"branching" conduits
Hello, I have found the Conduit abstraction to be very well suited to a set of problems I am facing. I am however wondering how to implement "branching" conduits, and even conduit pools. I am currently in the process of rewriting parts (the simple parts) of the Logstash tool. There is a sample program that I use here: https://github.com/bartavelle/hslogstash/blob/deprecateUtils/examples/RedisT... As it can be seen, it uses a "Redis" source, a conduit that decodes the JSON ByteString into a LogstashMessage, a conduit that stores it into Elasticsearch and outputs the result of that action as an Either, and finally a sink that prints the errors. My problem is that I would like more complex behaviour. For example, I would like to route messages to another server instead of putting them into Elasticsearch when the LogstashMessage has some tag set. But this is just an example, and it is probable I will want much more complex behavior soon. I am not sure how to proceed from here, but have the following ideas: * investigate how the Conduits are made internally to see if I can create a operator similar to $$, but that would have a signature like: Source m (Either a b) -> Sink a m r -> Sink b m r and would do the branching in a binary fashion. I am not sure this is even possible. * create a "mvars" connectors constructor, which might have a signature like this: Int -- ^ branch count (LogstashMessage -> Int) -- ^ branching function (Sink LogstashMessage m (), [Source m LogstashMessage]) -- ^ a suitable sink, several sources for the other conduits it would internally create a MVar (Maybe LogstashMessage) for each branch, and put putMVar accordingly to the branching function. When the Conduit is destroyed, it will putMVar Nothing in all MVars. the sources would takeMVar, check if it is Nothing, or just proceed as expected. The MVar should guarantee the constant space property, but there is the risk of inter branch blocking when one of the branches is significantly slower than the others. It doesn't really matter to me anyway. And all the branch Sinks would have to have some synchronization mechanism so that the main thread waits for them (as they are going to be launched by a forkIO). This is the simplest scheme I have thought of, and it is probably not a very good one. I am very interested in suggestions here.
On Thu, Jan 31, 2013 at 11:48 AM, Simon Marechal
Hello,
I have found the Conduit abstraction to be very well suited to a set of problems I am facing. I am however wondering how to implement "branching" conduits, and even conduit pools.
I am currently in the process of rewriting parts (the simple parts) of the Logstash tool. There is a sample program that I use here:
https://github.com/bartavelle/hslogstash/blob/deprecateUtils/examples/RedisT...
As it can be seen, it uses a "Redis" source, a conduit that decodes the JSON ByteString into a LogstashMessage, a conduit that stores it into Elasticsearch and outputs the result of that action as an Either, and finally a sink that prints the errors.
My problem is that I would like more complex behaviour. For example, I would like to route messages to another server instead of putting them into Elasticsearch when the LogstashMessage has some tag set. But this is just an example, and it is probable I will want much more complex behavior soon.
I am not sure how to proceed from here, but have the following ideas:
* investigate how the Conduits are made internally to see if I can create a operator similar to $$, but that would have a signature like: Source m (Either a b) -> Sink a m r -> Sink b m r and would do the branching in a binary fashion. I am not sure this is even possible.
* create a "mvars" connectors constructor, which might have a signature like this:
Int -- ^ branch count (LogstashMessage -> Int) -- ^ branching function (Sink LogstashMessage m (), [Source m LogstashMessage]) -- ^ a suitable sink, several sources for the other conduits
it would internally create a MVar (Maybe LogstashMessage) for each branch, and put putMVar accordingly to the branching function. When the Conduit is destroyed, it will putMVar Nothing in all MVars. the sources would takeMVar, check if it is Nothing, or just proceed as expected.
The MVar should guarantee the constant space property, but there is the risk of inter branch blocking when one of the branches is significantly slower than the others. It doesn't really matter to me anyway. And all the branch Sinks would have to have some synchronization mechanism so that the main thread waits for them (as they are going to be launched by a forkIO).
This is the simplest scheme I have thought of, and it is probably not a very good one. I am very interested in suggestions here.
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Hi Simon, For your first approach, I think what you're looking to do is combine two Sinks together, something like: combine :: Monad m => Sink i1 m r1 -> Sink i2 m r2 -> Sink (Either i1 i2) m (r1, r2) Then you'd be able to use the standard $$ and =$ operators on it. I've put up an example implementation here[1]. The majority of the code is simple pattern matching on the different possible combination, but some things to point out: * 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. 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. Michael [1] https://gist.github.com/4682609 [2] http://hackage.haskell.org/package/stm-conduit
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.... 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.
Hello, Simon.
On 31 January 2013 17:24, Simon Marechal
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....
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
participants (3)
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Alexander V Vershilov -
Michael Snoyman -
Simon Marechal