# ADR-052: Granular Debug Logging Strategy

**Date**: 2026-01-11

**Status**: Accepted (Fully Implemented)

**Related**: [ADR-004](0004-standardized-application-wide-structured-logging.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`) without being flooded by logs from the entire application.

## 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 (Completed 2026-01-11)

Implemented in [src/services/logger.server.ts:140-150](src/services/logger.server.ts#L140-L150):

```typescript
const debugModules = (process.env.DEBUG_MODULES || '').split(',').map((s) => s.trim());

export const createScopedLogger = (moduleName: string) => {
  // If DEBUG_MODULES contains "ai-service" or "*", force level to 'debug'
  const isDebugEnabled = debugModules.includes('*') || debugModules.includes(moduleName);

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

### Adopted Services (Completed 2026-01-26)

Services currently using `createScopedLogger`:

- `ai-service` - AI/Gemini integration ([src/services/aiService.server.ts:1020](src/services/aiService.server.ts#L1020))
- `flyer-processing-service` - Flyer upload and processing ([src/services/flyerProcessingService.server.ts:20](src/services/flyerProcessingService.server.ts#L20))

## Usage

### Enable Debug Logging for Specific Modules

To debug only AI and flyer processing:

```bash
DEBUG_MODULES=ai-service,flyer-processing-service npm run dev
```

### Enable All Debug Logging

Use wildcard to enable debug logging for all modules:

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

### Common Module Names

| Module Name                | Purpose                                  | File                                            |
| -------------------------- | ---------------------------------------- | ----------------------------------------------- |
| `ai-service`               | AI/Gemini API interactions               | `src/services/aiService.server.ts`              |
| `flyer-processing-service` | Flyer upload, validation, and processing | `src/services/flyerProcessingService.server.ts` |

## 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.

2. **Use Child Loggers for Contextual Data**: Even within scoped loggers, create child loggers with job/request-specific context:

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

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

3. **Module Naming Convention**: Use kebab-case suffixed with `-service` or `-worker` (e.g., `ai-service`, `email-worker`).

4. **Production Usage**: `DEBUG_MODULES` can be set in production for temporary debugging, but should not be used continuously due to increased log volume.

## Examples

### Development Debugging

Debug AI service issues during development:

```bash
# Dev container
DEBUG_MODULES=ai-service npm run dev

# Or via PM2
DEBUG_MODULES=ai-service pm2 restart flyer-crawler-api-dev
```

### Production Troubleshooting

Temporarily enable debug logging for a specific subsystem:

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

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

# View logs
pm2 logs flyer-crawler-api --lines 100

# Disable debug logging
pm2 unset DEBUG_MODULES flyer-crawler-api
pm2 restart flyer-crawler-api
```

## 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

**Negative**:

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