Cassandra vs MongoDB

Cassandra and MongoDB are both popular NoSQL databases but represent different data models and architectural philosophies. Cassandra is a wide-column store optimized for writes; MongoDB is a document store optimized for flexible queries.

Architecture Differences

Cassandra uses a masterless ring architecture where every node is equal. Writes go to any node (coordinator) and are replicated to N nodes based on the replication factor. The LSM-tree storage engine makes writes extremely fast: they are always sequential appends.

MongoDB uses a primary-secondary architecture within replica sets. One primary handles all writes within a shard, and secondaries replicate asynchronously. Sharding distributes data across replica sets using a shard key.

Performance Characteristics

Cassandra excels at write throughput. Its append-only LSM-tree storage and masterless architecture mean writes are distributed and never blocked. MongoDB's B-tree (WiredTiger) storage provides more balanced read/write performance with in-place updates.

For high-write system designs like messaging, activity feeds, and IoT ingestion, Cassandra's architecture is purpose-built. MongoDB handles these workloads well but Cassandra's linear write scaling is more predictable.

Trade-offs

Cassandra requires you to design your data model around your query patterns. If you need a new query pattern, you may need a new table with denormalized data. MongoDB allows ad-hoc queries on any field, providing much more query flexibility.

Cassandra's masterless architecture eliminates single points of failure: any node going down does not impact availability. MongoDB's primary-secondary model means a primary failure triggers an election, causing brief write unavailability.

Data Modeling

Cassandra's wide-column model organizes data by partition key and clustering columns. You query by partition key and optionally filter by clustering columns. MongoDB's document model stores related data together, with flexible indexes on any field.

Real-World Usage

Cassandra powers Apple (400K+ nodes), Netflix, and Instagram. MongoDB runs at eBay, Forbes, and Toyota. The choice depends on your primary access pattern: heavy writes with known queries (Cassandra) versus flexible querying (MongoDB).

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