Seamless Migration: How to Transition from MongoDB to Amazon DocumentDB Without Downtime

While many assume migrating databases requires downtime or complex rewrites, this guide reveals a strategic approach to shift from MongoDB to DocumentDB smoothly, keeping your application live and performant.

MongoDB and Amazon DocumentDB are both NoSQL document databases designed for flexibility and scalability. However, they differ in architecture, managed service offerings, and operational nuances. If your business is considering moving to Amazon DocumentDB—for its fully managed nature, better integration with AWS ecosystem, or built-in security features—understanding how to migrate without impacting your users is key.

This post walks you through a practical, step-by-step migration process from MongoDB to Amazon DocumentDB without downtime, ensuring your application's availability throughout the transition.

Why Move from MongoDB to Amazon DocumentDB?

Before diving into the how-to, it’s good to understand why an organization might want this shift:

Fully Managed Service: Amazon DocumentDB handles backups, patching, scaling, and replication automatically.
Seamless AWS Integration: Easy to use with other AWS services like Lambda, ECS, CloudWatch.
Scalability & Reliability: Designed for 99.99% availability with automated fault tolerance.
Security & Compliance: Integrates with IAM roles and KMS encryption options.

However, migrating between these two systems isn’t as simple as swapping connection strings due to differences in replication protocols and certain MongoDB features that DocumentDB doesn't fully support.

Challenges of Migrating From MongoDB to DocumentDB

Different Replication Mechanisms: MongoDB uses replica sets; DocumentDB uses a distributed storage cluster.
Feature Parity: Some MongoDB features (like multi-document ACID transactions in older versions) may be unavailable or behave differently.
Connection URIs: Authentication and SSL configurations differ.

Because of these factors, a naive migration might introduce downtime or require a heavy rewrite. The goal here is zero-downtime migration with minimal operational impact.

The Zero-Downtime Migration Strategy

The general approach is:

Provision Your Amazon DocumentDB Cluster
Initial Data Export from MongoDB
Continuous Data Synchronization
Read/Write Redirect – Dual Writes / Application Update
Cutover — Switch Traffic Fully to Amazon DocumentDB
Decommission Old MongoDB

1. Provision Your Amazon DocumentDB Cluster

Start by creating your Amazon DocumentDB cluster via the AWS Console or CLI:

aws docdb create-db-cluster \
    --db-cluster-identifier my-docdb-cluster \
    --engine docdb \
    --master-username adminuser \
    --master-user-password password123 \
    --vpc-security-group-ids sg-xxxxxx

Once the cluster is available, create instances (replicas) under it:

aws docdb create-db-instance \
    --db-instance-identifier my-docdb-instance1 \
    --db-cluster-identifier my-docdb-cluster \
    --engine docdb \
    --db-instance-class db.r5.large

Configure network access so your application servers can connect securely.

2. Initial Data Export from MongoDB

Perform a snapshot export of your current data using mongodump:

mongodump --uri="mongodb://mongo_user:mypassword@mongo-db-host:27017/mydatabase" --out=/data/dump/

This command backs up your database into BSON files stored locally (or on an EC2 instance you’re using).

To import into DocumentDB, use mongorestore:

mongorestore --host docdb-cluster.cluster-codaws123docdb.us-east-1.docdb.amazonaws.com:27017 \
  --ssl --sslCAFile rds-combined-ca-bundle.pem \
  --username adminuser --password password123 /data/dump/mydatabase

Note:

You need the Amazon root CA cert for SSL connections.
The user must have appropriate privileges on the DocumentDB cluster.

At this point, your Amazon DocumentDB contains a snapshot of the live data but is out of sync from new writes happening on MongoDB during migration.

3. Continuous Data Synchronization

To keep data in sync without downtime you need ongoing replication until cutover.

Two approaches exist:

Option A – Use Change Streams (Mongo 4+)

Change Streams allow you to track real-time operations happening in MongoDB:

Build or deploy a simple process that listens for change events on MongoDB collections.
For every insert/update/delete event, apply corresponding changes to Amazon DocumentDB.

