MongoDB Server 4.0: What's New, Release History & Support Lifecycle

What Is New in MongoDB 4.0

MongoDB 4.0 is a major release that introduced significant enhancements in transactions, sharding, and tooling. It marks a pivotal step towards unifying the developer experience across the entire database.

How do transactions work in MongoDB 4.0?

MongoDB 4.0 delivers full multi-document ACID transactions. This is the headline feature that allows you to perform complex operations with atomicity guarantees across your entire dataset.

In practice, you can now wrap operations on multiple documents, even across different collections and databases, within a single transaction. The syntax is familiar, using session.startTransaction(), session.commitTransaction(), and session.abortTransaction(). This matters because it brings MongoDB's consistency model in line with traditional relational databases for complex workflows.

Code Example

const session = db.getMongo().startSession();
session.startTransaction();
try {
    db.accounts.updateOne( { _id: 1 }, { $inc: { balance: -100 } }, { session } );
    db.accounts.updateOne( { _id: 2 }, { $inc: { balance: 100 } }, { session } );
    session.commitTransaction();
} catch (error) {
    session.abortTransaction();
}
session.endSession();

What sharding improvements were made?

Sharding got a major performance and reliability overhaul. The balancer is now more efficient, and the config server metadata management was completely refreshed.

A key change is the new way sharded clusters track metadata. Instead of a single source of truth, the config servers now act as a replica set, which simplifies failover and improves consistency. The balancer also got smarter, making chunk migrations faster and less intrusive on cluster performance.

For developers, a subtle but important change is that new collections shard on UUIDs (_id) by default instead of ObjectIds. This prevents potential hotspots on inserts when using monotonically increasing shard keys.

What new aggregation features are available?

The aggregation framework received powerful new type conversion and expression operators. This makes data transformation pipelines more robust and flexible.

The new $convert operator and its helper aliases like $toDouble or $toString allow you to handle type casting directly within your aggregation stages. You can specify on-error and on-null behavior, which is crucial for cleaning up messy data. The $type operator was also enhanced to return the BSON type directly as a string within an expression.

Code Example

// Convert a field to a double, handling errors
db.sales.aggregate([
    {
        $project: {
            price: { $toDouble: "$priceString" },
            quantity: { $convert: {
                input: "$qty",
                to: "int",
                onError: 0,
                onNull: 0
            }}
        }
    }
])

Were there any security enhancements?

Yes, security was tightened with more granular audit logging and a stronger default authentication mechanism.

You can now filter the audit log output by specific users or roles, which makes it much easier to track actions for compliance without being overwhelmed by noise. Additionally, while SCRAM-SHA-1 is still supported, SCRAM-SHA-256 became available, offering a more secure hashing algorithm for password storage.