One of the keys (pun intended) of achieving optimal performance out of CosmosDB is partitioning properly.  In partitioning, the items in a container are divided into distinct subsets called logical partitions.

Azure Cosmos DB transparently and automatically manages the placement of logical partitions on physical partitions to efficiently satisfy the scalability and performance needs of the container. As the throughput and storage requirements of an application increase, Azure Cosmos DB moves logical partitions to automatically spread the load across a greater number of servers.

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