RE: ETS: update list value without race condition

It seems ets:select_replace could be your solution.

 

The documentation contains this example usage to do a compare-and-swap for a single key:

 

[Old] = ets:lookup(T, Key),

New = update_object(Old),

Success = (1 =:= ets:select_replace(T, [{Old, [], [{const, New}]}])),

 

/Sverker

 

From: erlang-questions <[email protected]> On Behalf Of Frank Muller
Sent: den 22 april 2021 13:32
To: Erlang-Questions Questions <[email protected]>
Subject: ETS: update list value without race condition

 

Hi guys

 

I’ve a little issue I’m unable to solve with the current ETS API.

 

My table is of type set. Keys are integers and values are list of names:

123 -> [ jane, john, bob ]

456 -> [ joe, alice ]

…

 

Process A with Key=123 wants to delete ‘Jane’ while process B with Key=123 wants to add ‘Adam’.

 

First, they both needs to read the value associated with Key=123, update that list accordingly and set back the new value.

 

But this lead to race condition (ex. process B could put back ‘jane’ - last write wins).

 

Could this problem be solved atomically without involving explicit locking per Key?

 

Is there any other better way to represent this data set which will not suffer from the race condition?

 

Thanks

/Frank