Read concern levels (local, majority, snapshot) in MongoDB

Introduction

Read concern levels tell MongoDB how fresh or safe the data you read should be. They help you decide if you want the newest data or data confirmed by many servers.

When you want to read the most recent data even if it is not confirmed by other servers.

When you want to read data that is confirmed by most servers to avoid reading uncommitted changes.

When you want to read a consistent snapshot of data at a specific point in time during a transaction.

When you want to avoid reading data that might be rolled back in case of failures.

When you want to balance between speed and data accuracy depending on your application needs.

Syntax

MongoDB

db.collection.find().readConcern('level')

// where 'level' can be 'local', 'majority', or 'snapshot'

local returns the most recent data on the node, even if not confirmed by others.

majority returns data confirmed by most nodes, ensuring durability.

snapshot provides a consistent view of data during a transaction.

Examples

Reads the most recent data from the node, fast but might not be confirmed by others.

MongoDB

db.orders.find().readConcern('local')

Reads data confirmed by most nodes, safer but might be slightly older.

MongoDB

db.orders.find().readConcern('majority')

Reads a consistent snapshot of data inside a transaction.

MongoDB

session = db.getMongo().startSession({readConcern: {level: 'snapshot'}})
session.startTransaction()
session.getDatabase('shop').orders.find().toArray()
session.commitTransaction()
session.endSession()

Sample Program

This example inserts two orders and reads them twice: once with 'local' and once with 'majority' read concern.

MongoDB

use shop

// Insert sample data
db.orders.insertMany([
  {item: 'apple', qty: 5},
  {item: 'banana', qty: 10}
])

// Read with local read concern
print('Local read concern:')
var localData = db.orders.find().readConcern('local').toArray()
printjson(localData)

// Read with majority read concern
print('Majority read concern:')
var majorityData = db.orders.find().readConcern('majority').toArray()
printjson(majorityData)

Important Notes

Use local for fastest reads when you don't mind reading unconfirmed data.

majority is safer for critical data because it reads data confirmed by most servers.

snapshot is only available inside transactions and gives a consistent view of data.

Summary

Read concern controls how fresh and safe your read data is.

local is fastest but less safe, majority is safer but might be slower.

snapshot is for consistent reads inside transactions.