Partition key design: high cardinality, even spread, point reads; hierarchical partition keys

Cosmos DB scales out by partitioning. Every item's partition key value places it in a logical partition, the set of all items that share that value. Logical partitions are grouped onto physical partitions, each with its own storage and a share of the container's throughput. You pick the partition key path, such as /customerId, when creating the container, and it cannot be changed afterwards without migrating the data to a new container. That makes it the most important design decision in Cosmos DB.