Causal consistency concept in MongoDB

Introduction

Causal consistency helps keep data changes in order so users see updates in the right sequence, avoiding confusion.

When multiple users update shared data and you want everyone to see changes in the order they happened.

In chat apps where messages should appear in the order they were sent.

When tracking actions in a game so players see moves in the correct sequence.

For collaborative documents where edits must appear in the order made.

When reading data that depends on previous writes to avoid showing outdated info.

Syntax

MongoDB

const session = client.startSession({ causalConsistency: true });

session.withTransaction(async () => {
  // your read and write operations here
});

You enable causal consistency by starting a session with { causalConsistency: true }.

All reads and writes inside this session follow the causal order.

Examples

This example shows a session with causal consistency where a document is inserted and then read in the same order.

MongoDB

const session = client.startSession({ causalConsistency: true });

await session.withTransaction(async () => {
  await collection.insertOne({ name: 'Alice' }, { session });
  const doc = await collection.findOne({ name: 'Alice' }, { session });
  console.log(doc);
});

Here causal consistency is off, so reads might not reflect recent writes right away.

MongoDB

const session = client.startSession({ causalConsistency: false });

// Reads may not see the latest writes immediately.

Sample Program

This program connects to MongoDB, starts a session with causal consistency, inserts a user named Bob, then reads and prints that user inside the same transaction to ensure the read sees the write.

MongoDB

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

async function run() {
  const client = new MongoClient('mongodb://localhost:27017');
  await client.connect();

  const db = client.db('testdb');
  const collection = db.collection('users');

  const session = client.startSession({ causalConsistency: true });

  try {
    await session.withTransaction(async () => {
      await collection.insertOne({ name: 'Bob' }, { session });
      const user = await collection.findOne({ name: 'Bob' }, { session });
      console.log(user);
    });
  } finally {
    await session.endSession();
    await client.close();
  }
}

run().catch(console.dir);

Important Notes

Causal consistency ensures you see your own writes and related changes in order.

It is useful in distributed systems where data is replicated across servers.

Using causal consistency may add slight overhead but improves data correctness for users.

Summary

Causal consistency keeps data changes in the order they happened.

Enable it by starting a session with { causalConsistency: true } in MongoDB.

It helps users see updates in the right sequence, avoiding confusion.