import { Request, Response, NextFunction } from 'express';
/**
 * Express middleware for generating and managing correlation IDs across distributed systems.
 *
 * Provides request tracing capabilities by generating unique correlation IDs (request IDs)
 * that can be used to track requests as they flow through multiple services. Supports both
 * generating new IDs and propagating existing IDs from incoming headers.
 *
 * @class CorrelationMiddleware
 *
 * @example
 * ```typescript
 * // Basic usage
 * const correlationMiddleware = new CorrelationMiddleware();
 * app.use(correlationMiddleware.generateRequestId());
 *
 * // With existing request ID from headers
 * // If client sends 'x-request-id' header, it will be reused
 * // Otherwise, a new UUID will be generated
 *
 * // The middleware automatically:
 * // - Reads existing 'x-request-id' header from incoming request
 * // - Generates new UUID if no header exists
 * // - Sets the request ID in both request and response headers
 * // - Makes the request ID available throughout the request lifecycle
 * ```
 *
 * @example
 * ```typescript
 * // Accessing request ID in route handlers
 * app.get('/api/users', (req: Request, res: Response) => {
 *   const requestId = req.headers['x-request-id'];
 *   logger.info('Processing user request', { requestId });
 *   // Logs: { message: "Processing user request", requestId: "abc-123-xyz" }
 * });
 *
 * // Response headers will include:
 * // x-request-id: abc-123-xyz
 * ```
 */
export declare class CorrelationMiddleware {
    /**
     * Generates Express middleware for request ID correlation.
     *
     * Creates middleware that ensures every request has a unique correlation ID,
     * either by reusing an existing 'x-request-id' header from the client or
     * generating a new UUIDv4. The ID is propagated through both request and
     * response headers for distributed tracing.
     *
     * @returns {function} Express middleware function
     *
     * @example
     * ```typescript
     * // Application setup
     * const app = express();
     * const correlation = new CorrelationMiddleware();
     *
     * // Apply middleware globally
     * app.use(correlation.generateRequestId());
     *
     * // Or apply to specific routes
     * app.use('/api/*', correlation.generateRequestId());
     *
     * // The middleware will:
     * // 1. Check for existing 'x-request-id' in request headers
     * // 2. Use existing ID if present, generate new UUID if not
     * // 3. Set the ID in request headers for downstream use
     * // 4. Set the ID in response headers for client propagation
     * ```
     */
    generateRequestId(): (req: Request, res: Response, next: NextFunction) => void;
}