Why is partition key selection so important in Cosmos DB?

The partition key determines how data is distributed across physical partitions. A poor choice leads to hot partitions (uneven load), cross-partition queries (high cost), and storage limits. Unlike other settings, you cannot change the partition key after creating a container without migrating data to a new container.

What makes a good partition key?

A good partition key has high cardinality (many distinct values), even distribution (similar amounts of data per value), and aligns with your most frequent query patterns so most queries target a single partition. Common choices include userId, tenantId, or region depending on the workload.

What is a synthetic partition key?

A synthetic partition key is a computed property that combines multiple fields or adds a random suffix to improve distribution. For example, concatenating userId and orderId creates more unique values. Cosmos DB also supports hierarchical partition keys for multi-level distribution.

Azure Cosmos DB Partition Key Advisor

StorageAzure

Analyze Cosmos DB partition key choices for cardinality and distribution.

Cosmos DB Configuration

Analyze Cosmos DB partition key choices for cardinality and distribution.

Required Fields

containercandidateKeyscandidateKeys[0]estimatedCardinalityestimatedCardinality./customerId

Generated Output

Output will appear here...

About This Tool

The Cosmos DB Partition Key Advisor provides guidance on selecting the optimal partition key for your Azure Cosmos DB containers. The partition key is the most important design decision in Cosmos DB, affecting data distribution, query performance, and cost. This tool analyzes your access patterns, data model, and workload characteristics to recommend partition key candidates, estimate partition sizes, and warn about common anti-patterns like hot partitions.

When to Use This Tool

•Evaluate partition key candidates for a new Cosmos DB container by analyzing expected query patterns and data distribution.
•Identify potential hot partition issues before they affect production performance by modeling data distribution across logical partitions.
•Compare different partition key strategies for the same dataset to find the best balance of distribution and query efficiency.
•Get recommendations for synthetic partition keys when no single property provides even distribution.

Frequently Asked Questions

Why is partition key selection so important in Cosmos DB?: The partition key determines how data is distributed across physical partitions. A poor choice leads to hot partitions (uneven load), cross-partition queries (high cost), and storage limits. Unlike other settings, you cannot change the partition key after creating a container without migrating data to a new container.
What makes a good partition key?: A good partition key has high cardinality (many distinct values), even distribution (similar amounts of data per value), and aligns with your most frequent query patterns so most queries target a single partition. Common choices include userId, tenantId, or region depending on the workload.
What is a synthetic partition key?: A synthetic partition key is a computed property that combines multiple fields or adds a random suffix to improve distribution. For example, concatenating userId and orderId creates more unique values. Cosmos DB also supports hierarchical partition keys for multi-level distribution.

Disclaimer: This tool runs entirely in your browser. No data is sent to our servers. Always verify outputs before using them in production. AWS, Azure, and GCP are trademarks of their respective owners.

Azure Cosmos DB Partition Key Advisor

StorageAzure

Analyze Cosmos DB partition key choices for cardinality and distribution.

Cosmos DB Configuration

Analyze Cosmos DB partition key choices for cardinality and distribution.

Required Fields

containercandidateKeyscandidateKeys[0]estimatedCardinalityestimatedCardinality./customerId

Generated Output

Output will appear here...

About This Tool

When to Use This Tool

•Evaluate partition key candidates for a new Cosmos DB container by analyzing expected query patterns and data distribution.
•Identify potential hot partition issues before they affect production performance by modeling data distribution across logical partitions.
•Compare different partition key strategies for the same dataset to find the best balance of distribution and query efficiency.
•Get recommendations for synthetic partition keys when no single property provides even distribution.

Frequently Asked Questions

Why is partition key selection so important in Cosmos DB?: The partition key determines how data is distributed across physical partitions. A poor choice leads to hot partitions (uneven load), cross-partition queries (high cost), and storage limits. Unlike other settings, you cannot change the partition key after creating a container without migrating data to a new container.
What makes a good partition key?: A good partition key has high cardinality (many distinct values), even distribution (similar amounts of data per value), and aligns with your most frequent query patterns so most queries target a single partition. Common choices include userId, tenantId, or region depending on the workload.
What is a synthetic partition key?: A synthetic partition key is a computed property that combines multiple fields or adds a random suffix to improve distribution. For example, concatenating userId and orderId creates more unique values. Cosmos DB also supports hierarchical partition keys for multi-level distribution.