Databases for mobile apps React Native & Realm

Meyer distinguishes MongoDB, a server database built for many connections, from Realm, a local object database for a device. Realm returns live objects that developers can use directly, rather than query results that must be copied into JavaScript state.

AsyncStorage works for simple key-value data, but larger JSON stores can take time to load and track. State libraries keep data in memory, while SQLite and ORMs add query and relationship management. Meyer says Realm offers typed models and direct object access without requiring developers to write SQL for common tasks.

To get started, install `realm` and `@realm/react`, define data models, and wrap the app in a Realm provider. The `useQuery` hook returns collections that can be filtered and sorted; hooks also listen for relevant changes and trigger React re-renders. Writes happen inside transactions, which Realm processes and can roll back if needed.

With Flexible Sync, an app subscribes to the data it needs, such as records for a particular user or chat room. People can read and edit local data while offline, then synchronize changes when connectivity returns. Meyer gives delivery logistics and farming apps in areas with unreliable connections as examples.

Meyer says Realm avoids a separate hydration step by opening local data directly and loading it into memory as it is accessed. He also discusses work on WebAssembly support for browser use and a Realm.js version 12 release candidate that adds a TypeScript interface to Realm Core.