MongoDB Online Migration

Introduction

MongoDB online migration refers to the process of migrating data from one MongoDB deployment to another while keeping your application running with minimal or no downtime. This is crucial for production environments where service availability is critical, and traditional offline migration methods with extended downtime periods would be unacceptable.

In this guide, we'll explore various strategies, tools, and techniques for performing MongoDB online migrations successfully. Whether you're upgrading to a newer MongoDB version, moving to a different infrastructure, or simply scaling your deployment, understanding online migration approaches will help ensure a smooth transition with minimal impact on your users.

Why Consider Online Migration?

Before diving into the "how," let's understand the "why":

• Business Continuity: Keep your services running during migration
• Reduced Risk: Ability to test and verify before fully switching over
• Customer Satisfaction: Avoid disrupting user experience
• Meeting SLAs: Maintain your service level agreements for availability
• Geographic Expansion: Migrate data closer to your users

Online Migration Strategies

1. Replica Set Migration

One of the most common and reliable approaches for online migration is leveraging MongoDB's replica set functionality.

How it works:

Step-by-step implementation:

1. Set up target infrastructure

First, provision your new MongoDB servers with adequate resources.

bash

# Example: Setting up a new MongoDB instance
mongod --dbpath /data/db --port 27018 --replSet newReplicaSet

2. Initialize the target replica set

javascript

// Connect to the new MongoDB instance
mongo --port 27018

// Initialize the replica set
rs.initiate({
  _id: "newReplicaSet",
  members: [
    { _id: 0, host: "mongodb-new-server:27018" }
  ]
})

3. Add the source as a temporary member

javascript

// On the new replica set
rs.add({ host: "mongodb-source-server:27017", priority: 0, votes: 0 })

4. Wait for initial sync

The new replica set member will automatically sync all data from the primary. Monitor the sync progress:

javascript

rs.printReplicationInfo()

5. Test with read traffic

Direct a small portion of your read traffic to the new deployment while keeping writes on the original system.

javascript

// In your application code (pseudocode)
if (shouldUseNewDatabase()) {
  db = connectToNewMongoDB();
} else {
  db = connectToOldMongoDB();
}

6. Perform the cutover

Once satisfied with testing, switch all application traffic to the new deployment.

7. Remove temporary replication

javascript

// Safely remove the temporary replication link
rs.remove("mongodb-source-server:27017")

2. MongoDB Atlas Live Migration Service

If you're migrating to MongoDB Atlas (MongoDB's cloud service), you can use their Live Migration Service.

Prerequisites:

• Source MongoDB version 3.0 or later
• Network connectivity between source and Atlas
• Valid Atlas cluster

Steps:

1. Configure the live migration process in Atlas UI

Navigate to your project in Atlas, select "Migrate Data to this Cluster", and enter your source connection details.

2. Test the migration connection

Atlas will verify connectivity and access to your source database.

3. Start the initial sync

Atlas begins copying data from your source to the target cluster.

4. Set up the change stream

Once initial sync is complete, Atlas sets up a change stream processor to capture ongoing changes.

5. Cutover when ready

javascript

// Example: Checking migration status programmatically using Atlas API
const response = await axios.get(
  'https://cloud.mongodb.com/api/atlas/v1.0/groups/{PROJECT_ID}/liveMigrations/{MIGRATION_ID}',
  {
    headers: { 
      'Content-Type': 'application/json',
      'Authorization': 'Bearer ' + API_KEY 
    }
  }
);

console.log('Migration Status:', response.data.status);

3. Using mongomirror Tool

mongomirror is a tool designed specifically for online migration scenarios.

bash

# Basic syntax for mongomirror
mongomirror --host source-mongodb:27017 \
            --port 27017 \
            --username user \
            --password pass \
            --ssl \
            --target mongodb+srv://user:[email protected]/test

