Bug: Prevent a single metric have more than 100 data points

[leandrodamascena]

Expected Behaviour

Flush metric set when data point is reached to prevent potential data loss, where 100+ data points are added thus being ignored silently by CloudWatch EMF when processed in the background.

https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Embedded_Metric_Format_Specification.html#CloudWatch_Embedded_Metric_Format_Specification_structure_target

Current Behaviour

More than 100 metric data points can be added.

Code snippet

import { Metrics, MetricUnits } from '@aws-lambda-powertools/metrics';

const metrics = new Metrics({
  namespace: 'serverlessAirline',
  serviceName: 'orders',
});

export const handler = async (
  _event: unknown,
  _context: unknown
): Promise<void> => {
  metrics.addMetric('performedActionA', MetricUnits.Count, 1);
  metrics.addMetric('performedActionA', MetricUnits.Count, 2);
 // add other 100 values to the same metric
};

Steps to Reproduce

Run the code added in code snippet

Possible Solution

Serialize and flush metric data set when a single metric reaches 100 data points. Dimensions etc should be kept intact.

Powertools for AWS Lambda (TypeScript) version

latest

AWS Lambda function runtime

18.x

Packaging format used

Lambda Layers, npm

Execution logs

No response

[dreamorosi]

Hi @leandrodamascena thanks for reporting this.

I have added a (failing) test that looks like this:

test("it should publish metrics when the array of values reaches the maximum size", () => {
  // Prepare
  const metrics: Metrics = new Metrics({ namespace: TEST_NAMESPACE });
  const consoleSpy = jest.spyOn(console, "log");
  const metricName = "test-metric";

  // Act
  for (let i = 0; i <= MAX_METRICS_SIZE; i++) {
    metrics.addMetric(`${metricName}`, MetricUnits.Count, i);
  }
  metrics.publishStoredMetrics();

  // Assess
  // 2 calls to console.log: 1 for the first batch of metrics, 1 for the second batch (explicit call)
  expect(consoleSpy).toHaveBeenCalledTimes(2);
  const firstMetricsJson = JSON.parse(consoleSpy.mock.calls[0][0]) as EmfOutput;
  const secondMetricsJson = JSON.parse(consoleSpy.mock.calls[1][0]) as EmfOutput;

  expect(firstMetricsJson[metricName]).toHaveLength(MAX_METRICS_SIZE);
  expect(secondMetricsJson[metricName]).toHaveLength(1);
});

Based on my understanding of this section of the EMF Specification, the values array should not exceed length 100:

Valid values of target members depend on what is referencing them. A metric target MUST be a numeric value or an array of numeric values. Numeric array metric targets MUST NOT have more than 100 members. A dimension target MUST have a string value.

As a result, Metrics should flush the stored metrics and handle the next ones transparently - which is not the case at the moment.

We'll work on a fix and include it in the next release.

[github-actions]

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[github-actions]

This is now released under version 1.10.0!