Example pseudocode in Node.js using the official driver:

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

async function replicateChanges() {
  const sourceClient = new MongoClient("mongodb://mongo_user:mypassword@mongo-host");
  const targetClient = new MongoClient("mongodb://adminuser:password123@docdb-host", {
    sslCAFile: "rds-combined-ca-bundle.pem",
    ssl:true,
  });

  await sourceClient.connect();
  await targetClient.connect();

  const sourceDb = sourceClient.db("mydatabase");
  const targetDb = targetClient.db("mydatabase");

  const changeStream = sourceDb.collection("mycollection").watch();

  changeStream.on('change', async (change) => {
    console.log("Change detected:", change);

    switch(change.operationType) {
      case "insert":
        await targetDb.collection("mycollection").insertOne(change.fullDocument);
        break;
      case "update":
        await targetDb.collection("mycollection").updateOne(
          { _id: change.documentKey._id },
          { $set: change.updateDescription.updatedFields }
        );
        break;
      case "delete":
        await targetDb.collection("mycollection").deleteOne({ _id: change.documentKey._id });
        break;
      // handle other operation types as needed
      default:
        console.log(`Unhandled operation type ${change.operationType}`);
    }
  });
}

replicateChanges().catch(console.error);

This daemon keeps applying live changes until stopped at cutover.

Option B – Use Third-Party Tools

Alternatively, use tools like AWS Database Migration Service (DMS) which supports ongoing replication between MongoDB and Amazon DocumentDB with minimal setup.

DMS handles:

Initial full load of existing data,
Ongoing replication of changes,
Monitoring and failure recovery.

You configure DMS tasks via AWS Console specifying source/target endpoints along with migration settings (full load + CDC).

4. Read/Write Redirect — Dual Writes or Application Switch Preparation

While syncing data is ongoing:

Either implement dual writes where your application writes simultaneously to both DBs temporarily,

Continue writing primarily on Mongo but ensure continuous sync process described above handles syncing new data over seamlessly.

Ensure reads temporarily keep going against original Mongo for consistent low-latency queries until cutover triggers.

5. Cutover — Switching Traffic Fully to Amazon DocumentDB

After confirming that data between both systems is fully synced (no lag), proceed cautiously:

Briefly pause writes (using app maintenance mode or queues).
Wait a few seconds to allow last change events sync.
Point your application’s database connection strings/configuration files from old Mongo URI to the new Amazon DocumentDB endpoint.
Resume full application operation writing/reading exclusively against DocumentDB.
Monitor closely for errors or latency issues during initial phase.

This transition usually takes seconds—avoiding traditional prolonged downtime windows.

6. Decommission Your Old Mongo Cluster

Once confidence is high that all traffic routes correctly and no performance/regression issues occur over a few days/weeks:

Safely disable previous sync daemons/data pipelines.
Shut down or repurpose old infrastructure hosting original Mongo if applicable.

Always keep backups until you’re certain no rollback needed!

Tips for Smooth Migration Success

Test entire migration flow on staging environments before production run.
Instrument detailed logging on sync consumers/processes.
Leverage monitoring tools in AWS CloudWatch + custom app logs.
Be aware of feature differences (like aggregation pipeline limitations or transaction support).
Consider read-only users on old cluster during migration period for additional safety.

Summary

Migrating from MongoDB to Amazon DocumentDB without downtime requires careful orchestration but can be achieved by combining full initial export/import with continuous change replication—either custom-built with Change Streams or via AWS DMS services—alongside intelligent traffic switching strategies at cutover time.

With this approach you unlock benefits of managed cloud services while maintaining application uptime—empowering seamless modernization in today’s fast-paced business world!

Ready to migrate? Start by provisioning your cluster now and test initial mongodump / mongorestore steps today! And if you want sample code or AWS DMS setup guidance — just drop me a comment below! Happy migrating 🚀

Mongodb To Documentdb