azure-cosmosdb

What this skill does

## Core Principle

**Choose partition key wisely, design for your access patterns, understand consistency tradeoffs.**

Cosmos DB distributes data across partitions. Your partition key choice determines scalability, performance, and cost. Design for even distribution and query efficiency.

---

## Cosmos DB APIs

| API | Use Case |
|-----|----------|
| **NoSQL (Core)** | Document database, most flexible |
| **MongoDB** | MongoDB wire protocol compatible |
| **PostgreSQL** | Distributed PostgreSQL (Citus) |
| **Apache Cassandra** | Wide-column store |
| **Apache Gremlin** | Graph database |
| **Table** | Key-value (Azure Table Storage compatible) |

This skill focuses on **NoSQL (Core) API** - the most common choice.

---

## Key Concepts

| Concept | Description |
|---------|-------------|
| **Container** | Collection of items (like a table) |
| **Item** | Single document/record (JSON) |
| **Partition Key** | Determines data distribution |
| **Logical Partition** | Items with same partition key |
| **Physical Partition** | Storage unit (max 50GB, 10K RU/s) |
| **RU (Request Unit)** | Throughput currency |

---

## Partition Key Design

### Good Partition Keys
```typescript
// High cardinality, even distribution, used in queries

// E-commerce: userId for user data
{ "id": "order-123", "userId": "user-456", ... }  // PK: /userId

// Multi-tenant: tenantId
{ "id": "doc-1", "tenantId": "tenant-abc", ... }  // PK: /tenantId

// IoT: deviceId for telemetry
{ "id": "reading-1", "deviceId": "device-789", ... }  // PK: /deviceId

// Logs: synthetic key (date + category)
{ "id": "log-1", "partitionKey": "2024-01-15_errors", ... }  // PK: /partitionKey
```

### Hierarchical Partition Keys
```typescript
// For multi-level distribution (e.g., tenant → user)
// Container created with: /tenantId, /userId

{
  "id": "order-123",
  "tenantId": "acme-corp",
  "userId": "user-456",
  "items": [...]
}

// Query within tenant and user efficiently
```

### Bad Partition Keys
```typescript
// Avoid:
// - Low cardinality (status, type, boolean)
// - Monotonically increasing (timestamp, auto-increment)
// - Frequently updated fields
// - Fields not used in queries

// Bad: Only 3 values → hot partitions
{ "status": "pending" | "completed" | "cancelled" }

// Bad: All writes go to latest partition
{ "timestamp": "2024-01-15T10:30:00Z" }
```

---

## SDK Setup (TypeScript)

### Install
```bash
npm install @azure/cosmos
```

### Initialize Client
```typescript
// lib/cosmosdb.ts
import { CosmosClient, Database, Container } from '@azure/cosmos';

const endpoint = process.env.COSMOS_ENDPOINT!;
const key = process.env.COSMOS_KEY!;
const databaseId = process.env.COSMOS_DATABASE!;

const client = new CosmosClient({ endpoint, key });

// Or with connection string
// const client = new CosmosClient(process.env.COSMOS_CONNECTION_STRING!);

export const database: Database = client.database(databaseId);

export function getContainer(containerId: string): Container {
  return database.container(containerId);
}
```

### Type Definitions
```typescript
// types/cosmos.ts
export interface BaseItem {
  id: string;
  _ts?: number;      // Auto-generated timestamp
  _etag?: string;    // For optimistic concurrency
}

export interface User extends BaseItem {
  userId: string;    // Partition key
  email: string;
  name: string;
  createdAt: string;
  updatedAt: string;
}

export interface Order extends BaseItem {
  userId: string;    // Partition key
  orderId: string;
  items: OrderItem[];
  total: number;
  status: 'pending' | 'paid' | 'shipped' | 'delivered';
  createdAt: string;
}

export interface OrderItem {
  productId: string;
  name: string;
  quantity: number;
  price: number;
}
```

