verneuil VS Apache Ignite

It'd be process wide. If you want an example can check out the example using a vfs here. There's an explicit passing of vfs there and an implicit usage of it. https://github.com/backtrace-labs/verneuil/blob/main/examples/rusqlite_integration.rs

It doesn't even have to be WAL-based system. Backtrace Labs has a SQLite virtual file system (VFS) called Verneuil that works similarly but works with the rollback journal instead of the WAL.

https://github.com/backtrace-labs/verneuil/ is one way to address the diffing / read replica part of the problem. I believe it's compatible with gossipping: most of the data is in small content-addressed chunks, with small manifests that tell clients what chunks to fetch and how to reassemble them to recreate a sqlite database. There's already client-side caching to persistent storage, and chunks can be fetched on demand.

Sharing replication data P2P, while retaining the simplicity of a single authoritative writer per database, is explicitly part of the project's long-term goals!

Ignite works as you describe:

https://ignite.apache.org/

I wouldn't really recommend this approach, I would think more in terms of subscriptions and topics and less of a 'database'.

Apache Ignite https://ignite.apache.org/

Last days, I started using Apache Ignite as a cache strategy for some applications. Apache Ignite is an open-source In-Memory Data Grid, distributed database, caching, and high-performance computing platform.