Creating MongoDB Transactions in Python Applications

Review the following code, which demonstrates how to create multi-document transactions in MongoDB by using PyMongo.

Create a Transaction

Note that you must have an active database connection to create a transaction. Once you're connected to a database, complete the following steps:

1. Define the callback that specifies the sequence of operations to perform inside the transaction. Make sure to do the following:

   • In the callback, include the required parameter, which is the client session. You can also add additional parameters for your particular transaction if needed. In the following example, there are four keyword parameters specific to this bank transfer: transfer_id, account_id_receiver, account_id_sender, and transfer_amount.

   • Within the callback function, get references to the collections that the operations will take place on.

   • Write the transaction operations. Note that you must pass the session to each operation.

2. Start a client session by calling the start_session method on the client object in a with statement.

3. Carry out the transaction by calling with_transaction on the session object. with_transaction starts a transaction, runs the callback, and commits (or cancels if there's an error). For this step, consider the following:

   • with_transaction has one required parameter, which is the callback function that specifies the sequence of operations to perform inside the transaction. In the following example, we passed in the callback_wrapper function to pass the additional arguments to the callback: transfer_id, account_id_receiver, account_id_sender, and transfer_amount.

   • Note that the general best practice for passing arbitrary arguments to the callback is to use a lambda function. In this example, we used the callback wrapper for simplicity.

Here's the code to create the transaction:

# Connect to MongoDB cluster with MongoClient
client = MongoClient(MONGODB_URI)

# 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)


client.close()