CDC (Change Data Capture ) - rahul7838/quora.clone GitHub Wiki

Sync MongoDb database with elastic search full-text index

Let me help you evaluate both approaches for syncing MongoDB with Elasticsearch.

Approach 1: Using CDC (Change Data Capture)

Pros:

• Reliable data consistency as it captures all changes at the database level
• Lower application complexity - sync logic is separate from business logic
• Better handles failure scenarios and retries
• Can replay historical changes if needed
• Reduces application response time since writes happen asynchronously

Cons:

• Additional infrastructure to maintain (e.g., MongoDB Change Streams, Debezium)
• Potential slight delay in data availability in Elasticsearch
• More complex initial setup

Example CDC setup:

// Using MongoDB Change Streams
mongoClient.watch()
    .forEach(changeDocument -> {
        // Process change and sync to Elasticsearch
    });

Approach 2: Parallel CRUD Operations

Pros:

• Simpler initial setup
• Immediate consistency (if both operations succeed)
• Easier to debug as everything happens in the application layer

Cons:

• Higher risk of data inconsistency if one operation fails
• More complex error handling needed in application code
• Increased response time due to waiting for both operations
• Higher coupling between MongoDB and Elasticsearch operations

Example parallel operations:

@Transactional
public void saveProduct(Product product) {
    try {
        mongoRepository.save(product);
        elasticsearchRepository.save(product);
    } catch (Exception e) {
        // Need complex compensation logic here
        // What if MongoDB succeeds but Elasticsearch fails?
    }
}

Recommendation

I recommend using CDC approach for several reasons:

1. Better reliability and data consistency guarantees
2. Cleaner separation of concerns in your application code
3. Better scalability as your data grows
4. More robust handling of failure scenarios
5. Easier to maintain in the long run

Specific CDC Implementation Suggestions:

1. Use Debezium with MongoDB connector
2. Or implement using MongoDB Change Streams
3. Consider using Apache Kafka as a message buffer
4. Implement idempotent updates in Elasticsearch

Replica in Mongo db

Let me help you set up a single-node MongoDB replica set. While MongoDB typically uses replica sets for redundancy across multiple servers, you can create a single-node replica set for development purposes.

First, let's understand what we're doing: A replica set is MongoDB's way of maintaining multiple copies of your data. Even with just one node, configuring it as a replica set allows you to use features that require replication, like change streams, which are essential for CDC (Change Data Capture).

Here's how to set it up:

1. First, create a data directory:

mkdir ~/mongodb-data

2. Start MongoDB with replica set configuration. You can do this in two ways:

Using command line:

mongod --replSet rs0 --dbpath ~/mongodb-data --port 27017

Or if you prefer using a configuration file (recommended), create mongod.conf:

storage:
  dbPath: ~/mongodb-data

replication:
  replSetName: rs0

net:
  port: 27017
  bindIp: localhost

Then start MongoDB using this config:

mongod --config mongod.conf

3. Now, connect to MongoDB using the mongo shell:

mongosh

4. Initialize the replica set:

rs.initiate({
  _id: "rs0",
  members: [{
    _id: 0,
    host: "localhost:27017"
  }]
})

5. Verify the replica set status:

rs.status()

If you're using Docker, here's the equivalent docker-compose setup:

version: '3.8'
services:
  mongodb:
    image: mongo:latest
    command: ["--replSet", "rs0", "--bind_ip_all"]
    ports:
      - "27017:27017"
    volumes:
      - mongodb_data:/data/db
    healthcheck:
      test: mongosh --eval "try { rs.status() } catch (err) { rs.initiate({_id:'rs0',members:[{_id:0,host:'localhost:27017'}]}) }"
      interval: 10s
      timeout: 10s
      retries: 3

volumes:
  mongodb_data:

After setting this up, any applications connecting to MongoDB should still use the standard connection string, but now with replica set name:

mongodb://localhost:27017/?replicaSet=rs0

Remember: While this setup works great for development, in a production environment you'd typically want multiple nodes in your replica set for true redundancy and fault tolerance. Would you like me to explain more about replica sets or help with any specific issues you're encountering?