Cosmos DB Service Implementation

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

Installation

pip install azure-cosmos azure-identity

Environment Variables

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):

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

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

Emulator (local development):

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:

# 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

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

When to Use

This skill is applicable to execute the workflow or actions described in the overview.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.