LIST_DOCUMENTS doesn't work after restarting the server

Data Services
1

Question on behalf of @markus-async-dune:

I got an A12 data-service server (version: 32.2.6) up and running with a postgres backend. A model is present and I tried adding a document via ADD_DOCUMENT on /api/v2/batch, which worked.
I then tried to do LIST_DOCUMENTS on /api/v2/batch, which as expected returned the document I just added.
However when I stop and then start the server again, doing LIST_DOCUMENTS on /api/v2/batch yields an empty result list, even though the document is stored in the database.
So why is the result to LIST_DOCUMENTS “empty” and just yields the following?

{
    "results": [
        {
            "idRef": "myid",
            "result": {
                "fullSize": 0,
                "page": {
                    "offset": 0,
                    "limit": 0
                },
                "entries": []            }
        }
    ],
    "exceptions": {
        "global": [],
        "operationScoped": []
    }
}
2

Answer on behalf of @markus-async-dune:

The default search engine luceneram forgets everything when the server is stopped. Using lucene as the search engine fixed the problem, as it keeps the search index.