Partition Keys and Sort Keys

Choosing the right primary key is the single most important design decision in DynamoDB. This lesson explains how partition keys and sort keys work, how DynamoDB distributes data, and how to design keys that enable efficient access patterns.

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How DynamoDB Stores Data

DynamoDB uses partitioning to distribute data across multiple storage nodes:

1. When you write an item, DynamoDB hashes the partition key value
2. The hash determines which physical partition stores the item
3. Within a partition, items are sorted by the sort key (if one exists)

                     Hash Function
                         │
        ┌────────────────┼────────────────┐
        ▼                ▼                ▼
   Partition A      Partition B      Partition C
   ┌──────────┐    ┌──────────┐    ┌──────────┐
   │ Items     │    │ Items     │    │ Items     │
   │ sorted by │    │ sorted by │    │ sorted by │
   │ sort key  │    │ sort key  │    │ sort key  │
   └──────────┘    └──────────┘    └──────────┘

Each partition can store up to 10 GB of data and handle 3,000 read capacity units (RCUs) and 1,000 write capacity units (WCUs).

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The Partition Key

The partition key (also called the hash key) determines data distribution:

Good Partition Keys