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

[Bob Ippolito]

Well, they're actually quite useful (for performance) if you know what
you're doing and the application can tolerate it. Often times the
return value of a counter is not important. I suppose if they were
ripped out, maybe fewer people would use Mnesia, which might be a net
good :)

On Sun, Mar 6, 2011 at 9:00 AM, Ken Ellis <kaellis@REDACTED> wrote:
> Amen, the whole dirty_* set of functions should be ripped out of the release.
>
>
> On Sun, Mar 6, 2011 at 11:28 AM, Dan Gudmundsson <dgud@REDACTED> wrote:
>> 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
>>>>
>>>>
>>>>
>>>
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>>
>
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