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Re: Backend data model deserialization
There should be some old Titan resources that describe how the data model is binary coded into the row keys and row values. Of course, it is also implicit from the JanusGraph source code.
If you look back at this week's OLAP presentations (https://lists.lfaidata.foundation/g/janusgraph-users/topic/janusgraph_meetup_4/82939376) you will see that one of the presenters exactly did what you propose: they exported rows from scylladb and converted it to gryo format for import into TinkerPop HadoopGraph. You might want to contact them to coordinate a possible contribution to the JanusGraph project.
Best wishes, Marc