Data

Cassandra vs MongoDB, Which Application DB to Choose?

Overview

In the ever-evolving landscape of database technology, choosing between Apache Cassandra and MongoDB remains one of the most critical decisions for architects and developers. This comprehensive guide will walk you through everything from high-level concepts to hands-on technical implementations, helping you make an informed decision for your next project.

1. Understanding the Fundamentals

Cassandra at a Glance

Apache Cassandra operates on a masterless architecture, designed for handling massive amounts of data across multiple data centers. Think of it as a distributed system that’s always available, even when entire data centers go down.

MongoDB’s Approach

MongoDB takes a different route with its document-based model, offering flexibility and rich querying capabilities. It’s like having a dynamic filing system that can adapt to changing business needs on the fly.

2. Architecture Deep Dive

Cassandra’s Architecture

# Example Cassandra cluster configuration
cluster_name: 'ProductionCluster'
num_tokens: 256
hinted_handoff_enabled: true
max_hint_window_in_ms: 10800000
authenticator: PasswordAuthenticator
authorizer: CassandraAuthorizer
partitioner: Murmur3Partitioner

# Performance tuning
concurrent_reads: 32
concurrent_writes: 32
concurrent_counter_writes: 32

Key components:

  • Gossip protocol for node communication
  • Token ring for data distribution
  • Virtual nodes (vnodes) for balanced distribution
  • Tunable consistency levels

MongoDB’s Architecture

// Example MongoDB replica set configuration
config = {
    _id: "production_replica_set",
    members: [
        { _id: 0, host: "mongodb0.example.net:27017", priority: 1 },
        { _id: 1, host: "mongodb1.example.net:27017", priority: 0.5 },
        { _id: 2, host: "mongodb2.example.net:27017", priority: 0.5 }
    ],
    settings: {
        getLastErrorDefaults: { w: "majority", wtimeout: 5000 }
    }
}

3. Data Modeling Patterns

Cassandra Data Modeling

Let’s look at a real-world example for a user activity tracking system:

-- Cassandra data model for user activity tracking
CREATE KEYSPACE user_analytics WITH replication = {
    'class': 'NetworkTopologyStrategy',
    'DC1': 3,
    'DC2': 2
};

CREATE TABLE user_activities (
    user_id uuid,
    activity_date date,
    activity_timestamp timestamp,
    activity_type text,
    device_id text,
    ip_address inet,
    location map<text, text>,
    session_duration int,
    PRIMARY KEY ((user_id, activity_date), activity_timestamp)
) WITH CLUSTERING ORDER BY (activity_timestamp DESC);

-- Query pattern optimized table
CREATE TABLE daily_activity_summary (
    activity_date date,
    activity_type text,
    hour_bucket int,
    activity_count counter,
    PRIMARY KEY ((activity_date, activity_type), hour_bucket)
);

MongoDB Data Modeling

Here’s a corresponding MongoDB schema:

// MongoDB schema for user activity
const userActivitySchema = {
    user_id: ObjectId,
    activities: [{
        timestamp: Date,
        type: String,
        device: {
            id: String,
            type: String,
            os: String
        },
        location: {
            city: String,
            country: String,
            coordinates: {
                type: String,
                coordinates: [Number]
            }
        },
        session: {
            duration: Number,
            start: Date,
            end: Date
        },
        metadata: Schema.Types.Mixed
    }],
    summary: {
        total_sessions: Number,
        average_duration: Number,
        most_used_device: String,
        last_activity: Date
    }
}

4. Query Patterns and Examples

Cassandra Query Examples

-- Finding user activities for a specific date range
SELECT * FROM user_activities 
WHERE user_id = 123e4567-e89b-12d3-a456-426614174000 
AND activity_date >= '2025-01-01' 
AND activity_date <= '2025-01-31';

-- Counting activities by type for a specific day
SELECT activity_type, COUNT(*) 
FROM daily_activity_summary 
WHERE activity_date = '2025-01-27' 
GROUP BY activity_type;

-- Getting latest activities with paging
SELECT * FROM user_activities 
WHERE user_id = 123e4567-e89b-12d3-a456-426614174000 
AND activity_date = '2025-01-27' 
ORDER BY activity_timestamp DESC 
LIMIT 10;

MongoDB Query Examples

// Complex aggregation pipeline for user analytics
db.userActivities.aggregate([
    {
        $match: {
            timestamp: {
                $gte: ISODate("2025-01-01"),
                $lte: ISODate("2025-01-31")
            }
        }
    },
    {
        $group: {
            _id: {
                userId: "$user_id",
                activityType: "$type"
            },
            count: { $sum: 1 },
            avgDuration: { $avg: "$session.duration" }
        }
    },
    {
        $sort: { count: -1 }
    }
]);

