Changes Feed

/**
 * Stream database changes with various filtering options
 * @param opts - Changes feed options including filters, continuous mode, and callbacks
 * @returns Object with cancel method to stop the changes feed
 */
api._changes(opts): { cancel: function };

// Basic changes monitoring
const changes = db.changes({
  since: 'now',
  live: true,
  include_docs: true
}).on('change', (change) => {
  console.log('Document changed:', change.id);
  console.log('New revision:', change.changes[0].rev);
  if (change.doc) {
    console.log('Document content:', change.doc);
  }
}).on('error', (err) => {
  console.error('Changes feed error:', err);
});

// Stop monitoring after 30 seconds
setTimeout(() => {
  changes.cancel();
  console.log('Changes feed stopped');
}, 30000);

// One-time changes since last update
db.changes({
  since: lastUpdateSeq,
  include_docs: true
}, (err, result) => {
  if (err) {
    console.error('Error getting changes:', err);
    return;
  }
  
  result.results.forEach(change => {
    console.log('Changed document:', change.id);
    console.log('Current revision:', change.changes[0].rev);
  });
  
  // Store the last sequence for next query
  lastUpdateSeq = result.last_seq;
});

/**
 * Monitor changes with filtering options
 * @param opts - Options including filter functions and document IDs
 * @returns Changes feed controller with cancel method
 */
api._changes(opts): { cancel: function };

// Filter by document IDs
const userChanges = db.changes({
  live: true,
  include_docs: true,
  doc_ids: ['user:alice', 'user:bob', 'user:charlie']
}).on('change', (change) => {
  console.log('User document changed:', change.id);
  if (change.doc) {
    console.log('User data:', {
      name: change.doc.name,
      email: change.doc.email
    });
  }
});

// Filter by design document view
const filteredChanges = db.changes({
  live: true,
  include_docs: true,
  filter: '_view',
  view: 'users/active'
}).on('change', (change) => {
  console.log('Active user changed:', change.id);
});

// Custom server-side filter
const customFilterChanges = db.changes({
  live: true,
  include_docs: true,
  filter: 'myapp/important',
  query_params: {
    priority: 'high'
  }
}).on('change', (change) => {
  console.log('Important document changed:', change.id);
});

/**
 * Set up continuous changes monitoring with reconnection
 * @param opts - Continuous monitoring options
 * @returns Changes feed controller
 */
api._changes(opts): { cancel: function };

// Continuous monitoring with heartbeat
let changesController;
let reconnectTimeout;

function startChangesMonitoring() {
  changesController = db.changes({
    live: true,
    continuous: true,
    include_docs: true,
    heartbeat: 10000, // 10 seconds
    timeout: 60000,   // 60 seconds
    since: localStorage.getItem('lastSeq') || 'now'
  }).on('change', (change) => {
    console.log('Document updated:', change.id);
    
    // Store sequence for recovery
    localStorage.setItem('lastSeq', change.seq);
    
    // Process the change
    handleDocumentChange(change);
    
  }).on('error', (err) => {
    console.error('Changes feed error:', err);
    
    // Attempt to reconnect after 5 seconds
    if (!reconnectTimeout) {
      reconnectTimeout = setTimeout(() => {
        console.log('Attempting to reconnect changes feed...');
        reconnectTimeout = null;
        startChangesMonitoring();
      }, 5000);
    }
    
  }).on('complete', (info) => {
    console.log('Changes feed completed:', info);
  });
}

function handleDocumentChange(change) {
  if (change.deleted) {
    console.log('Document deleted:', change.id);
    removeFromLocalCache(change.id);
  } else {
    console.log('Document updated:', change.id);
    updateLocalCache(change.id, change.doc);
  }
}

function stopChangesMonitoring() {
  if (changesController) {
    changesController.cancel();
    changesController = null;
  }
  
  if (reconnectTimeout) {
    clearTimeout(reconnectTimeout);
    reconnectTimeout = null;
  }
}

// Start monitoring
startChangesMonitoring();

/**
 * Monitor changes with conflict detection
 * @param opts - Options including conflicts flag
 * @returns Changes feed controller
 */
api._changes(opts): { cancel: function };

// Monitor with conflict detection
const conflictAwareChanges = db.changes({
  live: true,
  include_docs: true,
  conflicts: true
}).on('change', (change) => {
  console.log('Document changed:', change.id);
  
  if (change.doc._conflicts) {
    console.warn('Document has conflicts:', change.doc._conflicts);
    
    // Handle conflicts by fetching all conflicting revisions
    handleDocumentConflicts(change.id, change.doc._conflicts);
  } else {
    // Normal document update
    console.log('Clean document update:', change.doc);
  }
});

async function handleDocumentConflicts(docId, conflicts) {
  console.log(`Resolving conflicts for ${docId}`);
  
  try {
    // Get all conflicting revisions
    const conflictingDocs = await Promise.all(
      conflicts.map(rev => 
        new Promise((resolve, reject) => {
          db.get(docId, { rev }, (err, doc) => {
            if (err) reject(err);
            else resolve(doc);
          });
        })
      )
    );
    
    // Implement conflict resolution logic
    const resolvedDoc = resolveConflicts(conflictingDocs);
    
    // Save resolved document
    db.put(resolvedDoc, (err, result) => {
      if (err) {
        console.error('Failed to resolve conflict:', err);
      } else {
        console.log('Conflict resolved:', result.rev);
      }
    });
    
