How to Tune Up Azure Cosmos DB for Performance and Cost Efficiency

Azure Cosmos DB is a fully managed NoSQL database designed for high availability, scalability, and low-latency access. However, without proper tuning, you may encounter performance bottlenecks and unexpected costs. Here’s how to optimise your Cosmos DB setup for peak efficiency.

1. Optimise Partitioning Strategy

Cosmos DB uses partitioning to scale horizontally, but a poor partition key choice can lead to uneven data distribution, causing hotspots and throttling. To improve performance:

• Choose a partition key that evenly distributes data and query load.

• Use synthetic keys (e.g., combining customerId and orderId) if no natural key ensures even distribution.

• Monitor physical partitions in the Azure Portal to identify imbalances.

2. Fine-Tune Request Units (RUs)

Request Units (RUs) determine the cost of queries, inserts, and updates. To optimise RUs:

• Enable Auto-Scale: Adjusts RU consumption dynamically based on demand.

• Optimise Queries: Use indexing selectively and avoid full document scans.

• Use Bulk Mode for Inserts: The .NET SDK and bulk executor library reduce RU consumption for batch operations.

• Leverage Dedicated Containers: If a specific query pattern is consuming high RUs, consider isolating it into a separate container.

3. Optimise Indexing Policies

By default, Cosmos DB indexes all properties, which can be costly. To improve performance:

• Use Custom Indexing Policies: Exclude unneeded properties with indexing paths.

• Use Lazy Indexing for Heavy Write Workloads: Reduces the impact on write-heavy applications.

• Monitor Index Utilisation: The Azure Portal and Query Metrics help identify unnecessary indexes.

4. Improve Query Performance

Slow queries lead to high RU consumption. Optimise them by:

• Using SELECT VALUE Instead of SELECT * to fetch only needed fields.

• Avoiding Cross-Partition Queries: Design queries to target a single partition.

• Using ORDER BY on Indexed Fields: Sorting on non-indexed fields increases RUs drastically.

• Using Stored Procedures for Complex Logic: They execute on the server, reducing round trips.

5. Enable TTL and Multi-Region Writes

• TTL (Time-to-Live): Automatically deletes old documents to free up storage.

• Multi-Region Writes: Distribute workloads globally and improve write latencies.

6. Monitor and Tune Regularly

Use Azure Monitor, Application Insights, and Cosmos DB Metrics to:

• Track RU consumption per operation.

• Identify hot partitions and inefficient queries.

• Analyse latency, availability, and consistency metrics to adjust configurations accordingly.

Final Thoughts

Tuning Azure Cosmos DB is an ongoing process that requires balancing performance, cost, and scalability. Regular monitoring, efficient indexing, and well-structured partitioning are key to getting the most out of your database.

By implementing these best practices, you’ll achieve faster query responses, reduced costs, and improved overall efficiency in your Cosmos DB setup.