veza/apps/web/src/services/offlineQueue.ts

/**
 * Offline Request Queue Service
 * FE-API-015: Queue requests when offline and replay when back online
 * 
 * Stores failed requests due to network issues and replays them when connection is restored
 */

import { AxiosRequestConfig, AxiosResponse } from 'axios';
import { apiClient } from './api/client';
import { logger } from '@/utils/logger';

/**
 * Queued request with metadata
 */
export interface QueuedRequest {
  id: string;
  config: AxiosRequestConfig;
  timestamp: number;
  retryCount: number;
  priority: 'high' | 'normal' | 'low';
}

/**
 * Options for queueing requests
 */
export interface QueueOptions {
  priority?: 'high' | 'normal' | 'low';
  maxRetries?: number;
  retryDelay?: number;
}

/**
 * Offline Queue Service
 * Manages a queue of failed requests to be retried when connection is restored
 */
class OfflineQueueService {
  private queue: QueuedRequest[] = [];
  private isProcessing = false;
  private maxQueueSize = 100; // Maximum number of queued requests
  private defaultMaxRetries = 3;
  private defaultRetryDelay = 1000; // 1 second

  /**
   * Initialize the service
   */
  constructor() {
    // Load queue from storage on initialization
    this.loadQueue();
    
    // Listen for online events to process queue
    if (typeof window !== 'undefined') {
      window.addEventListener('online', () => {
        logger.info('[OfflineQueue] Connection restored, processing queue');
        this.processQueue();
      });
    }
  }

  /**
   * Check if we're currently offline
   */
  private isOffline(): boolean {
    if (typeof navigator === 'undefined') {
      return false;
    }
    return !navigator.onLine;
  }

  /**
   * Generate a unique ID for a request
   */
  private generateRequestId(): string {
    return `req_${Date.now()}_${Math.random().toString(36).substr(2, 9)}`;
  }

  /**
   * Add a request to the queue
   */
  async queueRequest(
    config: AxiosRequestConfig,
    options: QueueOptions = {},
  ): Promise<string> {
    const {
      priority = 'normal',
      maxRetries = this.defaultMaxRetries,
    } = options;

    // Check queue size limit
    if (this.queue.length >= this.maxQueueSize) {
      // Remove oldest low-priority request
      const lowPriorityIndex = this.queue.findIndex((req) => req.priority === 'low');
      if (lowPriorityIndex !== -1) {
        this.queue.splice(lowPriorityIndex, 1);
      } else {
        // Remove oldest request if no low-priority found
        this.queue.shift();
      }
    }

    const queuedRequest: QueuedRequest = {
      id: this.generateRequestId(),
      config,
      timestamp: Date.now(),
      retryCount: 0,
      priority,
    };

    // Insert based on priority (high first, then normal, then low)
    const priorityOrder = { high: 0, normal: 1, low: 2 };
    const insertIndex = this.queue.findIndex(
      (req) => priorityOrder[req.priority] > priorityOrder[priority],
    );
    
    if (insertIndex === -1) {
      this.queue.push(queuedRequest);
    } else {
      this.queue.splice(insertIndex, 0, queuedRequest);
    }

    // Save to storage
    await this.saveQueue();

    logger.info(`[OfflineQueue] Request queued: ${config.method?.toUpperCase()} ${config.url}`, {
      requestId: queuedRequest.id,
      priority,
      queueSize: this.queue.length,
    });

    return queuedRequest.id;
  }

  /**
   * Process the queue when back online
   */
  async processQueue(): Promise<void> {
    if (this.isProcessing || this.isOffline() || this.queue.length === 0) {
      return;
    }

    this.isProcessing = true;
    logger.info(`[OfflineQueue] Processing ${this.queue.length} queued requests`);

    // Process requests in order (high priority first)
    while (this.queue.length > 0 && !this.isOffline()) {
      const request = this.queue[0];
      
      try {
        // Retry the request
        const response = await apiClient.request(request.config);
        
        // Success - remove from queue
        this.queue.shift();
        await this.saveQueue();
        
        logger.info(`[OfflineQueue] Request succeeded: ${request.config.method?.toUpperCase()} ${request.config.url}`, {
          requestId: request.id,
        });
        
