Enhancement: Support Custom Composite Alarm Expression in AlarmFactory.AddCompositeAlarmProps

[jeffersonbezerra]

Description

Currently, AlarmFactory.AddCompositeAlarmProps interface only supports a single compositeOperator (AND/OR) to be applied across all
alarms in the composite alarm. This limitation prevents the creation of more complex alarm conditions that require mixed operators or NOT
conditions.

Current Behavior

The current implementation only allows expressions like:

• (Alarm1 AND Alarm2 AND Alarm3)
• (Alarm1 OR Alarm2 OR Alarm3)

Desired Behavior

Support for custom composite alarm expressions would allow more sophisticated monitoring scenarios, such as:

• (Alarm1 AND NOT (Alarm2 OR Alarm3 OR Alarm4))
• ((Alarm1 AND Alarm2) OR (Alarm3 AND NOT Alarm4))

Use Case

Many monitoring scenarios require more complex logic than simple AND/OR operations. For example:

• Trigger an alarm when a service is down (Alarm1) but NOT during scheduled maintenance windows (Alarm2)
• Alert when system load is high (Alarm1) AND memory usage is critical (Alarm2) AND NOT during known batch processing periods (Alarm3)

Proposed Solution

Add a new optional property to AddCompositeAlarmProps:

interface AddCompositeAlarmProps {
  // existing properties
  compositeOperator?: CompositeAlarmOperator;
  // new property
  customExpression?: string;
}

When customExpression is provided, it would take precedence over compositeOperator, allowing users to define custom composite
alarm rules using the CloudWatch composite alarm expression syntax.

Seems like you left a lot of the GenAI prompt boilerplate in, so I'm not sure if the issue fully captures what you'd like to see.

That said, the general problem I can see is that given the existing APIs, there isn't an obvious way to necessarily indicate which alarms to do what with given all of the relevant alarms that are being filtered down to. There's nothing really stopping a user from using the various methods to then compose another CompositeAlarm with more complex expressions.

[jeffersonbezerra]

HI @echeung-amzn ,

given

const monitoring = new AmazonMonitoringFacade(...)
const alarmFactory = monitoring.createAlarmFactory("anAlarmPrefix");
alarmFactory.addCompositeAlarm([alarmWithAnnotationInstance1, alarmWithAnnotationInstance2, alarmWithAnnotationInstance3, alarmWithAnnotationInstance4], {
    compositeOperator: CompositeAlarmOperator.AND, //Or CompositeAlarmOperator.OR
})

On the alarm factory, I can pass a list of AlarmWithAnnotation, but to compose these alarms in a composite alarm, I can only inform a single CompositeAlarmOperator AND, or OR to be used between all alarms informed

addCompositeAlarm(alarms: AlarmWithAnnotation[], props: AddCompositeAlarmProps): CompositeAlarm;

[jeffersonbezerra]

There's nothing really stopping a user from using the various methods to then compose another CompositeAlarm with more complex expressions.

actually I saw that the addCompositeAlarm allows the user to inform an array of AlarmWithAnnotation

addCompositeAlarm(alarms: AlarmWithAnnotation[], props: AddCompositeAlarmProps): CompositeAlarm;

and that an AlamrWithAnnotation is backed by an AlarmBase which can be a compositeAlarm export declare class CompositeAlarm extends AlarmBase {, But I'm creating instances of this metric with

AlarmFactory.addAlarm(metric: MetricWithAlarmSupport, props: AddAlarmProps): AlarmWithAnnotation;. Doing that in order to inherit AmazonMonitoringFacade configurations by default.