name: cosmos-db-python-skill
description: Build Azure Cosmos DB NoSQL services with Python/FastAPI following production-grade patterns. Use when implementing database client setup with dual auth (DefaultAzureCredential + emulator), service layer classes with CRUD operations, partition key strategies, parameterized queries, or TDD patterns for Cosmos. Triggers on phrases like "Cosmos DB", "NoSQL database", "document store", "add persistence", "database service layer", or "Python Cosmos SDK".

Cosmos DB Service Implementation

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

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:

# 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:

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

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

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

Clean Code Conventions

1. Single Responsibility: Client module handles connection; services handle business logic
2. Graceful Degradation: Services return None/[] when Cosmos unavailable
3. Consistent Naming: _doc_to_model(), _model_to_doc(), _use_cosmos()
4. Type Hints: Full typing on all public methods
5. CamelCase Aliases: Use Field(alias="camelCase") for JSON serialization

TDD Requirements

Write tests BEFORE implementation using these patterns:

@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

Template Files

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