MongoDB[1] announced earlier today the acquisition of Realm[2], a lightweight database optimized for embedded for mobile device deployments. Realm, which was initially developed in the 2010 – 2014 timeframe and commercialized in 2017[3], is an object-based embedded database often positioned as an alternative to the 20-year old SQLite[4].
The rationale for MongoDB making the acquisition is that, while it has been often used as the back end for mobile applications, it did not have a compact version suited for embedding on the device. Realm has become a popular platform with more than 100,000 developers actively using it.
Realm is a compact engine with the kernel written in C++ for performance with APIs enabling developers to create the logic in the language of choice. The database is encapsulated in a compact container, called a Realm[5]. The Realm can be described by what it is not. It is not a table in the relational sense because the data objects within it are not uniformly typed. Likewise, it is not a document or key-value store because within each object, there is a defined schema.
The data is maintained as "live objects" that can be updated through change feeds, and memory-mapped, so it goes directly from code to storage, and when synchronizing with the back end server, there is no replication of data. Instead, the server pushes updates like a change-data-capture feed. On the back end is the event handler, business logic, and connectors to enterprise systems. There are pre-built connectors to PostgreSQL[6] and SQL Server[7]; Node.js and C# server SDKs; and GraphQL API[8] (instead of REST).
As noted, Realm is often compared to SQLite. Benchmarks, which