The general rule with Amazon DynamoDB is to choose a high cardinality partition key.
In DynamoDB, a partition key that doesn’t have a high cardinality can result in many requests targeting only a few partitions and resulting in a hot partition. A hot partition can cause throttling if the partition limits of 3000 RCU or 1000 WCU (or a combination of both) per second are exceeded.
Write sharding is a mechanism to distribute a collection across a DynamoDB table’s partitions effectively. In the context of DynamoDB, sharding refers to adjusting the partition key to increase it’s cardinality. Increased cardinality, means items are distributed across a larger number of partitions.