objectbox-java | Java Database for Mobile & IoT - fast and lightweight | Database library

What you likely want to do is to call closeThreadResources() once you are done with a web request from the thread serving it. This works fine with threads pools, which are also used by web servers. You can do this for all requests with e.g. servlet filters, but not sure if there are better approaches these days to do that.

Some background: each thread using ObjectBox "caches" some resources (thread local). The best way to clean up after a thread is "done" (at least for now, e.g. served the eb request) with ObjectBox, it to call closeThreadResources(). Once that thread is back (e.g. from thread pool) it will simply allocate the resources it needs again.

Queries: yes you can reuse them, but ensure to lock e.g. the Query object while you query as you might race with other threads serving requests too.

PagingSource has an .invalidate() function you can call to trigger Paging to create a new PagingData / PagingSource pair to reflect changes in your Realm DB.

In your RemoteMediator implementation, you should fetch items from network, then write them to db and then invalidate the PagingSource before returning MediatorResult.Success.

You should set endOfPaginationReached to true from MediatorResult, if there were no updates to your db and you therefore don't expect to invalidate.

Btw, Room automatically handles this in its PagingSource implementation, so you may want to look into whether Realm offers some callbacks your PagingSource can listen to or you may need to to keep track yourself.

EDIT: Root cause for missing remote APPEND call was not setting nextKey to null eventually.