const { MongoClient } = require('mongodb');

const client = new MongoClient('mongodb://localhost:27017', {
  useNewUrlParser: true,
  useUnifiedTopology: true,
  maxPoolSize: 10
});

async function main() {
  try {
    await client.connect();
    const db = client.db('myapp');
    const collection = db.collection('users');

    // Use collection

  } finally {
    await client.close();
  }
}

main().catch(console.error);

// Configure connection pool
const client = new MongoClient(
  'mongodb://localhost:27017',
  {
    maxPoolSize: 10,        // Maximum connections
    minPoolSize: 5,         // Minimum connections
    maxIdleTimeMS: 45000,   // Idle timeout
    serverSelectionTimeoutMS: 5000
  }
);

const session = client.startSession();

try {
  await session.withTransaction(async () => {
    // Multiple operations atomic together
    await users.insertOne({ name: 'John' }, { session });
    await accounts.insertOne({ userId: '...', balance: 100 }, { session });

    // If any fails, all roll back
  });
} catch (error) {
  console.error('Transaction failed:', error);
} finally {
  await session.endSession();
}

try {
  await collection.insertOne({ email: 'test@example.com' });
} catch (error) {
  if (error.code === 11000) {
    console.log('Duplicate key error');
  } else if (error.name === 'MongoNetworkError') {
    console.log('Network error - retry');
  } else if (error.name === 'MongoWriteConcernError') {
    console.log('Write concern failed');
  } else {
    console.log('Other error:', error);
  }
}

const bulk = collection.initializeUnorderedBulkOp();

bulk.insert({ name: 'John', age: 30 });
bulk.insert({ name: 'Jane', age: 28 });
bulk.find({ age: { $lt: 25 } }).update({ $set: { status: 'young' } });
bulk.find({ name: 'old_user' }).remove();

await bulk.execute();

// Watch collection for changes
const changeStream = collection.watch();

changeStream.on('change', (change) => {
  console.log('Change:', change.operationType);
  console.log('Document:', change.fullDocument);
});

// With pipeline filtering
const pipeline = [
  { $match: { 'operationType': 'insert' } },
  { $match: { 'fullDocument.status': 'active' } }
];

const filteredStream = collection.watch(pipeline);

from pymongo import MongoClient
from pymongo.errors import DuplicateKeyError
import asyncio

# Synchronous
client = MongoClient('mongodb://localhost:27017')
db = client['myapp']
users = db['users']

# CRUD
user = users.insert_one({'name': 'John', 'email': 'john@example.com'})
found = users.find_one({'_id': user.inserted_id})
users.update_one({'_id': user.inserted_id}, {'$set': {'age': 30}})
users.delete_one({'_id': user.inserted_id})

# Transactions
def transfer_funds(from_user_id, to_user_id, amount):
    with client.start_session() as session:
        with session.start_transaction():
            users.update_one(
                {'_id': from_user_id},
                {'$inc': {'balance': -amount}},
                session=session
            )
            users.update_one(
                {'_id': to_user_id},
                {'$inc': {'balance': amount}},
                session=session
            )

# Error handling
try:
    users.insert_one({'email': 'test@example.com'})
except DuplicateKeyError:
    print('Email already exists')

import com.mongodb.client.MongoClient;
import com.mongodb.client.MongoClients;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.MongoCollection;
import org.bson.Document;

public class MongoDB {
    public static void main(String[] args) {
        // Connect
        MongoClient mongoClient = MongoClients.create("mongodb://localhost:27017");
        MongoDatabase database = mongoClient.getDatabase("myapp");
        MongoCollection<Document> users = database.getCollection("users");

        // Insert
        Document user = new Document("name", "John")
            .append("email", "john@example.com")
            .append("age", 30);
        users.insertOne(user);

        // Find
        Document found = users.find(new Document("email", "john@example.com")).first();

        // Update
        users.updateOne(
            new Document("email", "john@example.com"),
            new Document("$set", new Document("age", 31))
        );

        // Delete
        users.deleteOne(new Document("email", "john@example.com"));

        // Close
        mongoClient.close();
    }
}

// Pagination
async function paginate(collection, page = 1, limit = 10) {
  const skip = (page - 1) * limit;
  return collection
    .find({})
    .skip(skip)
    .limit(limit)
    .toArray();
}

// Batch processing
async function processBatch(collection, batchSize = 1000) {
  const cursor = collection.find({});

  let batch = [];
  for (const doc of await cursor.toArray()) {
    batch.push(doc);

    if (batch.length === batchSize) {
      await processBatch(batch);
      batch = [];
    }
  }

  if (batch.length > 0) {
    await processBatch(batch);
  }
}

// Lazy loading
class LazyLoader {
  constructor(collection, query) {
    this.cursor = collection.find(query).batchSize(100);
  }

  async next() {
    return this.cursor.hasNext() ? this.cursor.next() : null;
  }
}