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 Skills

• walkeros-understanding-destinations - Destination interface
• walkeros-understanding-sources - Source interface

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