Question: What is bidirectional replication in MongoDB?

Answer

MongoDB does not natively support bidirectional replication out of the box. Bidirectional replication refers to a setup where two or more databases replicate data amongst each other, ensuring that any change made in one database is reflected in the others. This process is also known as multi-master replication.

However, achieving bidirectional or multi-master replication in MongoDB requires additional architectures or third-party tools because MongoDB's built-in replication mechanism is designed as a single-primary (master) and multiple-secondaries (slaves) model. In this model, only the primary node can accept write operations, while secondaries replicate the data from the primary to ensure high availability and data redundancy.

If you require a bidirectional replication setup with MongoDB for your use case, you might consider the following approaches:

1. Change Streams and Application Logic: Use MongoDB's change streams feature to monitor changes to data in real-time and then apply those changes to the other database(s) through application logic that you develop. This approach provides a lot of flexibility but requires building and maintaining the synchronization logic yourself.

2. Third-Party Tools: Look into third-party solutions or middleware that supports bidirectional synchronization for MongoDB. Some tools are designed to work with multiple database types and can be configured for MongoDB instances. Always ensure these tools meet your performance and consistency requirements.

3. Custom Replication Mechanism: For specific use cases, developing a custom replication engine based on your application's needs might make sense. This could involve using MongoDB's APIs to capture data changes and programmatically replicating those changes to other instances.

Example of monitoring changes with Change Streams (without direct code for bidirectional replication):

const MongoClient = require('mongodb').MongoClient;
const uri = 'your_mongodb_uri';
const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology: true });

async function main() {
    try {
        await client.connect();
        const db = client.db('your_database');
        const collection = db.collection('your_collection');

        const changeStream = collection.watch();
        changeStream.on('change', (next) => {
            console.log(next);
            // Implement your replication logic here based on the change event
        });
    } catch (err) {
        console.error(err);
    }
}

main();

While bidirectional replication can be implemented in various ways with MongoDB, each method comes with trade-offs regarding complexity, data consistency, and performance. Evaluating your specific needs and testing thoroughly before deployment is crucial.