MongoDB, being a NoSQL database, did not support multi-document transactions until version 4.0. However, starting with version 4.0, MongoDB introduced support for multi-document transactions, allowing developers to perform complex operations on multiple documents in an atomic manner. In this section, we will explore the implementation of transactions in MongoDB.
Understanding Transactions
In a database, a transaction is a sequence of operations that form a logical unit of work. Transactions allow users to perform multiple changes to a database as a single operation. This is important to maintain the consistency and integrity of data in a database.
Starting a Transaction
To start a transaction in MongoDB, you first need to start a session. A session is an object that groups operations and allows them to be sent to the database in a single batch. To start a session, you can use the startSession() method of the MongoClient object.
let session = client.startSession();
Once the session is started, you can start a transaction using the session's startTransaction() method.
session.startTransaction();
Performing Operations in a Transaction
After starting a transaction, you can perform various CRUD (Create, Read, Update, Delete) operations on various documents within the transaction. To do this, you need to pass session as an option for each operation.
collection.insertOne({ a: 1 }, { session });
collection.updateOne({ a: 1 }, { $set: { b: 1 } }, { session });
Confirming a Transaction
After performing all necessary operations, you can commit the transaction using the session's commitTransaction() method. This will apply all changes made in the transaction to the database.
session.commitTransaction();
Undoing a Transaction
If something goes wrong during the transaction and you decide not to apply the changes, you can undo the transaction using the session's abortTransaction() method. This will discard all changes made to the transaction.
session.abortTransaction();
Conclusion
Transactions in MongoDB provide a powerful way to perform complex operations across multiple documents in an atomic manner. However, they also add significant overhead and should be used sparingly. In many cases, MongoDB's document-level atomicity features, such as findAndModify operations and complex updates, may be sufficient for your needs.
It is important to note that transactions in MongoDB require data replication, which means they are not available in a single MongoDB instance. Additionally, transactions have a 60-second timeout to prevent long-running operations from locking up the system.
Finally, while transactions are a welcome addition to MongoDB, they are not a silver bullet. As always, careful schema design and understanding your application needs are crucial to the effective use of transactions.
