Replication and Synchronization

/**
 * Replicate from another database to this database
 * @param source - Source database (URL string or PouchDB instance)
 * @param options - Replication options
 * @returns Replication object with event emitter interface
 */
db.replicate.from(source: string | PouchDB, options?: ReplicationOptions): Replication;

/**
 * Replicate from this database to another database
 * @param target - Target database (URL string or PouchDB instance)
 * @param options - Replication options
 * @returns Replication object with event emitter interface
 */
db.replicate.to(target: string | PouchDB, options?: ReplicationOptions): Replication;

/**
 * Static method for one-way replication between any two databases
 * @param source - Source database
 * @param target - Target database
 * @param options - Replication options
 * @returns Replication object
 */
PouchDB.replicate(source: string | PouchDB, target: string | PouchDB, options?: ReplicationOptions): Replication;

// Replicate from remote to local
const replication = db.replicate.from('http://localhost:5984/remote-db', {
  continuous: true,
  retry: true
}).on('change', function(info) {
  console.log('Received changes:', info.docs.length);
}).on('complete', function(info) {
  console.log('Replication completed');
}).on('error', function(err) {
  console.error('Replication error:', err);
});

// Replicate from local to remote
db.replicate.to('http://localhost:5984/remote-db')
  .on('complete', function(info) {
    console.log('Upload completed:', info.docs_written);
  });

// Static replication method
PouchDB.replicate(localDb, remoteDb, {
  filter: 'myapp/active_users'
});

/**
 * Bidirectional synchronization with another database
 * @param remoteDB - Remote database to sync with
 * @param options - Synchronization options
 * @returns Sync object with event emitter interface
 */
db.sync(remoteDB: string | PouchDB, options?: SyncOptions): Sync;

/**
 * Static method for bidirectional sync between any two databases
 * @param source - First database
 * @param target - Second database
 * @param options - Synchronization options
 * @returns Sync object
 */
PouchDB.sync(source: string | PouchDB, target: string | PouchDB, options?: SyncOptions): Sync;

interface SyncOptions extends ReplicationOptions {
  // Inherits all replication options
}

interface Sync extends EventEmitter {
  cancel(): void;
  on(event: 'change', listener: (info: SyncChangeInfo) => void): Sync;
  on(event: 'complete', listener: (info: SyncComplete) => void): Sync;
  on(event: 'error', listener: (error: Error) => void): Sync;
  on(event: 'active', listener: () => void): Sync;
  on(event: 'paused', listener: () => void): Sync;
}

// Basic bidirectional sync
const sync = db.sync('http://localhost:5984/remote-db', {
  live: true,
  retry: true
}).on('change', function(info) {
  console.log('Sync change:', info.direction);
  if (info.direction === 'pull') {
    console.log('Downloaded:', info.change.docs.length);
  } else {
    console.log('Uploaded:', info.change.docs_written);
  }
}).on('active', function() {
  console.log('Sync active');
}).on('paused', function() {
  console.log('Sync paused');
});

// Stop sync
sync.cancel();

interface ReplicationOptions {
  continuous?: boolean;          // Continuous replication
  retry?: boolean;               // Retry on failure
  filter?: string | Function;    // Replication filter
  query_params?: object;         // Filter parameters
  doc_ids?: string[];            // Replicate specific documents only
  checkpoint?: string | boolean; // Checkpoint handling
  batch_size?: number;           // Number of docs per batch (default: 100)
  batches_limit?: number;        // Number of concurrent batches (default: 10)
  since?: number | string;       // Starting sequence number
  timeout?: number;              // Request timeout
  heartbeat?: number;            // Heartbeat interval
  selector?: MangoSelector;      // Mango query selector (if supported)
}

interface Replication extends EventEmitter {
  cancel(): void;
  on(event: 'change', listener: (info: ReplicationChangeInfo) => void): Replication;
  on(event: 'complete', listener: (info: ReplicationComplete) => void): Replication;
  on(event: 'error', listener: (error: Error) => void): Replication;
  on(event: 'active', listener: () => void): Replication;
  on(event: 'paused', listener: () => void): Replication;
}

interface ReplicationChangeInfo {
  docs: Document[];              // Documents that changed
  docs_read: number;             // Number of docs read
  docs_written: number;          // Number of docs written
  doc_write_failures: number;    // Number of write failures
  errors: Error[];               // Array of errors
  last_seq: number | string;     // Last sequence processed
  start_time: string;            // Replication start time
}

