azure-cosmosdb

Azure Cosmos DB Skill

Load with: base.md + [typescript.md | python.md]

Azure Cosmos DB is a globally distributed, multi-model database with guaranteed low latency, elastic scalability, and multiple consistency models.

Sources: Cosmos DB Docs | Partitioning | SDK

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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.

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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.

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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

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Partition Key Design

Good Partition Keys

// 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

// 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

// 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" }

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SDK Setup (TypeScript)

Install

npm install @azure/cosmos

Initialize Client

// 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

// 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;
}

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CRUD Operations

Create Item

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)

// 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

// 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

// 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

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

Optimistic Concurrency (ETags)

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(updated, {
      accessCondition: { type: 'IfMatch', condition: etag }
    });
    return resource as User;
  } catch (error: any) {
    if (error.code === 412) {
      throw new Error('Document was modified by another process');
    }
    throw error;
  }
}

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Consistency Levels

Level	Guarantees	Latency	Use Case
Strong	Linearizable reads	Highest	Financial, inventory
Bounded Staleness	Consistent within bounds	High	Leaderboards, counters
Session	Read your writes	Medium	User sessions (default)
Consistent Prefix	Ordered reads	Low	Social feeds
Eventual	No ordering guarantee	Lowest	Analytics, logs

Set Consistency Per Request

// Override default consistency
const { resource } = await usersContainer.item(id, userId).read<User>({
  consistencyLevel: 'Strong'
});

// For queries
const { resources } = await container.items.query(
  { query: 'SELECT * FROM c' },
  { consistencyLevel: 'BoundedStaleness' }
).fetchAll();

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Batch Operations

Transactional Batch (Same Partition)

async function createOrderWithItems(userId: string, order: Order, items: any[]): Promise<void> {
  const ordersContainer = getContainer('orders');

  const operations = [
    { operationType: 'Create' as const, resourceBody: order },
    ...items.map(item => ({
      operationType: 'Create' as const,
      resourceBody: { ...item, userId, orderId: order.orderId }
 

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