# ADR-004: Standardized Application-Wide Structured Logging

**Date**: 2025-12-12

**Status**: Accepted

**Implemented**: 2026-01-07

## Context

Our application currently uses a `logger` service, but the implementation of logging varies significantly across different modules. The `errorHandler` middleware produces high-quality, structured JSON logs for errors, but logging within route handlers and service layers is often ad-hoc, using plain strings or inconsistent object structures.

This inconsistency leads to several problems:
**Difficult Debugging**: It is hard to trace a single user request through the system or correlate events related to a specific operation.
**Ineffective Log Analysis**: Inconsistent log formats make it difficult to effectively query, filter, and create dashboards in a log management system (like Datadog, Splunk, or the ELK stack).
**Security Risks**: There is no enforced standard for redacting sensitive information (like passwords or tokens) in logs outside of the `errorHandler`, increasing the risk of accidental data exposure.
**Missing Context**: Logs often lack crucial context, such as a unique request ID, the authenticated user's ID, or the source IP address, making them less useful for diagnosing issues.

## Decision

We will adopt a standardized, application-wide structured logging policy. All log entries MUST be in JSON format and adhere to a consistent schema.

**Request-Scoped Logger with Context**: We will create a middleware that runs at the beginning of the request lifecycle. This middleware will:

- Generate a unique `request_id` for each incoming request.
- Create a request-scoped logger instance (a "child logger") that automatically includes the `request_id`, `user_id` (if authenticated), and `ip_address` in every log message it generates.
- Attach this child logger to the `req` object (e.g., `req.log`).

**Mandatory Use of Request-Scoped Logger**: All route handlers and any service functions called by them **MUST** use the request-scoped logger (`req.log`) instead of the global logger instance. This ensures all logs for a given request are automatically correlated.

**Standardized Log Schema**: All log messages should follow a base schema. The logger configuration will be updated to enforce this.
**Base Fields**: `level`, `timestamp`, `message`, `request_id`, `user_id`, `ip_address`.
**Error Fields**: When logging an error, the log entry MUST include an `error` object with `name`, `message`, and `stack`.

**Standardized Logging Practices**:
**INFO**: Log key business events, such as `User logged in` or `Flyer processed`.
**WARN**: Log recoverable errors or unusual situations that do not break the request, such as `Client Error: 404 on GET /api/non-existent-route` or `Retrying failed database connection`.
**ERROR**: Log only unhandled or server-side errors that cause a request to fail (typically handled by the `errorHandler`). Avoid logging expected client errors (like 4xx) at this level.
**DEBUG**: Log detailed diagnostic information useful during development, such as function entry/exit points or variable states.

### Example Usage

```typescript
// In a new middleware file: logger.middleware.ts
import { logger } from '../services/logger.server';
import { randomUUID } from 'crypto';

export const requestLogger = (req, res, next) => {
  const requestId = randomUUID();
  // Create a child logger with context for this request
  req.log = logger.child({
    request_id: requestId,
    user_id: req.user?.user_id, // Assumes user is attached by auth middleware
    ip_address: req.ip,
  });
  next();
};

// In server/app setup:
// app.use(requestLogger); // Add this early in the middleware chain

// In a route handler:
router.get('/:id', async (req, res, next) => {
  // Use the request-scoped logger
  req.log.info({ flyerId: req.params.id }, 'Fetching flyer by ID');
  try {
    // ... business logic ...
    res.json(flyer);
  } catch (error) {
    // The error itself will be logged with full context by the errorHandler
    next(error);
  }
});
```

## Consequences

### Positive

**Enhanced Observability**: Every log line from a single request can be instantly grouped and analyzed, dramatically speeding up debugging.
**Improved Security**: Centralizing the addition of context (like `user_id`) reduces the chance of developers manually logging sensitive data.
**Scalable Log Management**: Consistent JSON logs are easily ingested and indexed by any modern log aggregation tool.
**Clearer Code**: Removes the need to manually pass contextual information (like user ID) down to service functions just for logging purposes.

### Negative

**Refactoring Effort**: Requires adding the `requestLogger` middleware and refactoring all routes and services to use `req.log` instead of the global `logger`.
**Slight Performance Overhead**: Creating a child logger for every request adds a minor performance cost, though this is negligible for most modern logging libraries.

