Azure Cosmos DB

Cosmos DB is Azure’s planetary-scale database. It supports multiple APIs: NoSQL (Core/JSON), MongoDB, PostgreSQL, Cassandra, Gremlin (Graph), and Table.

When to Use

• Global Distribution: Replicate data to any Azure region with a click.
• Multi-Model: If you need Mongo or Postgres APIs but want “PaaS” management.
• Low Latency: Guaranteed <10ms read/write latency at the 99th percentile.

Quick Start (NoSQL API)

Container container = database.GetContainer("Items");

Item item = new Item
{
    Id = "1",
    Category = "Personal",
    Name = "Groceries"
};

await container.CreateItemAsync(item, new PartitionKey(item.Category));

Core Concepts

Request Units (RUs)

The currency of Cosmos DB. Use RUs to pay for throughput. 1 RU ≈ reading a 1KB doc.

Partition Key

Crucial. Determines how data is distributed. A bad partition key (“Date”) creates “Hot Partitions” (bottlenecks). A good key (“UserId”) distributes load evenly.

Consistency Levels

Offers 5 levels: Strong, Bounded Staleness, Session (Default), Consistent Prefix, Eventual. Trade off consistency for availability/latency.

Best Practices (2025)

Do:

• Use the NoSQL API: It is the native API with the most features.
• Use Hierarchical Partition Keys (2025): Supports up to 3 keys for better data distribution (TenantId -> UserId -> DeviceId).
• Use Analytical Store (Synapse Link): Run heavy analytics (BI) on your operational data without impacting performance.

Don’t:

• Don’t ignore RU consumption: Monitor it. Queries without a partition key (“Cross-partition queries”) are expensive.

References

• Azure Cosmos DB Documentation