Logger

walkerOS provides a centralized logging system that enables consistent logging across the collector, sources, and destinations. The logger supports four log levels and provides scoped logging for better debugging.

Configuration

Configure logging when initializing the collector:

import { startFlow } from '@walkeros/collector';

const { collector, elb } = await startFlow({
logger: {
  level: 'DEBUG', // 'ERROR' | 'WARN' | 'INFO' | 'DEBUG'
},
});

Log levels

Level	Value	Description
ERROR	0	Only errors (default)
WARN	1	Errors and warnings
INFO	2	Errors, warnings, and informational
DEBUG	3	All messages including debug information

Using the logger

The logger is automatically available in destination and source contexts:

In destinations

const myDestination = {
type: 'my-destination',
config: {},

push(event, { logger }) {
  logger.debug('Processing event', { name: event.name });

  try {
    // Send event to service
    sendToService(event);
    logger.info('Event sent successfully');
  } catch (error) {
    if (error.retryable) {
      logger.warn('Event send failed, will retry', error);
    } else {
      logger.error('Failed to send event', error);
    }
  }
},
};

In sources

const mySource = async (config, env) => {
const { logger } = env;

logger.info('Source initialized');

return {
  type: 'my-source',
  config,
  push: async (event) => {
    logger.debug('Received event', event);
    await env.push(event);
  },
};
};

Logger API

Methods

Method	Description
error(message, context?)	Log an error message
warn(message, context?)	Log a warning (degraded state, config issues)
info(message, context?)	Log an informational message
debug(message, context?)	Log a debug message
throw(message, context?)	Log an error and throw an exception (returns never)
json(data)	Output structured JSON data
scope(name)	Create a scoped logger with a prefixed scope

Error handling

The logger accepts Error objects directly and automatically extracts relevant information:

try {
await riskyOperation();
} catch (error) {
// Error properties (message, name, stack, cause) are extracted automatically
logger.error(error);

// Or with additional context
logger.error('Operation failed', { error, operation: 'sync' });
}

Boundary error logs

When an exception escapes the inner pipeline of collector.push or collector.command, the collector emits a structured error log and increments collector.status.failed. The messages are 'push failed' and 'command failed' respectively, with the full event/ingest or command/data context as structured fields. See Status — What failed counts for details and PII implications.

Internal pipeline failures from mapping, source startup, transformer init, and destination init are scoped under 'source', 'transformer:<type>', or the destination type. User-callback throws from on subscriptions are scoped under 'on'. The full list of error verbs and scopes is documented in Status — What failed counts.

Throwing errors

Use logger.throw() to log an error and throw in a single call:

function validateConfig(config) {
if (!config.apiKey) {
  // Logs the error AND throws - returns 'never' type
  logger.throw('API key is required');
}

// TypeScript knows apiKey exists here due to 'never' return type
return config.apiKey;
}

Scoped loggers

Scoped loggers prefix log messages with context information, making it easier to trace logs to their origin:

// In a destination, the logger is already scoped: [google-gtag:myGtag]
push(event, { logger }) {
logger.info('Processing'); // Output: [google-gtag:myGtag] Processing

// Create nested scope
const ga4Logger = logger.scope('ga4');
ga4Logger.debug('Sending to GA4'); // Output: [google-gtag:myGtag:ga4] Sending to GA4
}

Custom handler

For advanced use cases like sending logs to external services, configure a custom handler:

import { startFlow } from '@walkeros/collector';

const { collector, elb } = await startFlow({
logger: {
  level: 'INFO',
  handler: (level, message, context, scope, originalHandler) => {
    // Send to external logging service
    loggingService.log({
      level,
      message,
      context,
      scope: scope.join(':'),
      timestamp: Date.now(),
    });

    // Optionally call the original console handler
    originalHandler(level, message, context, scope);
  },
},
});

The handler receives:

Parameter	Type	Description
level	Level	Log level enum (ERROR=0, WARN=1, INFO=2, DEBUG=3)
message	string	The log message
context	LogContext	Additional context object
scope	string[]	Array of scope names
originalHandler	DefaultHandler	The default console handler

Testing

Use the mock logger utility for testing destinations and sources:

import { createMockLogger } from '@walkeros/core';

describe('MyDestination', () => {
it('logs events correctly', async () => {
  const mockLogger = createMockLogger();

  await myDestination.push(testEvent, {
    config: {},
    logger: mockLogger,
  });

  expect(mockLogger.info).toHaveBeenCalledWith(
    'Event sent successfully'
  );
});
});

The mock logger provides jest mock functions for all logger methods plus:

• scopedLoggers: Array of scoped loggers created via scope()