Bug: child logger with overwritten options clear all other options

Expected Behaviour

When creating a child logger (see “Code snippet” section) and passing some options the child logger should retain all the parent properties/options except the ones explicitly overwritten.

Current Behaviour

The child logger won’t have all the same options that the parent has whenever the user creates the logger while also passing some options to overwrite. For example:

const logger = new Logger({
  serviceName: "MyService",
  sampleRateValue: 0.01,
});
const auditLogger = logger.createChild({
  sampleRateValue: 1,
});

From the example above, I would expect auditLogger to have the same serviceName as logger and only differ in sampleRateValue - however as you can see from the logs in the “Debugging logs” section, serviceName is undefined in auditLogger.

Code snippet

import { Logger, injectLambdaContext } from "@aws-lambda-powertools/logger";
import middy from "@middy/core";

const logger = new Logger({
  serviceName: "MyService",
  sampleRateValue: 0.01,
});
const auditLogger = logger.createChild({
  sampleRateValue: 1,
});

export const handler = middy(async () => {
  logger.info("I log sometimes");
  auditLogger.info("I always log");
})
  .use(injectLambdaContext(logger))
  .use(injectLambdaContext(auditLogger));

Possible Solution

Rework the createChild function in Logger to account for passed options and add unit tests to confirm fix & prevent future regressions.

Steps to Reproduce

Deploy a function using the code shared in the “Code snippet” section, then execute it, finally observe the logs (see example in Debugging logs section).

AWS Lambda Powertools for TypeScript version

latest

AWS Lambda function runtime

16.x

Packaging format used

Npm

Debugging logs

{
    "cold_start": false,
    "function_arn": "arn:aws:lambda:eu-west-1:536254204126:function:lambda-powertools-playground-fn",
    "function_memory_size": 128,
    "function_name": "lambda-powertools-playground-fn",
    "function_request_id": "6bb4c509-31d0-4dc7-9e16-f08f22a70b59",
    "level": "INFO",
    "message": "I log sometimes",
    "sampling_rate": 0.01,
    "service": "MyService",
    "timestamp": "2022-11-13T16:47:24.460Z",
    "xray_trace_id": "1-63711f9c-4c1fc3284f97466b7ff88473"
}
{
    "cold_start": false,
    "function_arn": "arn:aws:lambda:eu-west-1:536254204126:function:lambda-powertools-playground-fn",
    "function_memory_size": 128,
    "function_name": "lambda-powertools-playground-fn",
    "function_request_id": "6bb4c509-31d0-4dc7-9e16-f08f22a70b59",
    "level": "INFO",
    "message": "I always log",
    "sampling_rate": 1,
    "service": "MyService",
    "timestamp": "2022-11-13T16:47:24.461Z",
    "xray_trace_id": "1-63711f9c-4c1fc3284f97466b7ff88473"
}

[dreamorosi]

Hey @shdq, indeed I copy-pasted the logs wrong. I just checked and it’s like you say (which is the issue I was trying to report).

Thank you for stepping up on this, I’ll assign the issue to you and will be happy to collaborate.

[github-actions[bot]]

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