[erlang-questions] Mnesia, disk_log and dets

wde <>
Thu Feb 18 10:41:55 CET 2010


are there other documents about mnesia internals, other than those : http://www.erlang.org/~hakan/ ?


>wde wrote:
>> ok thank you.
>> 
>> With Mnesiaex we can have our own storage backend for the  new
>> "external_copies" type.
>> 
>> But instead of specifying a new copy type, I'm currently trying to
>> understand how I can integrate a wrapper module like
>> "mnesia_disc_backend" that handle all disc access (for disc_copies
>> and disc_only_copies) , and do the same thing for the "ram_copy"
>> type, by using a "mnesia_ram_backend" module.
>> 
>> The idea is to offer more flexibility and easily change the backend
>> for ram and disc I/O.
>> 
>> 
>> Do you think I'm wasting my time or that there is no interest for
>> something like that ?
>
>The external_copies concept originated in my rdbms contrib, so
>it would be silly of me to deny that I at least at one time thought
>this was a good idea. Two table types I experimented with at the
>time was a log table and a read-only file system. I don't think that
>mnesiaex copied the idea of hooking into the table creation operation
>in mnesia_schema, which allowed the external copy callback to verify
>custom table properties (such as mount point for the read-only file
>system). One point of playing with these two tables was that I
>wanted to experiment with types that had semantics that differed
>significantly from those of sets and bags.
>
>One thing to be very mindful of is that the callbacks for external
>copies must be error-free. Any crash in these operations will cause
>a mnesia core dump. This is the only sensible thing to do, as the
>callback modules are called /after/ commit, i.e. the point of no
>return as far as consistency is concerned.
>
>BR,
>Ulf W
>-- 
>Ulf Wiger
>CTO, Erlang Solutions Ltd, formerly Erlang Training & Consulting Ltd
>http://www.erlang-solutions.com
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