How would read-only replicas actually work

[a-type]

Ok, I admit, I sorta dabbled at support for this but didn't think it through.

The main blocker here is migrations. All meaningful migrations require writing operations to migrate the data.

A read-only replica isn't allowed to do those writes. So if it's the first replica to connect to a library after a schema change, it's unable to write the migration operations necessary to make data conform to the new schema.

This is going to require special behavior...

[a-type]

The ability to store non-synced migration operations in-memory would probably make this possible.

A readonly replica can still generate operations, but it won't store or sync them anywhere. Migration can still work as normal, but it's lost on refresh and has to be re-applied each time.

I think this can be done with minimal added complexity.

[a-type]

Actually, I may be overthinking this.

A read only replica could still theoretically be allowed to write to the local database. It's only forbidden from syncing back.

As an app maker, you probably want to prevent local writes to not confuse users. Either that, or limit only to one initial sync. These would both be possible to do fairly easily.

This requires no changes except to forbid sync messages from readonly replicas. Local migrations are allowed and happen according to existing rules.