Transactions in NoSQL

from pymongo import MongoClient
from pymongo.errors import OperationFailure
import datetime

client  = MongoClient("mongodb://localhost:27017/")
db      = client["marketplace"]

def process_payment(buyer_id, seller_id, amount_gbp):
    # A session is required for multi-document transactions
    with client.start_session() as session:
        try:
            session.start_transaction()

            # Step 1: debit buyer — fail if insufficient funds
            result = db.wallets.update_one(
                {"_id": buyer_id, "balance": {"$gte": amount_gbp}},
                {"$inc": {"balance": -amount_gbp}},
                session=session
            )
            if result.modified_count == 0:
                raise ValueError("Insufficient funds")

            # Step 2: credit seller
            db.wallets.update_one(
                {"_id": seller_id},
                {"$inc": {"balance": amount_gbp}},
                session=session
            )

            # Step 3: insert audit record
            db.transactions.insert_one({
                "buyer_id":  buyer_id,
                "seller_id": seller_id,
                "amount":    amount_gbp,
                "status":    "completed",
                "ts":        datetime.datetime.utcnow()
            }, session=session)

            session.commit_transaction()
            return {"status": "ok", "amount": amount_gbp}

        except Exception as e:
            session.abort_transaction()
            return {"status": "failed", "reason": str(e)}

from cassandra.cluster import Cluster
from cassandra import ConsistencyLevel
from cassandra.query import SimpleStatement

session = Cluster(['localhost']).connect('ticketing')

def reserve_seat(event_id, seat_id, user_id):
    # IF status = 'available' — only succeeds if seat is currently available
    # If two requests arrive simultaneously, only one can win
    stmt = SimpleStatement(
        """UPDATE seats
           SET status = 'reserved', reserved_by = %s
           WHERE event_id = %s AND seat_id = %s
           IF status = 'available'""",
        consistency_level=ConsistencyLevel.QUORUM
    )
    result = session.execute(stmt, (user_id, event_id, seat_id))
    row = result.one()

    if row.applied:
        return {"status": "reserved", "seat": seat_id, "user": user_id}
    else:
        # Row contains current values — show who has it
        return {"status": "already_taken", "reserved_by": row.reserved_by}

import boto3
from botocore.exceptions import ClientError

dynamodb = boto3.client('dynamodb', region_name='eu-west-1')

def purchase_item(product_id, user_id, order_id):
    try:
        dynamodb.transact_write_items(
            TransactItems=[
                # Operation 1: decrement inventory, only if stock > 0
                {
                    'Update': {
                        'TableName': 'inventory',
                        'Key': {'product_id': {'S': product_id}},
                        'UpdateExpression': 'SET stock = stock - :one',
                        'ConditionExpression': 'stock > :zero',
                        'ExpressionAttributeValues': {
                            ':one':  {'N': '1'},
                            ':zero': {'N': '0'}
                        }
                    }
                },
                # Operation 2: create the order record
                {
                    'Put': {
                        'TableName': 'orders',
                        'Item': {
                            'order_id':   {'S': order_id},
                            'user_id':    {'S': user_id},
                            'product_id': {'S': product_id},
                            'status':     {'S': 'confirmed'}
                        },
                        # Prevent duplicate orders for the same order_id
                        'ConditionExpression': 'attribute_not_exists(order_id)'
                    }
                }
            ]
        )
        return {"status": "purchased", "order_id": order_id}

    except ClientError as e:
        if e.response['Error']['Code'] == 'TransactionCanceledException':
            return {"status": "out_of_stock_or_duplicate"}
        raise