[aws-cloudwatch] Support region and account in MathExpression

[arnulfojr]

When trying to specify a cross region/cross account math expression the MathExpression requires also reference to the region and account.

Use Case

Creating cross account and cross region math expressions.

Proposed Solution

The Metric class already supports specifying region and accountId but the MathExpression metric doesnt, we could simply just add it

Other

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

---

This is a 🚀 Feature Request

[arnulfojr]

Is there a reason why when generating the metrics graph JSON the check regionIfDifferentFromStack and accountIfDifferentFromStack is made?

https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-cloudwatch/lib/private/env-tokens.ts#L3,L15

this seems to be breaking xaxr stacks e.g.,

• metric A with data from [account x, region f]
• metric B with data from [account y, region g]
• Stack in [account z, region g]

regionIfDifferentFromStack will not render a region (g) for the metric B and the graph will not work.

[rix0rrr]

I would have expected account and region to be independent. Are you saying they must both be present or both be absent?

Otherwise I don't really see how:

{ metric: B, account: Y }

Will fail to locate the metric when deployed into (account Z, region G).

[priyajeet]

Hi, is this still being worked on?

[CDillinger]

I created a wrapper class as a workaround for my use case (using CDK version 1.97.0). Hoping this can help anyone else who is blocked.

import * as cloudwatch from 'monocdk/aws-cloudwatch';

export interface XaxrMathExpressionProps extends cloudwatch.MathExpressionProps {
    /**
     * (experimental) Account which this metric comes from.
     *
     * @default - Deployment account.
     * @experimental
     */
    readonly account?: string;
    /**
     * (experimental) Region which this metric comes from.
     *
     * @default - Deployment region.
     * @experimental
     */
    readonly region?: string;
}

export class XaxrMathExpression implements cloudwatch.IMetric {
    private mathExpression: cloudwatch.MathExpression;

    constructor(private readonly props: XaxrMathExpressionProps) {
        this.mathExpression = new cloudwatch.MathExpression(props);
    }

    toAlarmConfig(): cdk.aws_cloudwatch.MetricAlarmConfig {
        throw new Error('Method not implemented.');
    }
    toGraphConfig(): cdk.aws_cloudwatch.MetricGraphConfig {
        throw new Error('Method not implemented.');
    }

    public with(options: cloudwatch.MathExpressionOptions): cloudwatch.IMetric {
        return new XaxrMathExpression({
            ...this.props,
            ...options
        });
    }

    public toMetricConfig(): cloudwatch.MetricConfig {
        const defaultMetricConfig = this.mathExpression.toMetricConfig();
        return {
            ...defaultMetricConfig,
            renderingProperties: {
                ...defaultMetricConfig.renderingProperties,
                accountId: this.props.account,
                region: this.props.region
            }
        }
    }
}

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