Cloudwatch: Creating cloudwatch alarms on existing metrics

Hey guys,

I don’t know if this is a feature request or a gap in the current documentation but I was wondering if it would be possible to create cloudwatch alarms on existing metrics. For example, when creating an ELB with CDK specific metrics come out of the box like: Response times. It would be great to be able to reference an already created cloudwatch metric like:

    const myExistingMetric = fn.metricErrors({
      namespace: "SomeNameSpace",
      metricName: "someMetricName"
    });

    new Alarm(this, 'CustomeAlarm', {
      metric: myExistingMetric,
      EvaluationPeriods: 2,
      threshold: 1,
      Statistic: 'Average',
      Period: 300, // five minutes
    });

I don’t exactly know great error handling mechanisms when those metrics don’t exist, but perhaps it would even expose what metrics are available out of the box.

[zakwalters]

Is there any way to import existing metrics so that they can be used when creating alarms? I was thinking of something similar to Vpc.fromLookup.

As I understand it, the approach outlined in Doug’s link only works if you know the correct values needed to construct the metric (the namespace, metric name, and dimensions). We have a use case where a dimension of the metric is the ID of the container posting it, and so we cannot predict the dimensions from the CDK code.

It would be ideal if we could do something like:

import {Alarm, Metric, TreatMissingData} from "@aws-cdk/aws-cloudwatch";
import {App, Stack, StackProps} from "@aws-cdk/core";

export class AlarmStack extends Stack {
    constructor(scope: App, id: string, props?: StackProps) {
        super(scope, id, props);

        const containerErrorMetrics = Metric.importMetrics({ namespace: "MyContainers", metricName: "errors" });
        // containerErrorMetrics:
        // [
        //   Metric { namespace: "MyContainers", metricName: "errors", dimensions: { id: "1" } },
        //   Metric { namespace: "MyContainers", metricName: "errors", dimensions: { id: "2" } },
        // ]
        containerErrorMetrics.forEach(metric => {
            const props = {
                metric,
                threshold: 1,
                evaluationPeriods: 1,
                treatMissingData: TreatMissingData.NOT_BREACHING,
            };
            new Alarm(this, `ContainerErrors${metric.dimensions.id}Alarm`, props);
        });
    }
}

In the absence of such a method to import existing metrics, the best approach I can think of is to use ListMetrics from the CloudWatch SDK to get the metrics from the account. However, the SDK is async, so the call to ListMetrics (and the subsequent creation of alarms on these metrics) can’t be done in a constructor, as a constructor can’t be async. Instead, it has to be done in some sort of initalizer method. This feels like it breaks the normal CDK pattern of creating all child resources in the constructor.

Overall, my questions are:

1. Is there ever going to be a way of importing existing metrics without knowing all the dimensions?
2. What approach would you recommend in the meantime? Is it an anti-pattern to add an initialize method to our CDK classes?

[zakwalters]

We ended up deciding to create alarms with the CloudWatch SDK when the container is created rather than trying to keep these alarms in our CDK code. Answers to the questions above would still be interesting and potentially valuable others who end up on this thread though.