# ADR-031: Granular Debug Logging Strategy

**Date**: 2026-02-10

**Status**: Accepted

**Source**: Imported from flyer-crawler project (ADR-052)

**Related**: [ADR-027](ADR-027-application-wide-structured-logging.md), [ADR-017](ADR-017-structured-logging-with-pino.md)

## Context

Global log levels (INFO vs DEBUG) are too coarse. Developers need to inspect detailed debug information for specific subsystems (e.g., `ai-service`, `db-pool`, `auth-service`) without being flooded by logs from the entire application.

When debugging a specific feature:

- Setting `LOG_LEVEL=debug` globally produces too much noise
- Manually adding/removing debug statements is error-prone
- No standard way to enable targeted debugging in production

## Decision

We will adopt a namespace-based debug filter pattern, similar to the `debug` npm package, but integrated into our Pino logger.

1. **Logger Namespaces**: Every service/module logger must be initialized with a `module` property (e.g., `logger.child({ module: 'ai-service' })`).
2. **Environment Filter**: We will support a `DEBUG_MODULES` environment variable that overrides the log level for matching modules.

## Implementation

### Core Implementation

Implemented in `src/services/logger.server.ts`:

```typescript
import pino from 'pino';

// Parse DEBUG_MODULES from environment
const debugModules = (process.env.DEBUG_MODULES || '').split(',').map((s) => s.trim());

// Base logger configuration
export const logger = pino({
  level: process.env.LOG_LEVEL || (process.env.NODE_ENV === 'production' ? 'info' : 'debug'),
  // ... other configuration
});

/**
 * Creates a scoped logger for a specific module.
 * If DEBUG_MODULES includes this module or '*', debug level is enabled.
 */
export const createScopedLogger = (moduleName: string) => {
  // If DEBUG_MODULES contains the module name or "*", force level to 'debug'
  const isDebugEnabled = debugModules.includes('*') || debugModules.includes(moduleName);

  return logger.child({
    module: moduleName,
    level: isDebugEnabled ? 'debug' : logger.level,
  });
};
```

### Service Usage Examples

```typescript
// src/services/aiService.server.ts
import { createScopedLogger } from './logger.server';

const logger = createScopedLogger('ai-service');

export async function processWithAI(data: unknown) {
  logger.debug({ data }, 'Starting AI processing');
  // ... implementation
  logger.info({ result }, 'AI processing completed');
}
```

```typescript
// src/services/authService.server.ts
import { createScopedLogger } from './logger.server';

const logger = createScopedLogger('auth-service');

export async function validateToken(token: string) {
  logger.debug({ tokenLength: token.length }, 'Validating token');
  // ... implementation
}
```

### Module Naming Convention

Use kebab-case suffixed with `-service` or `-worker`:

| Module Name     | Purpose                          | File                                  |
| --------------- | -------------------------------- | ------------------------------------- |
| `ai-service`    | AI/external API interactions     | `src/services/aiService.server.ts`    |
| `auth-service`  | Authentication and authorization | `src/services/authService.server.ts`  |
| `db-pool`       | Database connection pooling      | `src/services/database.server.ts`     |
| `cache-service` | Redis/caching operations         | `src/services/cacheService.server.ts` |
| `queue-worker`  | Background job processing        | `src/workers/queueWorker.ts`          |
| `email-service` | Email sending                    | `src/services/emailService.server.ts` |

## Usage

### Enable Debug Logging for Specific Modules

To debug only AI and authentication:

```bash
DEBUG_MODULES=ai-service,auth-service npm run dev
```

### Enable All Debug Logging

Use wildcard to enable debug logging for all modules:

```bash
DEBUG_MODULES=* npm run dev
```

### Development Environment

In `.env.development`:

```bash
# Enable debug logging for specific modules during development
DEBUG_MODULES=ai-service
```

### Production Troubleshooting

Temporarily enable debug logging for a specific subsystem:

```bash
# SSH into production server
ssh root@example.com

# Set environment variable and restart
DEBUG_MODULES=ai-service pm2 restart app-api

# View logs
pm2 logs app-api --lines 100

# Disable debug logging
pm2 unset DEBUG_MODULES app-api
pm2 restart app-api
```

### With PM2 Configuration

In `ecosystem.config.js`:

```javascript
module.exports = {
  apps: [
    {
      name: 'app-api',
      script: 'dist/server.js',
      env: {
        NODE_ENV: 'production',
        // DEBUG_MODULES is unset by default
      },
      env_debug: {
        NODE_ENV: 'production',
        DEBUG_MODULES: 'ai-service,auth-service',
      },
    },
  ],
};
```

Start with debug logging:

```bash
pm2 start ecosystem.config.js --env debug
```

## Best Practices

### 1. Use Scoped Loggers for Long-Running Services

Services with complex workflows or external API calls should use `createScopedLogger` to allow targeted debugging:

```typescript
const logger = createScopedLogger('payment-service');

export async function processPayment(payment: Payment) {
  logger.debug({ paymentId: payment.id }, 'Starting payment processing');

  try {
    const result = await externalPaymentAPI.process(payment);
    logger.debug({ result }, 'External API response');
    return result;
  } catch (error) {
    logger.error({ error, paymentId: payment.id }, 'Payment processing failed');
    throw error;
  }
}
```

### 2. Use Child Loggers for Contextual Data

Even within scoped loggers, create child loggers with job/request-specific context:

```typescript
const logger = createScopedLogger('queue-worker');

async function processJob(job: Job) {
  const jobLogger = logger.child({ jobId: job.id, jobName: job.name });

  jobLogger.debug('Starting job processing');
  // ... processing
  jobLogger.info('Job completed successfully');
}
```

### 3. Consistent Debug Message Patterns

Use consistent patterns for debug messages:

```typescript
// Function entry
logger.debug({ params: sanitizedParams }, 'Function entry: processOrder');

// External API calls
logger.debug({ url, method }, 'External API request');
logger.debug({ statusCode, duration }, 'External API response');

// State changes
logger.debug({ before, after }, 'State transition');

// Decision points
logger.debug({ condition, result }, 'Branch decision');
```

### 4. Production Usage Guidelines

- `DEBUG_MODULES` can be set in production for temporary debugging
- Should not be used continuously due to increased log volume
- Always unset after troubleshooting is complete
- Monitor log storage when debug logging is enabled

## Consequences

### Positive

- Developers can inspect detailed logs for specific subsystems without log flooding
- Production debugging becomes more targeted and efficient
- No performance impact when debug logging is disabled
- Compatible with existing Pino logging infrastructure
- Follows familiar pattern from `debug` npm package

### Negative

- Requires developers to know module names (mitigated by documentation)
- Not all services have adopted scoped loggers yet (gradual migration)
- Additional configuration complexity

## References

- [ADR-027: Application-Wide Structured Logging](ADR-027-application-wide-structured-logging.md)
- [ADR-017: Structured Logging with Pino](ADR-017-structured-logging-with-pino.md)
- [debug npm package](https://www.npmjs.com/package/debug) - Inspiration for namespace pattern
- [Pino Child Loggers](https://getpino.io/#/docs/child-loggers)