  } catch (err) {
    console.error('Error handling conflicts:', err);
  }
}

function resolveConflicts(docs) {
  // Simple conflict resolution: use most recent document
  return docs.reduce((latest, current) => {
    const latestTime = new Date(latest.updated || latest._rev);
    const currentTime = new Date(current.updated || current._rev);
    return currentTime > latestTime ? current : latest;
  });
}

// Process changes in batches for better performance
let changesBatch = [];
const BATCH_SIZE = 10;
const BATCH_TIMEOUT = 5000;
let batchTimeout;

const batchChanges = db.changes({
  live: true,
  include_docs: true
}).on('change', (change) => {
  changesBatch.push(change);
  
  // Process batch when it reaches target size
  if (changesBatch.length >= BATCH_SIZE) {
    processBatch();
  } else {
    // Set timeout to process partial batch
    if (batchTimeout) {
      clearTimeout(batchTimeout);
    }
    batchTimeout = setTimeout(processBatch, BATCH_TIMEOUT);
  }
});

function processBatch() {
  if (changesBatch.length === 0) return;
  
  console.log(`Processing batch of ${changesBatch.length} changes`);
  
  // Process all changes in the batch
  const batch = changesBatch.slice();
  changesBatch = [];
  
  if (batchTimeout) {
    clearTimeout(batchTimeout);
    batchTimeout = null;
  }
  
  // Handle batch processing
  batch.forEach(change => {
    console.log('Processing change:', change.id);
    // Your batch processing logic here
  });
}

class RobustChangesMonitor {
  constructor(db, options = {}) {
    this.db = db;
    this.options = {
      maxRetries: 5,
      retryDelay: 1000,
      backoffMultiplier: 2,
      ...options
    };
    this.currentChanges = null;
    this.retryCount = 0;
    this.isRunning = false;
  }
  
  start() {
    if (this.isRunning) return;
    
    this.isRunning = true;
    this.connectChanges();
  }
  
  stop() {
    this.isRunning = false;
    
    if (this.currentChanges) {
      this.currentChanges.cancel();
      this.currentChanges = null;
    }
  }
  
  connectChanges() {
    if (!this.isRunning) return;
    
    console.log('Starting changes feed...');
    
    this.currentChanges = this.db.changes({
      live: true,
      include_docs: true,
      since: this.options.since || 'now',
      heartbeat: 30000
    }).on('change', (change) => {
      // Reset retry count on successful change
      this.retryCount = 0;
      this.handleChange(change);
      
    }).on('error', (err) => {
      console.error('Changes feed error:', err);
      this.handleError(err);
      
    }).on('complete', (info) => {
      console.log('Changes feed completed');
      if (this.isRunning) {
        this.handleError(new Error('Changes feed completed unexpectedly'));
      }
    });
  }
  
  handleChange(change) {
    console.log('Document changed:', change.id);
    
    // Emit change event for external handlers
    if (this.options.onChange) {
      this.options.onChange(change);
    }
  }
  
  handleError(err) {
    if (!this.isRunning) return;
    
    this.retryCount++;
    
    if (this.retryCount <= this.options.maxRetries) {
      const delay = this.options.retryDelay * Math.pow(this.options.backoffMultiplier, this.retryCount - 1);
      
      console.log(`Retrying changes feed in ${delay}ms (attempt ${this.retryCount}/${this.options.maxRetries})`);
      
      setTimeout(() => {
        if (this.isRunning) {
          this.connectChanges();
        }
      }, delay);
    } else {
      console.error(`Max retries (${this.options.maxRetries}) exceeded, stopping changes feed`);
      this.stop();
      
      if (this.options.onMaxRetriesReached) {
        this.options.onMaxRetriesReached(err);
      }
    }
  }
}

// Usage
const monitor = new RobustChangesMonitor(db, {
  since: 'now',
  maxRetries: 10,
  retryDelay: 2000,
  onChange: (change) => {
    console.log('Robust change handler:', change.id);
  },
  onMaxRetriesReached: (err) => {
    console.error('Changes monitoring failed permanently:', err);
  }
});

monitor.start();

// Changes options
interface ChangesOptions {
  conflicts?: boolean;
  include_docs?: boolean;
  attachments?: boolean;
  descending?: boolean;
  since?: string | number;
  limit?: number;
  timeout?: number;
  heartbeat?: number | boolean;
  live?: boolean;
  continuous?: boolean;
  filter?: string;
  view?: string;
  doc_ids?: string[];
  query_params?: { [key: string]: any };
  selector?: any;
  style?: 'main_only' | 'all_docs';
  seq_interval?: number;
  batch_size?: number;
  onChange?: (change: Change) => void;
  onError?: (error: Error) => void;
  onComplete?: (result: ChangesResult) => void;
}

// Change object
interface Change {
  id: string;
  seq: string | number;
  changes: ChangeRevision[];
  doc?: PouchDoc;
  deleted?: boolean;
}

interface ChangeRevision {
  rev: string;
}

// Changes result
interface ChangesResult {
  results: Change[];
  last_seq: string | number;
  pending?: number;
}

// Changes controller
interface ChangesController {
  cancel(): void;
  on(event: 'change', callback: (change: Change) => void): ChangesController;
  on(event: 'error', callback: (error: Error) => void): ChangesController;
  on(event: 'complete', callback: (result: ChangesResult) => void): ChangesController;
}