In the iOS implementation, a new dispatch queue is created on each 'set' or 'get' call, from which it switches to the main queue to call MMKV, wrapped in a try/catch block.

Inside the MMKV library, a NSData object is created, another class is used to write a single "int" value to it, then the data object is used as the "raw" value for the given key.

I haven't looked any futher but already this seems like a lot of work to set a number in memory. Is there any reason for this?

We are using SQLite on React Native with the JSI without writing a java or object C bridge. As a result, all operations are much faster and almost all of them don't need to Promise callback. I don't see any reason to use this over SQLite. We can do same thing on SQLite much faster way.

Just for the note:

Sciter implements what I call "an ultimate data persistence solution" - NoSQL DB integrated right into script VM.

You can open storage as:

    var storage = Storage.open("path/to/data/file.db"); 

Storage provides root object - normal script object (or array) that can be updated by normal script means:

    storage.root = { foo:[1,2,3], bar: {...} };
    storage.root.foo.push(4); 
    storage.root.bar.newProp = 42; 

All script objects that are accessible from the storage.root are persist-able - pushed to HD on storage.close() or storage.commit();

More details about architecture and implementation: https://sciter.com/data-persistence-in-sciter-database-integ...

Overall feature set is close to MongoDB (modulo sharding).

DB layer uses Konstantin Knizhnik's DyBASE pretty much as it is: http://www.garret.ru/dybase.html

What is the benefit over async storage? We’ve built a few RN apps and never felt it was too slow to work with.

How does react-native-mmkv-storage store data under-the-hood?

When I was building SwiftStore - a KV store [1] for iOS, I went with LevelDB as the choice for the underlying database.

LevelDB because it's written in c++ and used widely as an embedded database. Integrating it as a static library in Objective-C is also fairly straight forward.

[1] https://github.com/hemantasapkota/SwiftStore

Why use this over SQLite?

I’m surprised to not find any iteration or query primitives. No key scanning?

can someone tell me what this is used for? Is this like a replacement for localstorage?