[erlang-questions] mnesia:dirty_update_counter and replicated tables

[Serge Aleynikov]

Uffe,

You highlighted an interesting point of wrapping "dirty" operations in 
transactions.  I haven't thought of this possibility, always perceiving 
"dirty" operations being orthogonal to transaction-based operations.

Is there ever a point of using "dirty" updates inside transactions?

Serge


Ulf Wiger (TN/EAB) wrote:
> Even though this problem would seem to be addressed by 
> wrapping dirty_update_counter inside a transaction:
> 
> mnesia:activity(
>    transaction,
>    fun() ->
>       mnesia:write_lock_table(Tab),
>       mnesia:dirty_update_counter({Tab, Key}, Incr)
>    end).
> 
> It _still_ wouldn't be safe, for the following reasons:
> 
> - Dirty updates within a transaction are not recognized
>   as updates per se. So there will be failure situations
>   were the update may not propagate consistently to all
>   replicas.
> - Since the dirty update cannot be undone, and mnesia 
>   reserves the right to re-run transactions in the name
>   of deadlock prevention, you are not guaranteed that 
>   the update will happen only once.
> - If your transaction aborts, the update_counter will
>   not be rolled back, if it had time to execute before
>   the abort, which you have no way of knowing (unless
>   you add some additional dirty stuff).
> 
> BR,
> Ulf W
>  
> Serge Aleynikov wrote:
>> Scott,
>>
>> The answer is regretfully no.  Let's consider an example to 
>> illustrate this point.  You have two nodes A and B both 
>> running mnesia with a replicated table.
>>
>> 1. The content of a counter is 1.
>> 2. Node A does a mnesia:dirty_update_counter call to 
>> increment the value of the counter by 2.
>> 3. Very closely in time node B does 
>> mnesia:dirty_update_counter call to increment the counter by 3.
>>
>> After this the value of that field could be either one of:
>>
>> a) 3.
>> b) 4.
>> c) 6.
>>
>> No guarantees can be made as to which value the counter would 
>> hold without having cross-node locking involved by using transactions.
>>
>> Depending on the application you could use transactions to 
>> update data and dirty operations for read-only purposes.


-- 
Serge Aleynikov
R&D Telecom, MIS, IDT Corp
Tel: +1 (973) 438-3436
Fax: +1 (973) 438-1464