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docs(metrics): document InvokeStore isolation and local testing #5486

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docs(metrics): document InvokeStore isolation and local testing
leiwilson Jul 20, 2026
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docs(metrics): clarify in-memory store is default on Lambda
leiwilson Jul 22, 2026
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10 changes: 10 additions & 0 deletions docs/features/metrics.md
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Expand Up @@ -456,6 +456,16 @@ You can customize how Metrics logs warnings and debug messages to standard outpu

## Testing your code

### Lambda environment vs local

In AWS Lambda, Metrics writes [Embedded Metric Format (EMF)](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Embedded_Metric_Format.html) blobs to standard output. The Lambda runtime/CloudWatch agent picks those log lines up — you do not call CloudWatch `PutMetricData` yourself.

When Lambda multi-concurrency is enabled (`AWS_LAMBDA_MAX_CONCURRENCY` is set), Metrics (and related buffered state such as dimensions/metadata) automatically uses [`@aws/lambda-invoke-store`](https://www.npmjs.com/package/@aws/lambda-invoke-store) for **per-invocation isolation**. No application code changes are required; Powertools selects the store at runtime.

Outside that Lambda multi-concurrency context Metrics uses an in-memory store by default. That applies to standard Lambda functions, local runs and unit tests. Buffered metrics are still flushed as EMF to stdout when you flush (middleware/decorator/`logMetrics`), so asserting emitted metrics locally works the same way.

@svozza svozza Jul 23, 2026

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Outside that Lambda multi-concurrency context Metrics uses an in-memory store by default. That applies to standard Lambda functions, local runs and unit tests. Buffered metrics are still flushed as EMF to stdout when you flush (middleware/decorator/`logMetrics`), so asserting emitted metrics locally works the same way.
Outside that Lambda multi-concurrency context Metrics uses an in-memory store by default. This applies to Lambda on-demand functions, local runs, and unit tests. Buffered metrics are still flushed as EMF to stdout when you flush (middleware/decorator/`logMetrics`), so asserting emitted metrics locally works the same way.


### Asserting metrics in unit tests

When unit testing your code that uses the Metrics utility, you may want to silence the logs emitted by the utility. To do so, you can set the `POWERTOOLS_DEV` environment variable to `true`. This instructs the utility to not emit any logs to standard output.

If instead you want to spy on the logs emitted by the utility, you must set the `POWERTOOLS_DEV` environment variable to `true` in conjunction with the `POWERTOOLS_METRICS_DISABLED` environment variable set to `false`.
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