Write Concerns in MongoDB

Introduction

In MongoDB, a write concern describes the level of acknowledgment requested from MongoDB for write operations. It is a critical aspect of ensuring data integrity and consistency across distributed systems.

What is Write Concern?

Write concern determines the guarantee that MongoDB provides when confirming the success of a write operation. It can specify whether to wait for acknowledgment from the server and how many nodes must acknowledge the write.

Write Concern Levels

MongoDB supports several write concern levels:

• **0**: No acknowledgment (fire-and-forget).
• **1**: Acknowledgment from the primary only.
• **"majority"**: Acknowledgment from the majority of voting nodes.
• **n**: Acknowledgment from a specified number of nodes.

Setting Write Concern

You can set the write concern for individual operations or for the entire database.

Example: Setting Write Concern for a Single Operation

db.collection.insertOne(
    { name: "Alice" },
    { writeConcern: { w: "majority", wtimeout: 5000 } }
);
            

Example: Setting Default Write Concern for a Database

db.runCommand(
    { 
        "setDefaultRWConcern": 1, 
        "defaultWriteConcern": { w: "majority" } 
    }
);
            

Best Practices

• Always consider the trade-offs between performance and data safety.
• Use "majority" write concern for critical data to ensure durability.
• Test the impact of different write concerns on your application's performance.
• Monitor your MongoDB deployment to ensure that write concerns are being met.

FAQ