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Question: What are the scaling limits of MongoDB?


"Mongodb, a popular NoSQL database, is known for its scalability, both vertically and horizontally. However, like any system, it has its limits and considerations when scaling.

Horizontal Scaling (Sharding)

Horizontal scaling, or sharding, distributes data across multiple machines. MongoDB supports sharding out-of-the-box, but there are several factors to consider:

  • Shard Key Selection: The choice of shard key impacts the distribution of data and query performance. Poorly chosen shard keys can lead to unbalanced clusters or hotspots.
  • Number of Shards: While MongoDB can support a large number of shards, each additional shard increases the operational complexity and the overhead of managing the cluster.
  • Balancing Chunks: MongoDB automatically balances data across shards, but this process can impact performance if shards become imbalanced.

Vertical Scaling

Vertical scaling involves adding more resources to the existing machines in a MongoDB deployment. Limits include:

  • Hardware Maximums: Even high-performance servers have limits in terms of CPU, memory, and I/O capacity.
  • Cost: Beyond certain points, vertical scaling becomes cost-prohibitive compared to horizontal scaling.

Practical Limits

  • WiredTiger Storage Engine Limitations: MongoDB's default storage engine, WiredTiger, has a document size limit of 16 MB and a maximum BSON document size limit. Additionally, while MongoDB databases don't have a set limit on storage, individual collection sizes can be impacted by the underlying filesystem.
  • Replication: MongoDB uses an oplog for replication. The size of this log can impact how far back in time a secondary can lag behind the primary without requiring a full resynchronization.
  • Network Latency: In sharded clusters or replica sets distributed over large geographic distances, network latency can significantly affect read/write operations' performance.

MongoDB scales well with careful planning and consideration of the above factors. Regular monitoring and adjustments ensure that a MongoDB deployment can handle growing data and user loads effectively."

Example Consideration: Shard Key

Choosing an effective shard key is crucial for ensuring even data distribution across shards. For example, selecting a shard key based on a frequently accessed field that evenly splits the data can prevent hotspots and improve query performance.

db.createCollection("orders", { sharded: true, shardKey: { orderDate : 1 } });

This example demonstrates creating a sharded collection orders with orderDate as the shard key, aiming for an even distribution of documents across shards based on the date of orders.

Remember, continuous monitoring and adjusting your MongoDB setup is key to maintaining optimal performance and cost efficiency as your system scales.

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