        // Small delay between requests to avoid overwhelming the server
        await new Promise((resolve) => setTimeout(resolve, 100));
      } catch (error) {
        // Check if we should retry
        request.retryCount++;
        const maxRetries = this.defaultMaxRetries;
        
        if (request.retryCount >= maxRetries) {
          // Max retries reached - remove from queue
          logger.error(`[OfflineQueue] Request failed after ${maxRetries} retries: ${request.config.method?.toUpperCase()} ${request.config.url}`, {
            requestId: request.id,
            error,
          });
          this.queue.shift();
          await this.saveQueue();
        } else {
          // Move to end of queue for retry
          this.queue.shift();
          this.queue.push(request);
          await this.saveQueue();
          
          // Wait before retrying
          await new Promise((resolve) => setTimeout(resolve, this.defaultRetryDelay * request.retryCount));
        }
        
        // If we went offline again, stop processing
        if (this.isOffline()) {
          logger.warn('[OfflineQueue] Connection lost, stopping queue processing');
          break;
        }
      }
    }

    this.isProcessing = false;
    
    if (this.queue.length > 0) {
      logger.info(`[OfflineQueue] Queue processing complete, ${this.queue.length} requests remaining`);
    } else {
      logger.info('[OfflineQueue] All queued requests processed successfully');
    }
  }

  /**
   * Get the current queue size
   */
  getQueueSize(): number {
    return this.queue.length;
  }

  /**
   * Get all queued requests (for debugging/monitoring)
   */
  getQueue(): QueuedRequest[] {
    return [...this.queue];
  }

  /**
   * Clear the queue
   */
  async clearQueue(): Promise<void> {
    this.queue = [];
    await this.saveQueue();
    logger.info('[OfflineQueue] Queue cleared');
  }

  /**
   * Remove a specific request from the queue
   */
  async removeRequest(requestId: string): Promise<boolean> {
    const index = this.queue.findIndex((req) => req.id === requestId);
    if (index !== -1) {
      this.queue.splice(index, 1);
      await this.saveQueue();
      logger.info(`[OfflineQueue] Request removed from queue: ${requestId}`);
      return true;
    }
    return false;
  }

  /**
   * Save queue to localStorage
   */
  private async saveQueue(): Promise<void> {
    try {
      if (typeof window !== 'undefined' && window.localStorage) {
        const serialized = JSON.stringify(this.queue);
        localStorage.setItem('veza_offline_queue', serialized);
      }
    } catch (error) {
      logger.error('[OfflineQueue] Failed to save queue to localStorage', { error });
    }
  }

  /**
   * Load queue from localStorage
   */
  private async loadQueue(): Promise<void> {
    try {
      if (typeof window !== 'undefined' && window.localStorage) {
        const serialized = localStorage.getItem('veza_offline_queue');
        if (serialized) {
          const parsed = JSON.parse(serialized);
          // Validate and filter out old requests (older than 24 hours)
          const oneDayAgo = Date.now() - 24 * 60 * 60 * 1000;
          this.queue = parsed.filter((req: QueuedRequest) => req.timestamp > oneDayAgo);
          
          if (this.queue.length !== parsed.length) {
            await this.saveQueue();
          }
          
          logger.info(`[OfflineQueue] Loaded ${this.queue.length} requests from storage`);
        }
      }
    } catch (error) {
      logger.error('[OfflineQueue] Failed to load queue from localStorage', { error });
      this.queue = [];
    }
  }

  /**
   * Check if a request should be queued
   * Some requests (like GET) might not need to be queued
   */
  shouldQueueRequest(config: AxiosRequestConfig): boolean {
    const method = config.method?.toUpperCase();
    
    // Don't queue GET requests (they can be retried fresh)
    if (method === 'GET') {
      return false;
    }
    
    // Queue mutation requests (POST, PUT, DELETE, PATCH)
    return ['POST', 'PUT', 'DELETE', 'PATCH'].includes(method || '');
  }
}

// Singleton instance
export const offlineQueue = new OfflineQueueService();