Partitioning in Azure Cosmos DB

Partitioning is a fundamental concept in Azure Cosmos DB that enables massive scalability, high availability, and global distribution. It involves horizontally partitioning your data across multiple logical and physical partitions. This strategy ensures that your database can handle large volumes of data and high throughput requests.

What is a Partition Key?

A partition key is a property within your item's JSON document that determines which logical partition the item belongs to. Choosing an effective partition key is crucial for performance and scalability. The partition key value is hashed, and the result of the hash determines the physical partition to which the item is routed.

How Partitioning Works

Logical Partitions: Items with the same partition key value are grouped into a logical partition.
Physical Partitions: Logical partitions are mapped to one or more physical partitions. Azure Cosmos DB automatically manages the distribution and movement of logical partitions across physical partitions to balance load and storage.
Throughput Provisioning: Request Units (RUs) are provisioned at the container level and distributed across the physical partitions. This ensures that your application can achieve consistent performance.

Key Considerations for Partitioning

Cardinality: A high-cardinality partition key (many unique values) is generally preferred for better distribution of requests and data.
Uniformity: Aim for a partition key that distributes your read and write operations uniformly across partitions to avoid "hot spots" (partitions that receive a disproportionate amount of traffic).
Query Patterns: Consider your common query patterns. If you frequently filter or partition your data by a specific property, that property can be a good candidate for a partition key.
Max Item Count per Partition: Each logical partition has a maximum item count of 20 GB.
Max RU/s per Partition: Each physical partition has a maximum throughput limit (e.g., 10,000 RU/s).

Partition Key Selection Strategies

Common strategies for selecting partition keys include:

Using a unique identifier like UserID or SessionID for user-centric data.
Using a geographical identifier like Region or City for geographically distributed data.
Combining properties to create a unique partition key if a single property does not offer sufficient cardinality.

Example of Partition Key Usage

Consider a collection of user profiles. If UserID is chosen as the partition key, all items belonging to the same user will reside in the same logical partition. This is beneficial for queries that fetch all data for a specific user.


{
    "id": "user123",
    "name": "Alice Smith",
    "email": "alice.smith@example.com",
    "city": "New York",
    "registeredOn": "2023-10-27T10:00:00Z",
    "partitionKey": "user123" // Assuming UserID is the partition key
}

Automatic vs. Manual Partitioning

Azure Cosmos DB offers automatic partition management. You define the partition key at the time of container creation, and the service handles the distribution and scaling of data across physical partitions. You don't need to manually manage the underlying infrastructure.

Scalability and Performance

Effective partitioning is key to unlocking the full scalability potential of Azure Cosmos DB. By distributing data and workload across many physical partitions, you can achieve near-limitless throughput and storage capacity. The service automatically rebalances partitions as your data grows or your workload changes, ensuring consistent performance.