ACID Properties in MongoDB

ACID properties—Atomicity, Consistency, Isolation, and Durability—are critical in database systems to ensure reliable transactions and data integrity. MongoDB provides ACID compliance at various levels, especially for replica sets and multi-document transactions. Understanding these properties helps in designing robust and reliable applications.

Atomicity

• Definition: Atomicity ensures that a series of operations within a transaction are all completed successfully as a single unit. If any operation fails, the entire transaction is rolled back, leaving the database unchanged.
• MongoDB Implementation: MongoDB provides atomic operations at the document level by default. This means that any single document update is atomic. For multi-document transactions, MongoDB ensures atomicity across multiple documents.

db.collection.updateOne(
  { _id: 1 },
  { $set: { field: "value" } }
);

Consistency

• Definition: Consistency ensures that a database remains in a valid state before and after a transaction. All data must adhere to the defined rules and constraints, ensuring that transactions do not leave the database in an inconsistent state.
• MongoDB Implementation: MongoDB ensures consistency through its data model and operations. With multi-document transactions, MongoDB maintains consistency by applying all operations within a transaction in a way that the database state is consistent before and after the transaction.

const session = db.getMongo().startSession();
session.startTransaction();
 
try {
  db.collection1.insertOne({ _id: 1, field: "value" }, { session });
  db.collection2.updateOne({ _id: 1 }, { $set: { field: "value" } }, { session });
  
  session.commitTransaction();
} catch (error) {
  session.abortTransaction();
} finally {
  session.endSession();
}

Isolation

• Definition: Isolation ensures that concurrent transactions do not interfere with each other. Each transaction should execute in isolation, without being affected by other ongoing transactions.
• MongoDB Implementation: MongoDB provides snapshot isolation for multi-document transactions. This means that transactions work with a consistent snapshot of the data, ensuring that they are isolated from other concurrent transactions.

const session = db.getMongo().startSession();
session.startTransaction();
 
const result1 = db.collection.find({}).toArray(); // Snapshot 1
// Other concurrent transactions won't affect the result1 snapshot
 
const result2 = db.collection.find({}).toArray(); // Snapshot 2
// Transactions will work with their respective snapshots
 
session.commitTransaction();
session.endSession();

Durability

• Definition: Durability ensures that once a transaction is committed, the changes are permanent and will survive any system failures. The database must persist the transaction’s changes to non-volatile storage.
• MongoDB Implementation: MongoDB ensures durability through its write concern settings. By default, MongoDB writes data to the primary replica in a replica set, and you can configure write concerns to ensure that data is written to multiple nodes to enhance durability.

db.collection.insertOne(
  { _id: 1, field: "value" },
  { writeConcern: { w: "majority", wtimeout: 5000 } }
);

Multi-Document Transactions

MongoDB supports multi-document transactions, which allow for ACID compliance across multiple documents. This capability is available in replica sets and sharded clusters and ensures that operations spanning multiple documents are executed atomically, consistently, isolated, and durably.

Summary

MongoDB provides strong ACID properties, particularly for replica sets and multi-document transactions, ensuring reliable and consistent data management. By leveraging these properties, MongoDB enables applications to handle complex data operations while maintaining data integrity and resilience.