Tools to migrate from Mnesia

[dumbbell]

I think it would be useful to provide a module to help users migrate from Mnesia to Khepri. Perhaps the opposite too to make things even easier.

Here are some ideas:

• We could provide a module to fill a Khepri store with the data from Mnesia tables. The layout could be something like that:

  RecordIndexPositions = mnesia:table_info(MnesiaTableName, index),
  
  %% For each record in table `MnesiaTableName', we need to decide on the record
  %% ID to put in the path. It could be limited to Mnesia tables with a single
  %% index. Also the type could be limited to atoms and binaries, or converted to
  %% a binary.
  RecordId = compute_record_id(Record, RecordIndexPositions)
  Path = [MnesiaTableName, RecordId],
  khepri:put(StoreId, Path, Record).

  We already have some interesting things in the RabbitMQ integration of Khepri. Perhaps we can move some of this code to Khepri itself.

• We could let the user decide how to derive a path from a record, either by taking an anonymous function or by leveraging ETS matching specs which allow to format the output of a match:

  PathMatchSpec = {
    #record{name = '$1', email = '$2', _ = '_'},
    [],
    [from_mnesia, MnesiaTableName, '$1', '$2']}.

• We could provide a module with an API close to Mnesia API. Probably difficult, but something as close as possible to a drop-in replacement to mnesia calls.

[michaelklishin]

It would be interesting to consider a module that transfers data from a Mnesia table into a Khepri "directory" (set of keys). But I see that this is to reduce code changes needed (our team obviously has a lot of vested interest here), which is more important.