What is Partition Key in Cosmos DB?

In Cosmos DB, the partition key is a property within each item stored in a container that is used to determine the physical partition where the item will be stored. Cosmos DB uses a distributed database architecture that partitions data across multiple physical partitions to achieve scalability and performance.

Each container in Cosmos DB has one or more logical partitions, and each logical partition is mapped to a physical partition. The partition key value is hashed to determine the target physical partition. All items with the same partition key value are stored together within the same physical partition.

The partition key is important because it directly affects the scalability and performance characteristics of your Cosmos DB container. By choosing a good partition key, you can evenly distribute the data across multiple partitions, allowing for parallel processing and efficient distribution of workload. On the other hand, choosing a poor partition key can lead to uneven data distribution, known as a “hot partition,” which can cause performance bottlenecks.

When designing your data model in Cosmos DB, it is important to select a partition key that evenly distributes the data and aligns with your access patterns. Some common approaches for selecting a partition key include using a property that has a high cardinality (i.e., a wide range of distinct values) or combining multiple properties to form a composite partition key.

It’s worth noting that the partition key is chosen at the time of container creation and cannot be changed afterwards. Therefore, careful consideration should be given to selecting an appropriate partition key based on your data and query patterns to ensure optimal performance and scalability in Cosmos DB.

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