## Implementation Details

### Logger Configuration

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

```typescript
import pino from 'pino';

const isProduction = process.env.NODE_ENV === 'production';
const isTest = process.env.NODE_ENV === 'test';

export const logger = pino({
  level: isProduction ? 'info' : 'debug',
  transport:
    isProduction || isTest
      ? undefined
      : {
          target: 'pino-pretty',
          options: {
            colorize: true,
            translateTime: 'SYS:standard',
            ignore: 'pid,hostname',
          },
        },
  redact: {
    paths: [
      'req.headers.authorization',
      'req.headers.cookie',
      '*.body.password',
      '*.body.newPassword',
      '*.body.currentPassword',
      '*.body.confirmPassword',
      '*.body.refreshToken',
      '*.body.token',
    ],
    censor: '[REDACTED]',
  },
});
```

### Request Logger Middleware

Located in `server.ts`:

```typescript
const requestLogger = (req: Request, res: Response, next: NextFunction) => {
  const requestId = randomUUID();
  const user = req.user as UserProfile | undefined;
  const start = process.hrtime();

  // Create request-scoped logger
  req.log = logger.child({
    request_id: requestId,
    user_id: user?.user.user_id,
    ip_address: req.ip,
  });

  req.log.debug({ method: req.method, originalUrl: req.originalUrl }, 'INCOMING');

  res.on('finish', () => {
    const duration = getDurationInMilliseconds(start);
    const { statusCode, statusMessage } = res;
    const logDetails = {
      user_id: (req.user as UserProfile | undefined)?.user.user_id,
      method: req.method,
      originalUrl: req.originalUrl,
      statusCode,
      statusMessage,
      duration: duration.toFixed(2),
    };

    // Include request details for failed requests (for debugging)
    if (statusCode >= 400) {
      logDetails.req = { headers: req.headers, body: req.body };
    }

    if (statusCode >= 500) req.log.error(logDetails, 'Request completed with server error');
    else if (statusCode >= 400) req.log.warn(logDetails, 'Request completed with client error');
    else req.log.info(logDetails, 'Request completed successfully');
  });

  next();
};

app.use(requestLogger);
```

### TypeScript Support

The `req.log` property is typed via declaration merging in `src/types/express.d.ts`:

```typescript
import { Logger } from 'pino';

declare global {
  namespace Express {
    export interface Request {
      log: Logger;
    }
  }
}
```

### Automatic Sensitive Data Redaction

The Pino logger automatically redacts sensitive fields:

```json
// Before redaction
{
  "body": {
    "email": "user@example.com",
    "password": "secret123",
    "newPassword": "newsecret456"
  }
}

// After redaction (in logs)
{
  "body": {
    "email": "user@example.com",
    "password": "[REDACTED]",
    "newPassword": "[REDACTED]"
  }
}
```

### Log Levels by Scenario

| Level | HTTP Status | Scenario                                           |
| ----- | ----------- | -------------------------------------------------- |
| DEBUG | Any         | Request incoming, internal state, development info |
| INFO  | 2xx         | Successful requests, business events               |
| WARN  | 4xx         | Client errors, validation failures, not found      |
| ERROR | 5xx         | Server errors, unhandled exceptions                |

### Service Layer Logging

Services accept the request-scoped logger as an optional parameter:

```typescript
export async function registerUser(email: string, password: string, reqLog?: Logger) {
  const log = reqLog || logger; // Fall back to global logger

  log.info({ email }, 'Registering new user');
  // ... implementation

  log.debug({ userId: user.user_id }, 'User created successfully');
  return user;
}

// In route handler
router.post('/register', async (req, res, next) => {
  await authService.registerUser(req.body.email, req.body.password, req.log);
});
```

### Log Output Format

**Development** (pino-pretty):

```text
[2026-01-09 12:34:56.789] INFO (request_id=abc123): Request completed successfully
    method: "GET"
    originalUrl: "/api/flyers"
    statusCode: 200
    duration: "45.23"
```

**Production** (JSON):

```json
{
  "level": 30,
  "time": 1704812096789,
  "request_id": "abc123",
  "user_id": "user_456",
  "ip_address": "192.168.1.1",
  "method": "GET",
  "originalUrl": "/api/flyers",
  "statusCode": 200,
  "duration": "45.23",
  "msg": "Request completed successfully"
}
```

### Routes Using `req.log`

All route files have been migrated to use the request-scoped logger:

- `src/routes/auth.routes.ts`
- `src/routes/user.routes.ts`
- `src/routes/flyer.routes.ts`
- `src/routes/ai.routes.ts`
- `src/routes/admin.routes.ts`
- `src/routes/budget.routes.ts`
- `src/routes/recipe.routes.ts`
- `src/routes/gamification.routes.ts`
- `src/routes/personalization.routes.ts`
- `src/routes/stats.routes.ts`
- `src/routes/health.routes.ts`
- `src/routes/system.routes.ts`

## Key Files

- `src/services/logger.server.ts` - Pino logger configuration
- `src/services/logger.client.ts` - Client-side logger (for frontend)
- `src/types/express.d.ts` - TypeScript declaration for `req.log`
- `server.ts` - Request logger middleware

## Related ADRs

- [ADR-001](./0001-standardized-error-handling.md) - Error handler uses `req.log` for error logging
- [ADR-026](./0026-standardized-client-side-structured-logging.md) - Client-side logging strategy