Question: How do you implement replication between MongoDB clusters?

Answer

Replicating data between MongoDB clusters involves a set of procedures aimed at ensuring data is copied and synchronized from one cluster (source) to another (destination). This process can be crucial for disaster recovery, data locality, or scaling read operations across different geographical locations. MongoDB does not provide a built-in feature explicitly designed for cross-cluster replication, but there are strategies that can be employed to achieve this.

Strategy 1: Change Stream Processing

Change streams in MongoDB allow applications to access real-time data changes without the complexity and risk of tailing the oplog. Applications can use change streams to subscribe to all data changes on a collection and replicate those changes to another cluster.

const sourceMongoClient = new MongoClient(sourceUri);
const targetMongoClient = new MongoClient(targetUri);

async function replicateData() {
  try {
    await sourceMongoClient.connect();
    await targetMongoClient.connect();
    
    const sourceDb = sourceMongoClient.db('yourSourceDB');
    const targetDb = targetMongoClient.db('yourTargetDB');
    const changeStream = sourceDb.collection('yourCollection').watch();

    changeStream.on('change', async (next) => {
      // Implement logic based on operation type
      switch(next.operationType) {
        case 'insert':
          await targetDb.collection('yourCollection').insertOne(next.fullDocument);
          break;
        case 'update':
          await targetDb.collection('yourCollection').updateOne({ _id: next.documentKey._id }, { $set: next.updateDescription.updatedFields });
          break;
        case 'delete':
          await targetDb.collection('yourCollection').deleteOne({ _id: next.documentKey._id });
          break;
        // Handle other cases as needed
      }
    });
  } catch (err) {
    console.error('Error replicating data:', err);
  }
}

replicateData();

This code snippet sets up a basic framework for listening to changes in the source database and applying equivalent operations on the target database. Remember to handle error scenarios and edge cases not covered here, such as schema differences or network failures.

Strategy 2: Custom Replication Tool

For more sophisticated replication needs or when change streams are not sufficient, developing a custom replication tool might be necessary. This could involve:

• Periodically dumping data from the source cluster with mongodump.
• Transferring the dump to a location accessible by the destination cluster.
• Restoring the dump to the destination cluster using mongorestore.

This method can be resource-intensive and might only be suitable for low-frequency replication requirements.

Strategy 3: Third-party Tools

There are third-party tools and services designed to facilitate MongoDB data replication across clusters. These tools often offer more advanced features, like selective replication, conflict resolution, and automatic failover. Examples include MongoDB Atlas' Global Clusters feature (for customers using MongoDB's managed service) and community tools like mongomirror (now deprecated but may have successors).

In conclusion, while MongoDB doesn't support cross-cluster replication out-of-the-box, several strategies can be implemented to achieve similar outcomes. The choice of strategy depends on your specific requirements, such as latency sensitivity, consistency needs, and operational overhead.