Custom Online Migration Approach

Sometimes, you may need to build a custom migration solution for specific requirements. Here's a basic approach using Node.js:

javascript

const { MongoClient } = require('mongodb');

async function migrateCollection(collectionName) {
  // Connect to source and destination databases
  const sourceClient = new MongoClient('mongodb://source-server:27017');
  const destClient = new MongoClient('mongodb://dest-server:27017');
  
  await sourceClient.connect();
  await destClient.connect();
  
  const sourceDB = sourceClient.db('sourceDB');
  const destDB = destClient.db('destDB');
  
  // Set up a change stream on the source
  const changeStream = sourceDB.collection(collectionName).watch();
  
  // Initial data copy
  console.log(`Starting initial sync for ${collectionName}`);
  const cursor = sourceDB.collection(collectionName).find({});
  const batch = [];
  const batchSize = 1000;
  
  let count = 0;
  for await (const doc of cursor) {
    batch.push(doc);
    if (batch.length >= batchSize) {
      if (batch.length > 0) {
        await destDB.collection(collectionName).insertMany(batch);
        count += batch.length;
        console.log(`Migrated ${count} documents...`);
        batch.length = 0;
      }
    }
  }
  
  // Insert any remaining documents
  if (batch.length > 0) {
    await destDB.collection(collectionName).insertMany(batch);
    count += batch.length;
  }
  
  console.log(`Initial sync complete. Migrated ${count} documents.`);
  
  // Process change streams
  console.log('Now watching for changes...');
  for await (const change of changeStream) {
    const operation = change.operationType;
    
    try {
      switch (operation) {
        case 'insert':
          await destDB.collection(collectionName).insertOne(change.fullDocument);
          break;
        case 'update':
          await destDB.collection(collectionName).updateOne(
            { _id: change.documentKey._id },
            { $set: change.updateDescription.updatedFields }
          );
          break;
        case 'delete':
          await destDB.collection(collectionName).deleteOne({ _id: change.documentKey._id });
          break;
      }
      console.log(`Replicated ${operation} operation`);
    } catch (err) {
      console.error(`Error replicating change: ${err}`);
    }
  }
}

// Usage
migrateCollection('users')
  .catch(err => console.error(err));

The above code demonstrates:

1. Initial data copy from source to destination
2. Setting up a change stream to capture ongoing changes
3. Applying those changes to the destination in real-time

Best Practices for MongoDB Online Migrations

Performance Considerations

1. Network Bandwidth: Ensure sufficient bandwidth between source and destination.

   javascript

   // You can monitor network usage with MongoDB's serverStatus command
   db.adminCommand({ serverStatus: 1, network: 1 })

2. Index Build Strategy: Consider building indexes before or after migration.

   javascript

   // Build indexes in background to avoid blocking operations
   db.users.createIndex({ email: 1 }, { background: true })

3. Batch Processing: Migrate in batches to limit resource usage.

Data Integrity

1. Validation: Implement count checks between source and destination.

   javascript

   // Example validation script
   async function validateMigration(collectionName) {
     const sourceCount = await sourceDB.collection(collectionName).countDocuments();
     const destCount = await destDB.collection(collectionName).countDocuments();
     
     if (sourceCount === destCount) {
       console.log(`Collection ${collectionName} validation successful: ${sourceCount} documents`);
       return true;
     } else {
       console.error(`Collection ${collectionName} validation failed: Source=${sourceCount}, Dest=${destCount}`);
       return false;
     }
   }

2. Schema Validation: Ensure schema compatibility between systems.

3. Data Sampling: Compare random document samples between source and destination.

Monitoring and Rollback

1. Progress Monitoring: Track migration progress with metrics.

   javascript

   // Track progress using MongoDB's currentOp command
   db.adminCommand({ currentOp: 1, $ownOps: true })

2. Rollback Plan: Maintain the ability to revert to the source if needed.

3. Performance Metrics: Monitor system resource usage during migration.

Real-World Scenario: E-commerce Platform Migration

Let's examine a practical case study of migrating an e-commerce platform's MongoDB database from on-premises to MongoDB Atlas.

