CosmosDB vs MongoDB in Azure: Key Differences and When to Use Each

Azure CosmosDB is a fully managed, globally distributed database service supporting multiple APIs including MongoDB, while MongoDB on Azure is a managed instance of the popular open-source document database. CosmosDB offers turnkey global distribution and multi-model support, whereas MongoDB on Azure focuses on native MongoDB features and ecosystem compatibility.

Quick Comparison

This table summarizes the main differences between Azure CosmosDB and MongoDB on Azure.

Feature	Azure CosmosDB	MongoDB on Azure
Database Type	Multi-model (supports document, key-value, graph, column-family)	Document database (JSON-like documents)
API Support	Multiple APIs: SQL, MongoDB, Cassandra, Gremlin, Table	Native MongoDB API only
Global Distribution	Turnkey global distribution with multi-region writes	Limited to region replication, no turnkey global distribution
Scaling	Automatic horizontal scaling with low latency SLAs	Manual scaling, depends on cluster setup
Management	Fully managed by Azure with SLA-backed availability	Managed service but depends on MongoDB Atlas or Azure Cosmos API for MongoDB
Consistency Models	Multiple consistency levels (strong, bounded staleness, session, eventual)	Strong consistency by default
Pricing Model	Request Units (RU/s) based pricing	Instance-based pricing similar to MongoDB Atlas

Key Differences

Azure CosmosDB is designed as a globally distributed, multi-model database service that supports several APIs including MongoDB's. It provides turnkey global distribution, meaning you can replicate your data across multiple Azure regions easily with low latency and high availability guaranteed by SLAs. CosmosDB also offers multiple consistency models, allowing you to balance between performance and data accuracy.

On the other hand, MongoDB on Azure refers to running the native MongoDB database either via Azure's managed services or MongoDB Atlas on Azure. It focuses on the traditional MongoDB features and ecosystem, supporting the rich query language and tools MongoDB users expect. However, it lacks CosmosDB's built-in global distribution and multi-model capabilities.

In terms of pricing and scaling, CosmosDB uses a Request Unit (RU) model that abstracts compute and storage costs, while MongoDB pricing is typically based on instance size and cluster configuration. CosmosDB is fully managed by Azure with strong SLAs, whereas MongoDB on Azure depends on the chosen managed service provider.

Code Comparison

Here is an example of inserting a document into a collection using the Azure CosmosDB MongoDB API.

javascript

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

async function run() {
  const uri = "mongodb://<cosmos-account>.mongo.cosmos.azure.com:10255/?ssl=true&replicaSet=globaldb";
  const client = new MongoClient(uri, {
    auth: { user: '<username>', password: '<password>' }
  });

  try {
    await client.connect();
    const database = client.db('sampleDB');
    const collection = database.collection('items');

    const doc = { name: 'apple', color: 'red', quantity: 5 };
    const result = await collection.insertOne(doc);
    console.log(`Inserted document with _id: ${result.insertedId}`);
  } finally {
    await client.close();
  }
}

run().catch(console.dir);

Output

Inserted document with _id: 60c72b2f9b1e8a5f4d3e8b7a

MongoDB on Azure Equivalent

This is the equivalent code for inserting a document into a MongoDB collection hosted on Azure using the native MongoDB driver.

javascript

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

async function run() {
  const uri = "mongodb+srv://<username>:<password>@cluster0.mongodb.net/sampleDB?retryWrites=true&w=majority";
  const client = new MongoClient(uri);

  try {
    await client.connect();
    const database = client.db('sampleDB');
    const collection = database.collection('items');

    const doc = { name: 'apple', color: 'red', quantity: 5 };
    const result = await collection.insertOne(doc);
    console.log(`Inserted document with _id: ${result.insertedId}`);
  } finally {
    await client.close();
  }
}

run().catch(console.dir);

Output

Inserted document with _id: 60c72b2f9b1e8a5f4d3e8b7a

When to Use Which

Choose Azure CosmosDB when you need global distribution, multi-model support, and guaranteed low latency with strong SLAs. It is ideal for applications requiring worldwide scale and flexible consistency options.

Choose MongoDB on Azure when you want native MongoDB features, compatibility with existing MongoDB tools, or when your application relies heavily on MongoDB-specific capabilities. It suits projects that do not require global distribution or multi-model databases.

Key Takeaways

Azure CosmosDB offers turnkey global distribution and multi-model support with multiple APIs including MongoDB.

MongoDB on Azure provides native MongoDB experience with rich query capabilities but lacks built-in global distribution.

CosmosDB uses Request Units (RU/s) pricing and offers multiple consistency models; MongoDB pricing depends on cluster size.

Use CosmosDB for globally distributed, multi-model applications needing strong SLAs and low latency.

Use MongoDB on Azure for native MongoDB features and compatibility with MongoDB ecosystem tools.