[Haskell-cafe] "branching" conduits

[Alexander V Vershilov]

Hello, Simon.

On 31 January 2013 17:24, Simon Marechal <simon at banquise.net> wrote:
> On 31/01/2013 13:50, Michael Snoyman wrote:
>> * To simplify, we start off with a call to injectLeftovers. This means
>> that we can entirely ignore the Leftover constructor in the main function.
>> * Since a Sink will never yield values, we can also ignore the
>> HaveOutput constructor.
>> * As soon as either of the Sinks terminates, we terminate the other one
>> as well and return the results.
>
> Your gist is extremely informative to me. I figured it would be
> something along these lines, but was very scared to try it myself. I
> have however realized that my first use case doesn't cover my need, as I
> will want to feed an arbitrary set of sinks with any value ...
>
> I started coding right after I sent that mail and wrote this:
> https://github.com/bartavelle/hslogstash/blob/master/Data/Conduit/Branching.hs
>
> It is not very elegant as the "branching" functions outputs [Int].
>
> I haven't tested it yet, but it should branch with any number of sinks.
> Another point that might (or might not) be of interest is the
> distribution of distinct branches on separate threads.
>
>> You can also consider going the mutable container route if you like.
>> Instead of creating a lot of stuff from scratch with MVars, you could
>> use stm-conduit[2]. In fact, that package already contains some kind of
>> merging behavior for sources, it might make sense to ask the author
>> about including unmerging behavior for Sinks.
>
> I did not think of bounded channels. They are a indeed a better match
> than MVars !
>
> I can see it uses resourceForkIO, which I believe is OK for sources that
> will be used in your 'main' thread. But for multiple Sinks, you need a
> way to wait for the all Sinks to terminate. I used stuff from
> Control.Concurrent.ParallelIO, but I am not sure it is ideal.
>

Sorry I've sent my first email of the list, and have no copy to resend it.
If you will use stm-conduit you can meet next troubles:

1). early close: if you'll use modern conduit API (yield, await), then try
to use $$ on sinkT?Chan very accurate as it will close Channel and so
receiver will be also closed.

2). late close: if you'll use resourceForkIO you channel may leave to
long, hovewer in much cases you'll be save with forkIO, unless you
share resources that was allocated with resourcet.

3). pipeline notifying: if you'll map you branches to different threads
you'll need a way to notify that branch is closed (closing channels will
be good, but you'll need an additional steps to check if they were closed
and either close all pipeline or just forgot this channel). On the other hand
 you can use one pipeline (and thread) for every branch, then you need
approach that was shown in Michael's gist. Run each downstream pipe,
and save a result, possibly modifying a list.

I'll paste concept that I mailed to you here hovewer in may be not accurate:

branch :: [a -> m ()] -- it's better to place sink here
branch fs = do chs <- mapM (\f -> (,) f <$> newTBMChanIO) fs
                      bracket (mapM (\(f,ch) ->  forkIO $
sourceTBMChan ch $$ f) chs)
                                  (mapM_ killThread)
                                  (\_ -> do x <- await
                                               mapM_ (flip
writeTBMChan x . snd) chs)

as an additional step you can call isTBMChanClosed on channel to check if branch
is still alive and filter channel if so. (Previous version contained a
filter function that
allowes to send value to branches that needs it, hovewer it's less composable)

also you may like iochan-conduit package [1], it can give a better
results in some cases
hovewer, stm-channels is much more general.

[1] https://github.com/qnikst/iochan-conduit
-- 
Alexander Vershilov