title: Azure Cosmos DB partition key - is primary key acceptable?
body: <p>Our Azure Cosmos DB collection has gotten large enough to require a partition key. In doing some reading about this, I get the impression that the best partition key is one that provides for even distribution and higher cardinality. <a href="https://docs.microsoft.com/en-us/azure/cosmos-db/partition-data" rel="noreferrer">This article</a> from Microsoft discusses it.</p>
  
  <p>Using a primary key as a partition key provides for even distribution, but a cardinality of only 1. If this is my only option, is this a bad thing? The aforementioned article gives a few <a href="https://docs.microsoft.com/en-us/azure/cosmos-db/partition-data#design-for-scale" rel="noreferrer">examples</a> and seems to indicate that the primary key should be used as a partition key in those instances. In the case of Azure Cosmos DB, the partitions are logical, not physical. So it wouldn't lead to having each document on its own disk, but it seems like it could lead to a bloated index.</p>
  
  <p>Is using a primary key as a partition key a common practice? Are there any downsides to it?</p>