Scenario Details:

• 500GB of data across multiple collections
• 24/7 operational requirements
• Peak traffic hours: 9AM-5PM

Implementation Plan:

1. Pre-migration Analysis:

   • Identify largest collections
   • Analyze access patterns
   • Document all indexes and special configurations

2. Initial Setup:

   javascript

   // Create Atlas connection string
   const atlasUri = "mongodb+srv://user:[email protected]/ecommerce";
   
   // Test connection
   const client = new MongoClient(atlasUri);
   await client.connect();
   console.log("Connected to Atlas successfully");

3. Staged Migration Approach:

   • Migrate static collections first (products, categories)
   • Migrate user data next
   • Migrate transactional data last (orders, carts)

4. Progressive Traffic Shifting:

   javascript

   // Application code for gradual traffic shift
   function getDatabaseConnection(requestType) {
     // During migration phase, direct read traffic gradually to new system
     if (requestType === 'READ' && Math.random() < MIGRATION_READ_PERCENTAGE) {
       return newDbConnection;
     }
     
     // All writes still go to the original system
     return originalDbConnection;
   }

5. Cutover Strategy:

   • Schedule during off-peak hours (3AM)
   • Implement application read-only mode for 15 minutes
   • Update connection strings and restart services
   • Verify first transactions on the new system

Troubleshooting Common Issues

Sync Delays

If your replication is falling behind:

javascript

// Check replication lag
rs.printSlaveReplicationInfo()

// If experiencing lag, consider:
// 1. Increasing network bandwidth
// 2. Upgrading MongoDB instance sizes
// 3. Optimizing indexes on the destination

Connection Issues

javascript

// Verify network connectivity
mongo --host destination-server --eval "db.serverStatus()"

// Check firewall settings
telnet destination-server 27017

// Ensure authentication details are correct
mongo --host destination-server -u username -p password --authenticationDatabase admin

Data Inconsistency

If your data validation shows inconsistencies:

javascript

// Find and fix inconsistencies
const sourceDoc = await sourceDB.collection('users').findOne({ _id: problematicId });
const destDoc = await destDB.collection('users').findOne({ _id: problematicId });

// Compare differences
const differences = findDifferences(sourceDoc, destDoc);
console.log('Differences:', differences);

// Fix if needed
await destDB.collection('users').replaceOne({ _id: problematicId }, sourceDoc);

Summary

MongoDB online migration allows you to move your database with minimal downtime, ensuring continuous service for your users. We've explored several approaches including:

1. Replica set migration - Using MongoDB's native replication
2. MongoDB Atlas Live Migration - For migrating to the cloud
3. Custom migration solutions - For specific requirements
4. Best practices - For ensuring successful migrations

Each approach has its advantages and considerations. The best choice depends on your specific requirements, including:

• Size of your database
• Acceptable downtime window
• Technical expertise
• Target environment
• Budget constraints

By carefully planning and implementing an online migration strategy, you can successfully transition your MongoDB deployment with minimal disruption to your services.

Additional Resources

• Official MongoDB Documentation: Further reading on replication and migration
• MongoDB University: Free courses on MongoDB operations
• MongoDB Forums: Community support for migration scenarios

Practice Exercises

1. Basic Migration Simulation: Set up two local MongoDB instances and practice migrating a sample dataset between them using replica sets.

2. Change Stream Processing: Implement a simple Node.js application that uses change streams to replicate data modifications from one collection to another.

3. Migration Validation: Write a script that validates the success of a migration by comparing document counts and sampling random documents for equivalence.

4. Performance Testing: Analyze the performance impact of migration on your database by running benchmarks before, during, and after migration.

5. Rollback Strategy: Design and document a rollback strategy for a failed migration scenario, and practice implementing it in a test environment.

Happy migrating!