With read-only it's not possible to modify the data. Updates to the data
are possible via a primary instance of oxigraph, but will not be
reflected. The data is frozen at the time the read-only server is
started.
The database can be opened once in primary mode, but may be opened
multiple times in secondary mode. It's not possible to write data to the
database in secondary mode.
Secondary mode does not support checkpoints so there might be data
inconsistencies when data is changed while said data is queried. It
might happen that the result data mixes both elements before and after
the change that can not happen in primary mode.
RocksDB provides a great cheap backup feature thanks to the immutable SST storage.
Sadly it is not compatible with in-memory databases which do not rely on the SST files.
Benedikt Seidl