Transactions

Overview

In this guide, you can learn how to use the PyMongo driver to perform transactions. Transactions allow you to run a series of operations that do not change any data until the transaction is committed. If any operation in the transaction returns an error, the driver cancels the transaction and discards all data changes before they ever become visible.

In MongoDB, transactions run within logical sessions. A session is a grouping of related read or write operations that you intend to run sequentially. Sessions enable causal consistency for a group of operations and allow you to run operations in an ACID-compliant transaction, which is a transaction that meets an expectation of atomicity, consistency, isolation, and durability. MongoDB guarantees that the data involved in your transaction operations remains consistent, even if the operations encounter unexpected errors.

When using PyMongo, you can create a new session from a MongoClient instance as a ClientSession type. We recommend that you reuse your MongoClient for multiple sessions and transactions instead of creating a new client each time.

Methods

After you start a session by using the start_session() method, you can manage the session state by using the following methods provided by the returned ClientSession:

A ClientSession also has methods to retrieve session properties and modify mutable session properties. To learn more about these methods, see the API documentation.

Example

The following example shows how you can create a session, create a transaction, and commit a multi-document insert operation through the following steps:

1. Create a session from the client by using the start_session() method.

2. Use the with_transaction() method to start a transaction.

3. Insert multiple documents. The with_transaction() method runs the insert operation and commits the transaction. If any operation results in errors, with_transaction() cancels the transaction. This method ensures that the session closes properly when the block exits.

4. Close the connection to the server by using the client.close() method.

# Establishes a connection to the MongoDB server
client = MongoClient("<connection string>")
# Defines the database and collection
restaurants_db = client["sample_restaurants"]
restaurants_collection = restaurants_db["restaurants"]
# Function performs the transaction
def insert_documents(session):
    restaurants_collection_with_session = restaurants_collection.with_options(
        write_concern=WriteConcern("majority"),
        read_concern=ReadConcern("local")
    )
    
    # Inserts documents within the transaction
    restaurants_collection_with_session.insert_one(
        {"name": "PyMongo Pizza", "cuisine": "Pizza"}, session=session
    )
    restaurants_collection_with_session.insert_one(
        {"name": "PyMongo Burger", "cuisine": "Burger"}, session=session
    )
# Starts a client session
with client.start_session() as session:
    try:
        # Uses the with_transaction method to start a transaction, execute the callback, and commit (or abort on error).
        session.with_transaction(insert_documents)
        print("Transaction succeeded")
    except (ConnectionFailure, OperationFailure) as e:
        print(f"Transaction failed: {e}")
# Closes the client connection
client.close()

If you require more control over your transactions, you can use the start_transaction() method. You can use this method with the commit_transaction() and abort_transaction() methods described in the preceding section to manually manage the transaction lifecycle.

Additional Information

To learn more about the concepts mentioned in this guide, see the following pages in the Server manual:

• Transactions

• Server Sessions

• Read Isolation, Consistency, and Recency

To learn more about ACID compliance, see the What are ACID Properties in Database Management Systems? article on the MongoDB website.