---

## CRUD Operations

### Create Item
```typescript
import { getContainer } from './cosmosdb';
import { User } from './types';

const usersContainer = getContainer('users');

async function createUser(data: Omit<User, 'id' | 'createdAt' | 'updatedAt'>): Promise<User> {
  const now = new Date().toISOString();
  const user: User = {
    id: crypto.randomUUID(),
    ...data,
    createdAt: now,
    updatedAt: now
  };

  const { resource } = await usersContainer.items.create(user);
  return resource as User;
}
```

### Read Item (Point Read)
```typescript
// Most efficient read - requires id AND partition key
async function getUser(userId: string, id: string): Promise<User | null> {
  try {
    const { resource } = await usersContainer.item(id, userId).read<User>();
    return resource || null;
  } catch (error: any) {
    if (error.code === 404) return null;
    throw error;
  }
}

// If id equals partition key value
async function getUserById(userId: string): Promise<User | null> {
  try {
    const { resource } = await usersContainer.item(userId, userId).read<User>();
    return resource || null;
  } catch (error: any) {
    if (error.code === 404) return null;
    throw error;
  }
}
```

### Query Items
```typescript
// Query within partition (efficient)
async function getUserOrders(userId: string): Promise<Order[]> {
  const ordersContainer = getContainer('orders');

  const { resources } = await ordersContainer.items
    .query<Order>({
      query: 'SELECT * FROM c WHERE c.userId = @userId ORDER BY c.createdAt DESC',
      parameters: [{ name: '@userId', value: userId }]
    })
    .fetchAll();

  return resources;
}

// Cross-partition query (use sparingly)
async function getOrdersByStatus(status: string): Promise<Order[]> {
  const ordersContainer = getContainer('orders');

  const { resources } = await ordersContainer.items
    .query<Order>({
      query: 'SELECT * FROM c WHERE c.status = @status',
      parameters: [{ name: '@status', value: status }]
    })
    .fetchAll();

  return resources;
}

// Paginated query
async function getOrdersPaginated(
  userId: string,
  pageSize: number = 10,
  continuationToken?: string
): Promise<{ items: Order[]; continuationToken?: string }> {
  const ordersContainer = getContainer('orders');

  const queryIterator = ordersContainer.items.query<Order>(
    {
      query: 'SELECT * FROM c WHERE c.userId = @userId ORDER BY c.createdAt DESC',
      parameters: [{ name: '@userId', value: userId }]
    },
    {
      maxItemCount: pageSize,
      continuationToken
    }
  );

  const { resources, continuationToken: nextToken } = await queryIterator.fetchNext();

  return {
    items: resources,
    continuationToken: nextToken
  };
}
```

### Update Item
```typescript
// Replace entire item
async function updateUser(userId: string, id: string, updates: Partial<User>): Promise<User> {
  const existing = await getUser(userId, id);
  if (!existing) throw new Error('User not found');

  const updated: User = {
    ...existing,
    ...updates,
    updatedAt: new Date().toISOString()
  };

  const { resource } = await usersContainer.item(id, userId).replace(updated);
  return resource as User;
}

// Partial update (patch operations)
async function patchUser(userId: string, id: string, operations: any[]): Promise<User> {
  const { resource } = await usersContainer.item(id, userId).patch(operations);
  return resource as User;
}

// Usage:
await patchUser('user-123', 'user-123', [
  { op: 'set', path: '/name', value: 'New Name' },
  { op: 'set', path: '/updatedAt', value: new Date().toISOString() },
  { op: 'incr', path: '/loginCount', value: 1 }
]);
```

### Delete Item
```typescript
async function deleteUser(userId: string, id: string): Promise<void> {
  await usersContainer.item(id, userId).delete();
}
```

### Optimistic Concurrency (ETags)
```typescript
async function updateUserWithETag(
  userId: string,
  id: string,
  updates: Partial<User>,
  etag: string
): Promise<User> {
  const existing = await getUser(userId, id);
  if (!existing) throw new Error('User not found');

  const updated: User = {
    ...existing,
    ...updates,
    updatedAt: new Date().toISOString()
  };

  try {
    const { resource } = await usersContainer.item(id, userId).replace(updat