MongoDB ACID Transactions

MongoDB introduced multi-document ACID (Atomicity, Consistency, Isolation, Durability) transactions in version 4.0. ACID transactions ensure that database transactions are processed reliably and consistently, even in the presence of failures.

Here's how MongoDB ACID transactions work:

1. Atomicity: MongoDB transactions are atomic, meaning that either all operations within a transaction are applied successfully, or none of them are applied at all. If any operation within a transaction fails, MongoDB will rollback the entire transaction to maintain data integrity.

2. Consistency: MongoDB ensures consistency by enforcing transactional isolation levels. Transactions in MongoDB are executed under snapshot isolation, which ensures that each transaction sees a consistent snapshot of the database. This prevents transactions from reading or writing inconsistent data caused by concurrent transactions.

3. Isolation: MongoDB supports snapshot isolation for transactions. With snapshot isolation, each transaction works with a consistent snapshot of the data, isolated from the effects of other transactions occurring concurrently. This prevents dirty reads, non-repeatable reads, and other anomalies that can occur when transactions interact with each other.

4. Durability: MongoDB guarantees durability by ensuring that committed transactions are permanently stored in the database. Once a transaction is committed, its changes are durably written to disk and are not lost even in the event of a system failure.

It's important to note that MongoDB ACID transactions are supported in replica set deployments, where data is replicated across multiple nodes for fault tolerance and high availability. Transactions are coordinated by the MongoDB replica set primary node, which ensures that all replica set members apply transactions in the same order, maintaining consistency across the cluster.

MongoDB ACID transactions allow developers to build complex applications with transactional requirements, such as e-commerce platforms, financial systems, and applications requiring strong data integrity guarantees. They provide the benefits of relational database transactions while leveraging MongoDB's flexibility, scalability, and document-oriented data model.