// Filtered replication
const filteredReplication = db.replicate.from(remoteDb, {
  filter: function(doc) {
    return doc.type === 'user' && doc.active === true;
  },
  continuous: true,
  retry: true,
  batch_size: 50,
  batches_limit: 5
});

// Replicate specific documents
const specificReplication = db.replicate.from(remoteDb, {
  doc_ids: ['user1', 'user2', 'config'],
  continuous: true
});

// Replicate with design document filter
const designDocReplication = db.replicate.from(remoteDb, {
  filter: 'myapp/active_users',
  query_params: {
    department: 'engineering'
  }
});

// Basic authentication
const authReplication = db.replicate.from('http://username:password@localhost:5984/remote-db');

// URL with credentials
const remoteUrl = 'http://localhost:5984/remote-db';
const replication = db.replicate.from(remoteUrl, {
  auth: {
    username: 'myuser',
    password: 'mypassword'
  }
});

// Custom headers
const headerReplication = db.replicate.from(remoteUrl, {
  headers: {
    'Authorization': 'Bearer ' + token,
    'X-Custom-Header': 'value'
  }
});

// Monitor for conflicts during sync
const conflictSync = db.sync(remoteDb, {
  live: true,
  retry: true
}).on('change', function(info) {
  // Check for conflicts in replicated documents
  if (info.direction === 'pull') {
    info.change.docs.forEach(async (doc) => {
      if (doc._conflicts) {
        await handleConflict(doc);
      }
    });
  }
});

async function handleConflict(doc) {
  console.log(`Conflict in document ${doc._id}`);
  
  // Get all conflicting revisions
  const conflicts = await Promise.all(
    doc._conflicts.map(rev => db.get(doc._id, { rev }))
  );
  
  // Resolve conflict (example: merge strategy)
  const resolved = mergeDocuments(doc, conflicts);
  
  // Save resolved version
  await db.put(resolved);
  
  // Delete conflicting revisions
  for (const conflict of conflicts) {
    await db.remove(conflict._id, conflict._rev);
  }
}

function mergeDocuments(main, conflicts) {
  // Implement your merge strategy
  // This example combines all unique fields
  const merged = { ...main };
  
  conflicts.forEach(conflict => {
    Object.keys(conflict).forEach(key => {
      if (!key.startsWith('_') && !merged[key]) {
        merged[key] = conflict[key];
      }
    });
  });
  
  return merged;
}

function startContinuousSync(localDb, remoteUrl) {
  let syncHandler;
  
  function createSync() {
    syncHandler = localDb.sync(remoteUrl, {
      live: true,
      retry: true,
      heartbeat: 10000,
      timeout: 30000
    });
    
    syncHandler.on('change', function(info) {
      console.log(`Sync ${info.direction}:`, info.change.docs.length, 'docs');
      updateSyncStatus('active', info);
    });
    
    syncHandler.on('active', function() {
      console.log('Sync resumed');
      updateSyncStatus('active');
    });
    
    syncHandler.on('paused', function(err) {
      if (err) {
        console.log('Sync paused due to error:', err);
        updateSyncStatus('error', err);
      } else {
        console.log('Sync paused');
        updateSyncStatus('paused');
      }
    });
    
    syncHandler.on('error', function(err) {
      console.error('Sync error:', err);
      updateSyncStatus('error', err);
      
