Request ID

import { Request, Response, NextFunction } from 'express';
import { v4 as uuidv4 } from 'uuid';

/**
 * Header names for request ID
 */
const REQUEST_ID_HEADER = 'X-Request-ID';
const CORRELATION_ID_HEADER = 'X-Correlation-ID';

/**
 * Extended request with ID fields
 */
export interface RequestWithId extends Request {
  id: string;
  correlationId?: string;
}

/**
 * Request ID Middleware
 * 
 * Generates a unique ID for each request and attaches it to:
 * - req.id (for use in application code)
 * - Response header X-Request-ID (for client correlation)
 * 
 * If the client sends X-Request-ID, it will be preserved.
 * If X-Correlation-ID is sent, it's stored for distributed tracing.
 */
export const requestId = (
  req: RequestWithId,
  res: Response,
  next: NextFunction
) => {
  // Use existing request ID from client or generate new one
  const existingId = req.headers[REQUEST_ID_HEADER.toLowerCase()] as string;
  const id = existingId || uuidv4();

  // Store correlation ID if provided (for distributed tracing)
  const correlationId = req.headers[CORRELATION_ID_HEADER.toLowerCase()] as string;

  // Attach to request object
  req.id = id;
  if (correlationId) {
    req.correlationId = correlationId;
  }

  // Set response header so client can correlate
  res.setHeader(REQUEST_ID_HEADER, id);
  if (correlationId) {
    res.setHeader(CORRELATION_ID_HEADER, correlationId);
  }

  next();
};

/**
 * Get request ID from request object (type-safe)
 */
export const getRequestId = (req: Request): string => {
  return (req as RequestWithId).id || 'unknown';
};

/**
 * Create a child logger context with request ID
 * Useful for structured logging
 * 
 * @example
 * const context = createLogContext(req);
 * logger.info('User logged in', context);
 * // Output: { requestId: '...', correlationId: '...', message: 'User logged in' }
 */
export const createLogContext = (req: Request) => {
  const typedReq = req as RequestWithId;
  return {
    requestId: typedReq.id,
    correlationId: typedReq.correlationId,
    method: req.method,
    path: req.path,
    ip: req.ip,
  };
};