// Geospatial query example
db.userActivities.find({
    "location.coordinates": {
        $near: {
            $geometry: {
                type: "Point",
                coordinates: [-73.9667, 40.78]
            },
            $maxDistance: 5000
        }
    }
});

5. Performance Optimization Examples

Cassandra Performance Tuning

# cassandra.yaml performance optimizations
compaction_throughput_mb_per_sec: 64
concurrent_reads: 32
concurrent_writes: 32
concurrent_counter_writes: 32
memtable_allocation_type: heap_buffers
memtable_flush_writers: 4
concurrent_compactors: 4

# JVM settings
-XX:+UseG1GC
-XX:G1RSetUpdatingPauseTimePercent=5
-XX:MaxGCPauseMillis=500
-XX:InitiatingHeapOccupancyPercent=70

MongoDB Performance Tuning

// Index creation with options
db.userActivities.createIndex(
    { "timestamp": 1, "type": 1 },
    { 
        background: true,
        partialFilterExpression: {
            "type": { $exists: true }
        }
    }
);

// Compound index for common queries
db.userActivities.createIndex(
    { 
        "user_id": 1, 
        "timestamp": -1,
        "type": 1 
    }
);

// Collection configuration
db.runCommand({
    collMod: "userActivities",
    validator: {
        $jsonSchema: {
            bsonType: "object",
            required: ["user_id", "timestamp", "type"]
        }
    },
    validationLevel: "moderate"
});

6. Deployment Best Practices

Cassandra Deployment Example

# Docker Compose for Cassandra cluster
version: '3'
services:
  cassandra-node1:
    image: cassandra:latest
    environment:
      - CASSANDRA_CLUSTER_NAME=ProductionCluster
      - CASSANDRA_DC=DC1
      - CASSANDRA_RACK=RACK1
      - CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch
    ports:
      - "9042:9042"
    volumes:
      - cassandra_data1:/var/lib/cassandra

  cassandra-node2:
    image: cassandra:latest
    environment:
      - CASSANDRA_SEEDS=cassandra-node1
      - CASSANDRA_CLUSTER_NAME=ProductionCluster
      - CASSANDRA_DC=DC1
      - CASSANDRA_RACK=RACK1
      - CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch
    volumes:
      - cassandra_data2:/var/lib/cassandra

MongoDB Deployment Example

# Docker Compose for MongoDB replica set
version: '3'
services:
  mongodb-primary:
    image: mongo:latest
    command: mongod --replSet rs0 --bind_ip_all
    ports:
      - "27017:27017"
    volumes:
      - mongodb_data1:/data/db

  mongodb-secondary1:
    image: mongo:latest
    command: mongod --replSet rs0 --bind_ip_all
    volumes:
      - mongodb_data2:/data/db

  mongodb-secondary2:
    image: mongo:latest
    command: mongod --replSet rs0 --bind_ip_all
    volumes:
      - mongodb_data3:/data/db

7. Making the Right Choice

Decision Matrix

Here’s a detailed comparison matrix to help you make your decision:

FeatureCassandraMongoDB
Write PerformanceExcellent (100k+ ops/sec)Good (10k+ ops/sec)
Read PerformanceGood (10k+ ops/sec)Excellent (100k+ ops/sec)
ConsistencyTunableStrong
ScalabilityLinearHorizontal
Query FlexibilityLimitedExtensive
Learning CurveSteepModerate
Use CasesTime-series, IoT, LogsContent Management, Real-time Analytics
Global DistributionBuilt-inRequires Configuration

Recommendation Framework

  1. Choose Cassandra when:
    • You need to handle massive write operations (>10k/second)
    • Your data is time-series based
    • You require multi-datacenter support
    • You can plan queries in advance
    • Linear scalability is crucial
  2. Choose MongoDB when:
    • You need flexible querying capabilities
    • Your schema might evolve frequently
    • You’re building content-heavy applications
    • You need rich indexing options
    • Development speed is crucial

Conclusion

Both Cassandra and MongoDB are powerful databases with distinct strengths. Your choice should align with your specific use case, team expertise, and scalability requirements. Remember to:

  1. Start with a proof of concept
  2. Test with realistic data volumes
  3. Consider your team’s expertise
  4. Plan for future scaling needs
  5. Account for operational costs

The examples and configurations provided in this guide should give you a solid foundation to start implementing either database system. Remember that the best choice depends on your specific requirements and constraints.

Have questions about implementing either database? Share them in the comments below!

Leave a comment