Bug: @metrics.logMetrics and @logger. decorator throws error if code runs outside Lambda

[tavosansal]

Expected Behavior

Tracer module handles the code potentially running outside Lambda context (like locally). When context is needed it can guard that and not let the code error.

Current Behavior

However, the Metrics and Logger module do not.

Specifically the @metrics.logMetrics decorator throws an exception right here because context is undefined.

This is what you get:

TypeError: Cannot read properties of undefined (reading 'functionName')

For Logger, using the @logger.injectLambdaContext also ends up in an error:

TypeError: Cannot read properties of undefined (reading 'invokedFunctionArn')

As a workaround for now I have two blocks in my handler to get around this behavior:

public async handler(event: any, context?: Context): Promise<void> {
    if (context) {
      logger.addContext(context);
    }
   // .... business logic for the handler

  if (context) {
      metrics.publishStoredMetrics();
    }
}

This works but it does not feel as natural as just adding the decorators and letting power tools handle the defaults.

Code snippet

Here is a sample. Just run this code locally:

import { LambdaInterface } from '@aws-lambda-powertools/commons/types';
import { Logger } from '@aws-lambda-powertools/logger';
import { Metrics } from '@aws-lambda-powertools/metrics';

const logger = new Logger({ serviceName: 'Lambda' });
const metrics = new Metrics({ serviceName: 'Lambda', namespace: 'Lambda' });

export class Lambda implements LambdaInterface {
  @logger.injectLambdaContext()
  @metrics.logMetrics()
  public async handler(event: any, context?: any): Promise<void> {
    console.log('You will never get here.');
  }
}

const lambda = new Lambda();
export const handler = lambda.handler.bind(lambda);

(async () => {
  const event = {};
  await handler(event);
})();

Steps to Reproduce

1. Use the snippet above
2. Create new file
3. Run it locally (not in Lambda)
4. See error

Possible Solution

It would be nice if all modules had a way to guard local development like the Tracer module does.

Powertools for AWS Lambda (TypeScript) version

latest

AWS Lambda function runtime

22.x

Packaging format used

npm

Execution logs

[boring-cyborg]

Thanks for opening your first issue here! We'll come back to you as soon as we can.
In the meantime, check out the #typescript channel on our Powertools for AWS Lambda Discord: Invite link

[dreamorosi]

Hi @tavosansal - thank you for opening this issue.

I'd like to better understand your use case before making a decision. Since you're using class method decorators, you must be using TypeScript already (they're not in the JavaScript language yet, and only available in TS).

From the code snippet you shared, I see that you've set the signature of the handler having the context as optional. In practice this can never happen in Lambda, so the correct type would be to not make it optional since Lambda always passes a context object to your function handler, so the signature should be:

public async handler(event: unknown, context: Context): Promise<void> {

With that said, if you still want to consider it optional you could also do this when you call your function in your tests, and the error would go away:

(async () => {
  const event = {};
  await handler(event, {} as Context);
})();

For full context, the Tracer doesn't handle the lack of context either - it just gets automatically disabled when running outside of Lambda because it doesn't have a backend to send data to. When this happens, everything of it becomes a no-op.

Besides the local runtime behavior we're discussing, I would still suggest you pass a context to the function you're testing - this is mentioned also in the Logger docs under testing.

---

Finally as a side note, I also don't 100% agree with this being a bug. According to our tenets we intentionally optimize for AWS Lambda environments, and this does not occur when following recommendations at runtime.

[tavosansal]

Hey @dreamorosi Thank you for the explanation. I do see now that there are places in the documentation where a dummy context is passed. I made the changes you suggested and it is indeed working. Appreciate the time for the explanation. Have a great rest of your day.