Re: ETS: update list value without race condition

If you just want to add a single element you can use `select_replace` like in the example in the docs https://erlang.org/doc/man/ets.html#select_replace-2 since a matchspec can include a cons:

MS = ets:fun2ms(fun({K, L}) when is_list(L) -> {K, [marker | L]} end).

A bag might still be more appropriate, but if the goal is to just have a single call that handles this for you then `select_replace` can work. The third option would be a compare and swap with `select_replace`, which is the second example in the docs.

On Thu, Apr 22, 2021, at 05:32, Frank Muller wrote:
> 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