# Azure Cosmos DB — Azure

Source: https://www.skillbyai.com/en/azure/d-cosmos

> Model data around a partition key and choose a consistency level.

## Globally distributed NoSQL

**Azure Cosmos DB** is a fully managed NoSQL database with single-digit-millisecond reads at any scale and optional multi-region writes. It offers several APIs: **NoSQL** (the native JSON document API, queried with a SQL-like language), **MongoDB**, **Apache Cassandra**, **Apache Gremlin** and **Table**. Throughput is measured in **Request Units (RU/s)**, either provisioned (manual or autoscale) or **serverless**. Data is spread across physical partitions by a **partition key** you choose per container; a good key has many distinct values, spreads writes evenly and appears in most queries. Cosmos DB offers five **consistency levels**: **Strong**, **Bounded staleness**, **Session** (the default, which gives read-your-own-writes within a session), **Consistent prefix** and **Eventual**, trading latency and availability for stronger guarantees.

## Querying within one partition

Including the partition key in a query keeps it to one partition, which is cheaper in RUs than a cross-partition query.

```python
from azure.identity import DefaultAzureCredential
from azure.cosmos import CosmosClient

client = CosmosClient("https://cosmos-shop-dev.documents.azure.com", DefaultAzureCredential())
orders = client.get_database_client("shop").get_container_client("orders")  # partition key: /customerId

orders.upsert_item({"id": "o-1001", "customerId": "c-42", "total": 1499})

items = orders.query_items(
    query="SELECT * FROM o WHERE o.customerId = @c AND o.total > @min",
    parameters=[{"name": "@c", "value": "c-42"}, {"name": "@min", "value": 1000}],
    partition_key="c-42",
)
for order in items:
    print(order["id"], order["total"])
```

## A bad partition key is expensive to fix

Partitioning by something like `status` or `country` creates a few huge, hot partitions. The partition key cannot be changed in place, so fixing it means copying data to a new container. Model the queries you need before creating containers.

**Quiz:** Which consistency level is the Cosmos DB default and guarantees a client reads its own writes?

- [ ] Strong
- [ ] Eventual
- [x] Session
- [ ] Consistent prefix

*Answer:* Session. Session consistency is the default and gives read-your-own-writes within a client session.