Atomicity and Transactions in mongodb?

Atomicity and transactions are key concepts in database management systems, including MongoDB, a popular NoSQL database.

1. Atomicity:
   • In the context of databases, atomicity is a part of the ACID properties (Atomicity, Consistency, Isolation, Durability) which are a set of principles for database transactions.
   • Atomicity ensures that each transaction is treated as a single unit, which either completely succeeds or completely fails. In other words, if any part of the transaction fails, the entire transaction is rolled back, and the database state remains unchanged.
   • In MongoDB, atomicity is applied at the level of a single document. This means changes to a single document are atomic. However, in earlier versions of MongoDB, operations involving multiple documents did not have atomicity guarantees.
2. Transactions:
   • Transactions are sequences of database operations that are executed as a single unit. They allow multiple operations to be performed in an all-or-nothing fashion, adhering to the atomicity principle.
   • With the introduction of MongoDB 4.0, multi-document transactions were supported in replica sets, and this support was extended to sharded clusters in MongoDB 4.2. This means that it is now possible to apply atomic operations across multiple documents and collections.
   • Transactions in MongoDB are similar to transactions in relational databases. They can be started with a startTransaction command, and they can be committed or aborted. If the transaction is committed, all the operations within the transaction are applied to the database. If the transaction is aborted, none of the operations affect the database.

These features make MongoDB more flexible in handling complex operations and maintaining data integrity, especially in systems where consistency of operations across multiple documents is crucial.