[erlang-questions] non-atomic nature of mnesia:dirty_update_counter

[Dan Gudmundsson]

Nothing is promised with dirty_write don't use it.

Dirty read may be ok if you know what you are doing
but avoid dirty_write in multiple node system.

/Dan

On Sun, Mar 6, 2011 at 5:12 PM, Ken Ellis <kaellis@REDACTED> wrote:
> Hi Ulf,
>
> Thanks for the clarification.  I also went back to that User Guide and
> it does an admirable job of defining atomic as meaning the ACID
> all-or-nothing variety and not what for example you'd call atomic when
> incrementing counters within the linux kernel.  My bad, forgot I was
> dealing with a database.  So I think in that sense the
> dirty_update_counter is atomic across all nodes.  I guess the question
> is when the users guide says atomicity is lost with dirty operations,
> what exactly is non-atomic about dirty_write or any dirty operation
> given that "Mnesia also ensures that all replicas of a table are
> updated if a dirty write operation is [successfully?] performed on a
> [any?] table".  Isn't that the definition of all-or-nothing atomicity?
>  Unless that statement is inaccurate, that would seem to mean all
> dirty_* operations are atomic, and that dirty is a reference to what
> an RDBMAS would call dirty reads, with an additional qualification
> that it can break isolation of transactions.  Which by the way means
> there is no isolation guarantee for any transaction, since dirty
> operations compromise their isolation.  (IMaybe filthy_* would be a
> better prefix :).  But I suppose for now I'll take the statement about
> lack of atomicity as true, and that dirty writes can be only partially
> executed and can leave tables on different nodes in an inconsistent
> state.
>
> Ken
>
>
> On Sun, Mar 6, 2011 at 2:19 AM, Ulf Wiger
> <ulf.wiger@REDACTED> wrote:
>>
>> On 6 Mar 2011, at 06:54, Ken Ellis wrote:
>>
>> After running test_counter:run(5000,1) on each node simultaneously,
>> and comparing the files, i get 2893 duplicates out of 5000.  Although
>> the counter has been incremented by 10000.  I can't repro running on a
>> single node with a large number of processes, so it seems atomic in
>> that case.
>>
>> It is exactly the single-node case that is atomic. The documentation should
>> spell this out.
>> The User Guide does say that you lose mnesia's atomicity and isolation
>> properties if you use dirty operations, which does seem to contradict what
>> is said about dirty_update_counter(), but what the Reference Manual (*and*
>> User Guide) should say is that dirty_update_counter() is atomic only in a
>> very limited context.
>> The User Guide does say something else that is, strictly speaking, wrong:
>> "It is not possible to have transaction protected updates of counter
>> records."
>> It is of course possible - they are only records after all. You can do an
>> update counter inside a transaction through a combination of read() and
>> write().
>> BR,
>> Ulf W
>> Ulf Wiger, CTO, Erlang Solutions, Ltd.
>> http://erlang-solutions.com
>>
>>
>>
>
> ________________________________________________________________
> erlang-questions (at) erlang.org mailing list.
> See http://www.erlang.org/faq.html
> To unsubscribe; mailto:erlang-questions-unsubscribe@REDACTED
>
>