from OK to ALARM at t3 before it suppresses the composite alarm's actions until t6. Because the suppressor alarm changes states from OK to ALARM at t3, the WaitPeriod that started at t2 is discarded. At t6, the suppressor alarm changes to OK. An ExtensionPeriod starts at t6 and ends at t9. After the ExtensionPeriod expires, the composite alarm takes its actions.

Example 6: State transition when actions are suppressed by ExtensionPeriod

In the image, the composite alarm changes states from OK to ALARM at t2. A WaitPeriod starts at t2 and ends at t4. This gives the suppressor alarm time to change states from OK to ALARM at t3 before it suppresses the composite alarm's actions until t6. Because the suppressor alarm changes states from OK to ALARM at t3, the WaitPeriod that started at t2 is discarded. At t6, the suppressor alarm changes back to OK. An ExtensionPeriod starts at t6 and ends at t8. When the composite alarm changes back to OK at t7, the ExtensionPeriod is discarded, and a new WaitPeriod starts at t7 and ends at t9. After the new WaitPeriod expires, the composite alarm can take its actions.

Tip

If you replace the action suppressor alarm, any active WaitPeriod or ExtensionPeriod is discarded.