      // Restart sync after delay
      setTimeout(() => {
        console.log('Restarting sync...');
        createSync();
      }, 5000);
    });
    
    syncHandler.on('complete', function(info) {
      console.log('Sync completed');
      updateSyncStatus('complete', info);
    });
    
    return syncHandler;
  }
  
  function updateSyncStatus(status, data) {
    // Update UI or application state
    document.dispatchEvent(new CustomEvent('syncStatusChange', {
      detail: { status, data }
    }));
  }
  
  // Start initial sync
  return createSync();
}

// Usage
const syncHandler = startContinuousSync(db, 'http://localhost:5984/remote-db');

// Stop sync when needed
function stopSync() {
  if (syncHandler) {
    syncHandler.cancel();
  }
}

// Replicate only user documents
const userReplication = db.replicate.from(remoteDb, {
  filter: function(doc) {
    return doc._id.startsWith('user_');
  },
  continuous: true
});

// Replicate documents modified after specific date
const recentReplication = db.replicate.from(remoteDb, {
  filter: function(doc) {
    const modifiedDate = new Date(doc.modified);
    const cutoffDate = new Date('2023-01-01');
    return modifiedDate > cutoffDate;
  }
});

// Use query parameters with design document filter
const parameterizedFilter = db.replicate.from(remoteDb, {
  filter: 'myapp/by_department',
  query_params: {
    department: 'sales',
    active: true
  }
});

function replicateOnce(source, target, options = {}) {
  return new Promise((resolve, reject) => {
    PouchDB.replicate(source, target, {
      ...options,
      continuous: false
    }).on('complete', resolve).on('error', reject);
  });
}

// Usage
try {
  const result = await replicateOnce(localDb, remoteDb);
  console.log('Replication completed:', result.docs_written);
} catch (error) {
  console.error('Replication failed:', error);
}

class SyncManager {
  constructor(localDb, remoteUrl) {
    this.localDb = localDb;
    this.remoteUrl = remoteUrl;
    this.syncHandler = null;
    this.status = 'stopped';
    this.listeners = new Set();
  }
  
  start() {
    if (this.syncHandler) {
      this.stop();
    }
    
    this.syncHandler = this.localDb.sync(this.remoteUrl, {
      live: true,
      retry: true
    });
    
    this.syncHandler.on('change', (info) => {
      this.emit('change', info);
    });
    
    this.syncHandler.on('active', () => {
      this.status = 'active';
      this.emit('statusChange', 'active');
    });
    
    this.syncHandler.on('paused', () => {
      this.status = 'paused';
      this.emit('statusChange', 'paused');
    });
    
    this.syncHandler.on('error', (err) => {
      this.status = 'error';
      this.emit('error', err);
    });
  }
  
  stop() {
    if (this.syncHandler) {
      this.syncHandler.cancel();
      this.syncHandler = null;
    }
    this.status = 'stopped';
    this.emit('statusChange', 'stopped');
  }
  
  on(event, callback) {
    this.listeners.add({ event, callback });
  }
  
  emit(event, data) {
    this.listeners.forEach(listener => {
      if (listener.event === event) {
        listener.callback(data);
      }
    });
  }
}

// Usage
const syncManager = new SyncManager(db, 'http://localhost:5984/remote-db');
syncManager.on('statusChange', (status) => {
  console.log('Sync status:', status);
});
syncManager.start();

const replication = db.replicate.from(remoteDb)
  .on('error', function(err) {
    if (err.status === 401) {
      console.log('Authentication failed');
      // Handle auth error
    } else if (err.status === 404) {
      console.log('Remote database not found');
      // Handle missing database
    } else if (err.name === 'timeout') {
      console.log('Replication timeout');
      // Retry or handle timeout
    } else if (err.error === 'forbidden') {
      console.log('Insufficient permissions');
      // Handle permissions error
    } else {
      console.error('Replication error:', err);
    }
  });