MongoDB in the NoSQL Ecosystem

// Firestore real-time listener — JavaScript (web/mobile)
import { getFirestore, doc, onSnapshot } from "firebase/firestore"

const db = getFirestore()

// Subscribe to a task document — fires on every change
const unsubscribe = onSnapshot(
  doc(db, "tasks", "task_8821"),
  (snapshot) => {
    const task = snapshot.data()
    // This callback runs automatically whenever ANY client updates this doc
    console.log("Task updated:", task.title, "— Status:", task.status)
  }
)

// To stop listening:
// unsubscribe()

// Firestore write — optimistic offline support built in
import { updateDoc, serverTimestamp } from "firebase/firestore"

await updateDoc(doc(db, "tasks", "task_8821"), {
  status:     "completed",
  completed_by: "u_441",
  completed_at: serverTimestamp()   // server-side timestamp — not client clock
})

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017')
db = client['storefront']

# Watch for any insert or update on the orders collection
pipeline = [{'$match': {'operationType': {'$in': ['insert', 'update']}}}]

with db.orders.watch(pipeline) as stream:
    print("Watching for order changes...")
    for change in stream:                            # blocks, waiting for changes
        op   = change['operationType']               # 'insert' or 'update'
        doc  = change.get('fullDocument', {})        # the changed document
        keys = change.get('updateDescription', {})   # what fields changed

        if op == 'insert':
            print(f"New order: {doc.get('order_id')} — £{doc.get('total')}")
        elif op == 'update':
            print(f"Order updated. Changed fields: {keys.get('updatedFields', {}).keys()}")

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017/?replicaSet=rs0')
db = client['storefront']

def purchase_product(user_id, product_id, quantity):
    # Start a session for the transaction
    with client.start_session() as session:
        with session.start_transaction():

            # Step 1: Check and decrement inventory
            result = db.products.find_one_and_update(
                {'_id': product_id, 'stock': {'$gte': quantity}},
                {'$inc': {'stock': -quantity}},
                session=session
            )

            if not result:
                # Insufficient stock — transaction auto-aborts
                raise ValueError("Insufficient stock")

            # Step 2: Create the order record
            db.orders.insert_one({
                'user_id':    user_id,
                'product_id': product_id,
                'quantity':   quantity,
                'status':     'confirmed'
            }, session=session)

            # Both operations committed atomically