MongoDB Transactions

def init_database():
    # TODO: create a migration script for creating db and its collections
    orders = get_client().get_database().orders
    inventory = get_client().get_database().inventory

    orders.delete_many({})
    inventory.delete_many({})

    # suppose inventory has 200 items of abc123
    inventory.insert_one({"sku": "abc123", "qty": 200})
    # suppose orders for abc123 is 0 yet
    orders.insert_one({"sku": "abc123", "qty": 0})


def sample_transaction():
    init_database()
    # https://pymongo.readthedocs.io/en/stable/api/pymongo/client_session.html
    # 1. Increments orders by 100
    # 2. Decrements inventory by 100
    # 3. If either 1 or 2 fails, then the transaction should be aborted
    with get_client().start_session() as session:
        session.with_transaction(increment_orders_decrement_inventory_callback)

    print("Successful transaction!")


def increment_orders_decrement_inventory_callback(session):
    orders = session.client.get_database().orders
    inventory = session.client.get_database().inventory

    # increments 100 items to orders
    orders.update_one({"sku": "abc123"}, {"$inc": {"qty": 100}}, session=session)

    # decrements 100 items from inventory
    inventory.update_one(
        {"sku": "abc123", "qty": {"$gte": 0}},
        {"$inc": {"qty": -100}},
        session=session,
    )

Run example

make sample_transaction

• executes sample transaction

Test

make test

• executes sample transaction
• make sure the correct data were created/updated in the database collections

Others

make up_database

• spins up the mongodb replica set container

make up

• spins up the mongodb replica set and opens pymongo-app's bash

make down

• stops the mongodb and pymongo-app containers