[erlang-questions] Mnesia: Error on insert object with same key in Set

Kostis Sagonas <>
Mon Oct 17 08:45:14 CEST 2011


On 10/17/11 09:40, Dan Gudmundsson wrote:
> This should be fixed now and thus safe to use now.

But doesn't the code below have an obvious race condition?

Kostis

> On Sun, Oct 16, 2011 at 11:29 PM, Tristan Sloughter
> <>  wrote:
>> I wanted to make sure there was still no way to tell Mnesia to return an
>> error if it is given a record to write that has the same key as an object
>> already stored in the table.
>> Reading through the docs it looks like there is no way through configuration
>> but you must simply read and then write if its not there, like I found when
>> this was asked in 2004 on this mailing list:
>> insert(K,V) ->
>>    fun() ->
>>    case mnesia:wread(K) of
>>      [Exist] ->  {abort, Exist};
>>      _ ->  mnesia:write(V)
>>    end
>>    end.
>> Which doesn't seem safe, as Ulf points on in that
>> thread: http://erlang.org/pipermail/erlang-questions/2004-May/012307.html
>> As well as slow.
>> So I was hoping maybe in 2011 there is a better way to do this? :)
>> Thanks,
>> Tristan
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