Cosmos DB Service Implementation

Build production-grade Azure Cosmos DB NoSQL services following clean code, security best practices, and TDD principles.

Installation

bash

pip install azure-cosmos azure-identity

Environment Variables

bash

COSMOS_ENDPOINT=https://<account>.documents.azure.com:443/
COSMOS_DATABASE_NAME=<database-name>
COSMOS_CONTAINER_ID=<container-id>
# For emulator only (not production)
COSMOS_KEY=<emulator-key>

Authentication

DefaultAzureCredential (preferred):

python

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

client = CosmosClient(
    url=os.environ["COSMOS_ENDPOINT"],
    credential=DefaultAzureCredential()
)

Emulator (local development):

python

from azure.cosmos import CosmosClient

client = CosmosClient(
    url="https://localhost:8081",
    credential=os.environ["COSMOS_KEY"],
    connection_verify=False
)

Architecture Overview

┌─────────────────────────────────────────────────────────────────┐
│                         FastAPI Router                          │
│  - Auth dependencies (get_current_user, get_current_user_required)
│  - HTTP error responses (HTTPException)                         │
└──────────────────────────────┬──────────────────────────────────┘
                               │
┌──────────────────────────────▼──────────────────────────────────┐
│                        Service Layer                            │
│  - Business logic and validation                                │
│  - Document ↔ Model conversion                                  │
│  - Graceful degradation when Cosmos unavailable                 │
└──────────────────────────────┬──────────────────────────────────┘
                               │
┌──────────────────────────────▼──────────────────────────────────┐
│                     Cosmos DB Client Module                     │
│  - Singleton container initialization                           │
│  - Dual auth: DefaultAzureCredential (Azure) / Key (emulator)   │
│  - Async wrapper via run_in_threadpool                          │
└─────────────────────────────────────────────────────────────────┘

Quick Start

1. Client Module Setup

Create a singleton Cosmos client with dual authentication:

python

# db/cosmos.py
from azure.cosmos import CosmosClient
from azure.identity import DefaultAzureCredential
from starlette.concurrency import run_in_threadpool

_cosmos_container = None

def _is_emulator_endpoint(endpoint: str) -> bool:
    return "localhost" in endpoint or "127.0.0.1" in endpoint

async def get_container():
    global _cosmos_container
    if _cosmos_container is None:
        if _is_emulator_endpoint(settings.cosmos_endpoint):
            client = CosmosClient(
                url=settings.cosmos_endpoint,
                credential=settings.cosmos_key,
                connection_verify=False
            )
        else:
            client = CosmosClient(
                url=settings.cosmos_endpoint,
                credential=DefaultAzureCredential()
            )
        db = client.get_database_client(settings.cosmos_database_name)
        _cosmos_container = db.get_container_client(settings.cosmos_container_id)
    return _cosmos_container

Full implementation: See references/client-setup.md

2. Pydantic Model Hierarchy

Use five-tier model pattern for clean separation:

python

class ProjectBase(BaseModel):           # Shared fields
    name: str = Field(..., min_length=1, max_length=200)

class ProjectCreate(ProjectBase):       # Creation request
    workspace_id: str = Field(..., alias="workspaceId")

class ProjectUpdate(BaseModel):         # Partial updates (all optional)
    name: Optional[str] = Field(None, min_length=1)

class Project(ProjectBase):             # API response
    id: str
    created_at: datetime = Field(..., alias="createdAt")

class ProjectInDB(Project):             # Internal with docType
    doc_type: str = "project"

3. Service Layer Pattern

python

class ProjectService:
    def _use_cosmos(self) -> bool:
        return get_container() is not None
    
    async def get_by_id(self, project_id: str, workspace_id: str) -> Project | None:
        if not self._use_cosmos():
            return None
        doc = await get_document(project_id, partition_key=workspace_id)
        if doc is None:
            return None
        return self._doc_to_model(doc)

Full patterns: See references/service-layer.md

Core Principles

Security Requirements

RBAC Authentication: Use
```
DefaultAzureCredential
```
in Azure — never store keys in code
Emulator-Only Keys: Hardcode the well-known emulator key only for local development
Parameterized Queries: Always use
```
@parameter
```
syntax — never string concatenation
Partition Key Validation: Validate partition key access matches user authorization

Clean Code Conventions

Single Responsibility: Client module handles connection; services handle business logic
Graceful Degradation: Services return
```
None
```
/
```
[]
```
when Cosmos unavailable

Consistent Naming:

_doc_to_model()

_model_to_doc()

_use_cosmos()

Type Hints: Full typing on all public methods
CamelCase Aliases: Use
```
Field(alias="camelCase")
```
for JSON serialization

TDD Requirements

Write tests BEFORE implementation using these patterns:

python

@pytest.fixture
def mock_cosmos_container(mocker):
    container = mocker.MagicMock()
    mocker.patch("app.db.cosmos.get_container", return_value=container)
    return container

@pytest.mark.asyncio
async def test_get_project_by_id_returns_project(mock_cosmos_container):
    # Arrange
    mock_cosmos_container.read_item.return_value = {"id": "123", "name": "Test"}
    
    # Act
    result = await project_service.get_by_id("123", "workspace-1")
    
    # Assert
    assert result.id == "123"
    assert result.name == "Test"

Full testing guide: See references/testing.md

Reference Files

File	When to Read
references/client-setup.md	Setting up Cosmos client with dual auth, SSL config, singleton pattern
references/service-layer.md	Implementing full service class with CRUD, conversions, graceful degradation
references/testing.md	Writing pytest tests, mocking Cosmos, integration test setup
references/partitioning.md	Choosing partition keys, cross-partition queries, move operations
references/error-handling.md	Handling CosmosResourceNotFoundError, logging, HTTP error mapping

Template Files

File	Purpose
assets/cosmos_client_template.py	Ready-to-use client module
assets/service_template.py	Service class skeleton
assets/conftest_template.py	pytest fixtures for Cosmos mocking

Quality Attributes (NFRs)

Reliability

Graceful degradation when Cosmos unavailable
Retry logic with exponential backoff for transient failures
Connection pooling via singleton pattern

Security

Zero secrets in code (RBAC via DefaultAzureCredential)
Parameterized queries prevent injection
Partition key isolation enforces data boundaries

Maintainability

Five-tier model pattern enables schema evolution
Service layer decouples business logic from storage
Consistent patterns across all entity services

Testability

Dependency injection via
```
get_container()
```
Easy mocking with module-level globals
Clear separation enables unit testing without Cosmos

Performance

Partition key queries avoid cross-partition scans
Async wrapping prevents blocking FastAPI event loop
Minimal document conversion overhead

azure-cosmos-db-py

NPX Install

Tags

SKILL.md Content

Cosmos DB Service Implementation

Installation

Environment Variables

Authentication

Architecture Overview

Quick Start

1. Client Module Setup

2. Pydantic Model Hierarchy

3. Service Layer Pattern

Core Principles

Security Requirements

Clean Code Conventions

TDD Requirements

Reference Files

Template Files

Quality Attributes (NFRs)

Reliability

Security

Maintainability

Testability

Performance