[erlang-questions] pg2...a warning

Ulf Wiger <>
Thu Apr 29 09:29:22 CEST 2010


Evans, Matthew wrote:
> 
> The problem is that an asynchronous operation, beyond our control,
> can cause pg2:join/2 to be called many times for the same process.
> The result of which is that pg2:get_closest_pid/1 will not be random
> (e.g. process on node 1 gets 5 "joins", and node 2 gets 3 "joins").
> Or rather it will not be random in how we consider it to be (i.e. we
> only want a process to join a group a single time).

This made me curious.

I will admit to not having used pg2, but the other day I was inspired
to explore how to emulate pg2's behaviour using gproc [1].

I noted the part in the documentation stating that you can join
several times, but didn't catch the fact that get_members/1 would
include each pid once for each time it has joined. This seems to imply
that joining several times serves an entirely different purpose than
relieving the programmer of the trouble of keeping track of whether
or not it has joined the group before.

OTOH, the man page doesn't mention this at all, which makes me believe
that it's a bug rather than a feature. It talks about how you should
use pg2:get_members/1 when you want to send a message to all members
of a group. This would be a good place to highlight the fact that
you need to remove duplicates from the list if you want the message
to be sent only once to each member.

BR,
Ulf W

[1] Making a pg2-like module on top of gproc is actually quite easy,
but requires the distributed gproc to work, which has not been the
case until now. I am in the process of verifying it, and will
hopefully be able to push a new version very soon.

-- 
Ulf Wiger
CTO, Erlang Solutions Ltd, formerly Erlang Training & Consulting Ltd
http://www.erlang-solutions.com
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