Is it possible to perform multiple DB operations in a single transaction in MongoDB? - mongodb

Question

Suppose I have two collections A and B
I want to perform an operation
db.A.remove({_id:1});
db.B.insert({_id:"1","name":"dev"})
I know MongoDB maintains atomicity at the document level. Is it possible to perform the above set of operation in a single transaction?

Answer 1

Yes, now you can!
MongoDB has had atomic write operations on the level of a single document for a long time. But, MongoDB did not support such atomicity in the case of multi-document operations until v4.0.0. Multi-document operations are now atomic in nature thanks to the release of MongoDB Transactions.
But remember that transactions are only supported in replica sets using the WiredTiger storage engine, and not in standalone servers (but may support on standalone servers too in future!)
Here is a mongo shell example also provided in the official docs:
// Start a session.
session = db.getMongo().startSession( { readPreference: { mode: "primary" } } );
employeesCollection = session.getDatabase("hr").employees;
eventsCollection = session.getDatabase("reporting").events;
// Start a transaction
session.startTransaction( { readConcern: { level: "snapshot" }, writeConcern: { w: "majority" } } );
//As many operations as you want inside this transaction
try {
employeesCollection.updateOne( { employee: 3 }, { $set: { status: "Inactive" } } );
eventsCollection.insertOne( { employee: 3, status: { new: "Inactive", old: "Active" } } );
} catch (error) {
// Abort transaction on error
session.abortTransaction();
throw error;
}
// Commit the transaction using write concern set at transaction start
session.commitTransaction();
session.endSession();
I recommend you reading this and this to better understand how to use!

Answer 2

MongoDB can not guarantee atomicity when more than one document is involved.
Also, MongoDB does not offer any single operations which affect more than one collection.
When you want to do whatever you actually want to do in an atomic manner, you need to merge collections A and B into one collection. Remember that MongoDB is a schemaless database. You can store documents of different types in one collection and you can perform single atomic update operations which perform multiple changes to a document. That means that a single update can transform a document of type A into a document of type B.
To tell different types in the same collection apart, you could have a type field and add this to all of your queries, or you could use duck-typing and identify types by checking if a certain field $exists.