How is the transaction mechanism implemented in NoSQL databases?

In NoSQL databases, the transaction mechanism is implemented differently than in traditional relational DBMSs. Many NoSQL systems are initially oriented towards high scalability and fault tolerance, so support for transactions may be limited.

Main approaches:

• Single-operation transactions — atomicity is guaranteed at the level of a single operation (e.g., writing a single document).
• Multi-operation transactions — supported not in all NoSQL, but some (e.g., MongoDB starting from version 4.0) implement transactions that cover multiple documents or collections.
• Using consistency patterns — for example, sagas or compensating operations to ensure data integrity without traditional transactions.

Example in MongoDB (Java):

ClientSession session = client.startSession();
session.startTransaction();
try {
    collection1.insertOne(session, doc1);
    collection2.updateOne(session, filter, update);
    session.commitTransaction();
} catch (Exception e) {
    session.abortTransaction();
}
session.close();

Thus, transactions in NoSQL are often limited in functionality or require special approaches to ensure consistency.