Transactions

• Transactions that take longer than 60s will be cancelled
• It any operations fail the transaction won't be completed and no changes will be made

# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
def callback(
    session,
    transfer_id=None,
    account_id_receiver=None,
    account_id_sender=None,
    transfer_amount=None,
):

    # Get reference to 'accounts' collection
    accounts_collection = session.client.bank.accounts

    # Get reference to 'transfers' collection
    transfers_collection = session.client.bank.transfers

    transfer = {
        "transfer_id": transfer_id,
        "to_account": account_id_receiver,
        "from_account": account_id_sender,
        "amount": {"$numberDecimal": transfer_amount},
    }

    # Transaction operations
    # Important: You must pass the session to each operation

    # Update sender account: subtract transfer amount from balance and add transfer ID
    accounts_collection.update_one(
        {"account_id": account_id_sender},
        {
            "$inc": {"balance": -transfer_amount},
            "$push": {"transfers_complete": transfer_id},
        },
        session=session,
    )

    # Update receiver account: add transfer amount to balance and add transfer ID
    accounts_collection.update_one(
        {"account_id": account_id_receiver},
        {
            "$inc": {"balance": transfer_amount},
            "$push": {"transfers_complete": transfer_id},
        },
        session=session,
    )

    # Add new transfer to 'transfers' collection
    transfers_collection.insert_one(transfer, session=session)

    print("Transaction successful")

    return


def callback_wrapper(s):
    callback(
        s,
        transfer_id="TR218721873",
        account_id_receiver="MDB343652528",
        account_id_sender="MDB574189300",
        transfer_amount=100,
    )


# Step 2: Start a client session
with client.start_session() as session:
    # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or cancel on error)
    session.with_transaction(callback_wrapper)

Using lambda expression is recommended:

with client.start_session() as session:
    # Step 3: Use with_transaction with a lambda function
    session.with_transaction(
        lambda s: (
            # Transaction operations within the lambda
            s.client.bank.accounts.update_one(
                {"account_id": account_id_sender},
                {
                    "$inc": {"balance": -transfer_amount},
                    "$push": {"transfers_complete": transfer_id},
                },
                session=s,
            ),
            s.client.bank.accounts.update_one(
                {"account_id": account_id_receiver},
                {
                    "$inc": {"balance": transfer_amount},
                    "$push": {"transfers_complete": transfer_id},
                },
                session=s,
            ),
            s.client.bank.transfers.insert_one(
                {
                    "transfer_id": transfer_id,
                    "to_account": account_id_receiver,
                    "from_account": account_id_sender,
                    "amount": {"$numberDecimal": transfer_amount},
                },
                session=s,
            ),
            print("Transaction successful")
        )
    )