(aws-cloudwatch): AlarmRule.anyOf returns invalid alarm rule

[andsok75]

AlarmRule.anyOf(...alarms) where alarms is empty yields "()" which is not a valid alarm rule.

Reproduction Steps

const alarms: Array<cw.Alarm> = [];
new cw.CompositeAlarm(this, 'CompositeAlarm', {
  compositeAlarmName: "CompositeAlarm",
  alarmRule: cw.AlarmRule.anyOf(...alarms),
});

Adding the above to the basic example (generated by cdk init) and running cdk synth

What did you expect to happen?

I expected a valid alarm rule to be created even for cases when the list of alarms passed to anyOf is empty.

What actually happened?

Running cdk synth and checking the generated template shows the AlarmRule as () which is not valid

CompositeAlarmF4C3D082:
Type: AWS::CloudWatch::CompositeAlarm
Properties:
AlarmName: CompositeAlarm
AlarmRule: ()

Environment

• CDK CLI Version : 1.72.0
• Framework Version:
• Node.js Version: v11.13.0
• OS :
• Language (Version):

Other

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This is 🐛 Bug Report

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[kimsa]

I ran into the same issue, especially when a service might have multiple environments and is dynamically providing the alarms in the AlarmRule.anyOf(...)

e.g.

• Alpha/Beta stage -> ends up with 0 alarms
• Gamma/Prod stage -> ends up with >0 alarms

Ideally, if AlarmRule.anyOf(...) has an empty input, it at least throws an error at build time...

Or (as I'm doing to get around this issue), provide an AlarmRule.fromBoolean(false):

    const alarms = [...someOtherFunctionThatCouldReturnAnEmptyArray()];

    return new CompositeAlarm(this.stack!, compositeAlarmName, {
      alarmRule:
        alarms.length === 0
          ? AlarmRule.fromBoolean(false)
          : AlarmRule.anyOf(...alarms),
      compositeAlarmName: compositeAlarmName,
    });

[jacklin213]

This is a very valid callout. Can we consider having CDK throw an error if expression is undefined? If we don't plan on giving alarmRule some fallback default

aws-cdk/packages/aws-cdk-lib/aws-cloudwatch/lib/alarm-rule.ts

Lines 111 to 121 in dc30faa

	private static concat(operator: Operator, ...operands: IAlarmRule[]): IAlarmRule {
	return new class implements IAlarmRule {
	public renderAlarmRule(): string {
	const expression = operands
	.map(operand => `${operand.renderAlarmRule()}`)
	.join(` ${operator} `);
	return `(${expression})`;
	}
	};
	}
	}

With something like

private static concat(operator: Operator, ...operands: IAlarmRule[]): IAlarmRule {
    return new class implements IAlarmRule {
      public renderAlarmRule(): string {
        const expression = operands
          .map(operand => `${operand.renderAlarmRule()}`)
          .join(` ${operator} `);
        if (!expression) {
            // Throw exception here
        }
        return `(${expression})`;
      }
    };
}

Otherwise users only find out during deployment time when CloudFormation fails with

Resource handler returned message: "Error in AlarmRule [Unsupported token ')' at char 1] (Service: AmazonCloudWatch ...)