What is the purpose of synthetic partition key in Cosmos DB?

In Cosmos DB, a synthetic partition key is an additional partition key introduced to satisfy certain requirements in data modeling or query scenarios. It is not directly derived from the data itself but is created or assigned based on specific logic or rules defined by the application.

The purpose of using a synthetic partition key in Cosmos DB can vary depending on the specific use case, but here are a few common scenarios:

  1. Custom Partitioning Strategy: In some cases, the natural properties of your data may not be suitable for an effective partition key. In such situations, you can introduce a synthetic partition key that aligns better with your desired partitioning strategy. This allows you to distribute and organize data in a way that suits your access patterns or scalability requirements.
  2. Data Sharding: A synthetic partition key can be used to perform data sharding, which involves dividing a large dataset into smaller subsets for easier management and improved performance. The synthetic partition key can be designed to distribute the data across multiple partitions based on specific criteria, such as time ranges, geographical regions, or any other logical grouping.
  3. Cross-Entity Joins: In Cosmos DB, cross-document or cross-entity joins across partitions are not directly supported. However, by introducing a synthetic partition key, you can create a common value or relationship between related entities that exist in different partitions. This enables the ability to perform logical joins across the synthetic partition key, even though the data physically resides in different partitions.
  4. Logical Grouping or Aggregation: Sometimes, you may need to logically group or aggregate related data across partitions for reporting or analysis purposes. By using a synthetic partition key, you can define logical groups or categories that span multiple partitions and then perform operations such as aggregations, rollups, or analytics on these logical groups.

It’s important to note that the choice and design of a synthetic partition key should align with your specific application requirements, data model, and query patterns. Careful consideration and testing are necessary to ensure that the synthetic partition key achieves the intended purpose and provides the desired performance and scalability benefits in your Cosmos DB application.

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