---

name: using-logger description: Use when working with sources/destinations to understand standard logging patterns, replace console.log, or add logging to external API calls. Covers DRY principles, when to log, and migration patterns.

Using the walkerOS Logger

Overview

The logger is walkerOS's standard logging system, available in all sources and destinations via env.logger or logger parameter. It provides scoped, level-aware logging that replaces console.log.

Core principle: Don't log what the collector already logs. Only log meaningful operations like external API calls, transformations, and validation errors.

Logger Access

In Sources

export const sourceFetch = async (
  config: PartialConfig,
  env: Types['env'], // env.logger is available here
): Promise<FetchSource> => {
  // Logger is scoped automatically by collector: [type:sourceId]
  env.logger.info('Server listening on port 3000');
};

In Destinations

export const destinationDataManager: DestinationInterface = {
  async init({ config, env, logger }) {
    // logger parameter is scoped automatically: [datamanager]
    logger.debug('Auth client created');
  },

  async push(event, { config, data, env, logger }) {
    // logger parameter is scoped: [datamanager]
    logger.debug('API response', { status: 200 });
  },
};

Note: You don't need to create or configure the logger—it's provided automatically with proper scoping.

Logger Methods

interface Logger.Instance {
  error(message: string | Error, context?: unknown | Error): void;
  info(message: string | Error, context?: unknown | Error): void;
  debug(message: string | Error, context?: unknown | Error): void;
  throw(message: string | Error, context?: unknown): never;
  scope(name: string): Logger.Instance;
}

Log Levels

• ERROR (0): Always visible—use for errors only
• INFO (1): High-level operations (server startup, event processed)
• DEBUG (2): Low-level details (API calls, transformations)

Default: ERROR only (must configure to see INFO/DEBUG)

Context Parameter

All methods accept optional structured context:

logger.debug('Sending to API', {
  endpoint: '/events',
  method: 'POST',
  eventCount: 5,
});
// Output: DEBUG [datamanager] Sending to API { endpoint: '/events', method: 'POST', eventCount: 5 }

When to Log (and When NOT to)

❌ DON'T Log These (Collector Handles)

• Init status: "Initializing...", "Init started", "Init complete"
• Push status: "Processing event...", "Event received"
• Generic status: "Settings validated", "Config loaded"
• Duplicate scoping: Don't add source/dest name (already in scope)

Why: Collector can log these automatically since it calls init/push and has scoped logger.

✅ DO Log These (Meaningful Operations)

• External API calls: Before/after with request/response details
• Auth operations: Token refresh, client creation/failures
• Transformations: Complex mappings or data processing
• Validation errors: Always use logger.throw for fatal errors

Usage Patterns

Pattern 1: Validation Errors (Always Use logger.throw)

// ✅ GOOD - Fatal configuration error
async init({ config, logger }) {
  const { apiKey, projectId } = config.settings || {};

  if (!apiKey) {
    logger.throw('Config settings apiKey missing');
  }

  if (!projectId) {
    logger.throw('Config settings projectId missing');
  }
}

Why logger.throw:

• Logs the error at ERROR level (always visible)
• Throws Error automatically (no separate throw needed)
• Collector catches and handles gracefully
• Never returns (TypeScript type: never)

Pattern 2: External API Calls

// ✅ GOOD - Log external calls with context
async push(event, { config, logger }) {
  const endpoint = 'https://api.vendor.com/events';

  // Log before call
  logger.debug('Calling API', {
    endpoint,
    method: 'POST',
    eventId: event.id,
  });

  const response = await fetch(endpoint, {
    method: 'POST',
    body: JSON.stringify(event),
  });

  // Log after call
  logger.debug('API response', {
    status: response.status,
    ok: response.ok,
  });

  if (!response.ok) {
    const errorText = await response.text();
    logger.throw(`API error (${response.status}): ${errorText}`);
  }
}

Pattern 3: Auth Operations

// ✅ GOOD - Log auth client creation
async init({ config, logger }) {
  try {
    const authClient = await createAuthClient(config.settings);
    logger.debug('Auth client created');

    return {
      env: { authClient },
    };
  } catch (error) {
    logger.throw(
      `Authentication failed: ${error instanceof Error ? error.message : 'Unknown error'}`,
    );
  }
}

Pattern 4: Server Startup (Sources)

// ✅ GOOD - Log server listening (high-level info)
if (settings.port !== undefined) {
  server = app.listen(settings.port, () => {
    env.logger.info(
      `Express source listening on port ${settings.port}\n` +
        `   POST ${settings.path} - Event collection (JSON body)\n` +
        `   GET ${settings.path} - Pixel tracking (query params)\n` +
        `   OPTIONS ${settings.path} - CORS preflight`,
    );
  });
}

Anti-Patterns (What NOT to Do)

❌ BAD: Verbose Init Logging

async init({ logger }) {
  logger.debug('Data Manager init started'); // Redundant
  logger.info('Data Manager initializing...'); // Redundant
  logger.debug('Settings validated'); // Redundant

  const authClient = await createAuthClient();
  logger.debug('Auth client created'); // OK

  logger.info('Data Manager ready'); // Redundant
}

Problem: Collector knows when init is called. Only log meaningful operations (auth client creation).

❌ BAD: Redundant Push Logging

async push(event, { logger }) {
  logger.debug('Processing event', {
    // Redundant
    name: event.name,
    id: event.id,
  });

  // Do work...

  logger.info('Event processed'); // Redundant
}

Problem: Collector knows when push is called and can log automatically.

❌ BAD: Using console.log

// ❌ NEVER use console.log in sources/destinations
console.log('Processing event:', event.name);

// ✅ Use logger instead
logger.debug('API call', { endpoint });

Migration Checklist

When updating a source/destination to use the logger:

• Remove all console.log, console.warn, console.error statements
• Remove verbose init/push status logging (let collector handle)
• Add logger.throw for all validation errors (apiKey missing, etc.)
• Add logger.debug before external API calls (with endpoint, method)
• Add logger.debug after external API calls (with response status)
• Add logger.debug for auth operations (client creation, token refresh)
• Use structured context (objects) instead of string concatenation
• Verify tests still pass with logger mocked

Testing with Logger

Use createMockLogger from @walkeros/core in tests:

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

test('throws on missing apiKey', () => {
  const logger = createMockLogger();

  expect(() => {
    destination.init({ config: {}, logger });
  }).toThrow('Config settings apiKey missing');

  expect(logger.throw).toHaveBeenCalledWith('Config settings apiKey missing');
});

Log Level Configuration

Default log level is ERROR. To see INFO/DEBUG logs:

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

const { elb } = await startFlow({
  logger: {
    level: 'DEBUG', // Show all logs
  },
  destinations: {
    /* ... */
  },
});

Levels:

• 'ERROR': Only errors (default)
• 'INFO': Errors + info
• 'DEBUG': Everything (errors + info + debug)

Related

Key Files:

• packages/core/src/logger.ts - Logger implementation
• packages/core/src/types/logger.ts - Logger types
• packages/core/src/mockLogger.ts - Testing utilities

Best Practice Examples:

• packages/server/destinations/datamanager/src/push.ts - API logging patterns
• packages/server/sources/express/src/index.ts - Server startup logging

Needs Improvement:

• packages/server/sources/fetch/src/index.ts - Missing all logging
• packages/server/destinations/meta/src/push.ts - Missing push logging
• packages/server/destinations/aws/src/firehose/push.ts - Missing push logging

Skills:

• understanding-destinations - Destination interface
• understanding-sources - Source interface
• testing-strategy - Testing with mockLogger