[Haskell-cafe] child thread control design patterns (was: GHC.Conc.threadStatus - documentation of ThreadDied :: ThreadStatus)

Michael Peyton Jones me at michaelpj.com
Fri Mar 11 10:50:52 UTC 2022

I hadn't come across structured concurrency before, that was an interesting

A quick google turns up a Haskell structured concurrency package, but it
seems to be relatively new: https://hackage.haskell.org/package/ki


On Thu, Mar 10, 2022 at 5:59 PM Olaf Klinke <olf at aatal-apotheke.de> wrote:

> On Thu, 2022-03-10 at 07:44 +0000, Oliver Charles wrote:
> > On Wed, 9 Mar 2022, at 7:45 PM, Olaf Klinke wrote:
> > > That is indeed a neat pattern. What are its advantages/disadvantages?
> > > The main thread in my case is the (Yesod) webserver. Perhaps Yesod does
> > > exactly what you propose, internally.
> >
> > Mostly that one doesn't have to think about linking threads together,
> that just comes by design. If you want "these things must always be
> running", I find it's a nice fit. Also, this pattern implements the
> "structured concurrency" paradigm, which I'm a big fan of (see
> https://vorpus.org/blog/notes-on-structured-concurrency-or-go-statement-considered-harmful/
> )
> >
> > > Your idiom would be something like this?
> > >
> > > main = foldr race_ (warp port site) workers
> >
> > Yes, something like that.
> >
> > > I also do want to include the possibility of a child (daemon) thread to
> > > crash without crashing the webserver (hence my original question),
> > > which your race solution does not cater for. In fact I will have a mix
> > > of perpetually running actions and actions that are fired on command of
> > > the user and then exit gracefully.
> >
> > Right, if you _expect_ the daemon to terminate, then this isn't a good
> fit. However, I would still be striving to get some kind of structured
> concurrency solution. To that end, I would probably have some over-arching
> daemon that has a shared job queue (e.g., an STM TChan), and web requests
> can write to this TChan (maybe with a TMVar to fill in responses). Then,
> the worker daemon would pop jobs off this TChan and itself spawn new
> threads and manage the lifetime of them. This gives you the best of both
> worlds - structured concurrency in main, and ephemeral worker threads.
> >
> > Hope this helps,
> > Ollie
> Hmm, but this over-arching daemon (nursery, as it is called in the blog
> post) watching over the queue *is* my webserver. Instead of a TChan it
> has routes that correspond to start and pause commands of specific
> daemon threads. Inter-thread signalling is indeed handled via TVars,
> that's what they're for. It would be cool to have a TVar or TChan that
> is read-only for one side and write-only for the other. And thank to
> Control.Concurrent.Async.link I have full control over which exceptions
> get propagated from child to parent. The library designers probably
> should not have made async and link two distinct functions, because
> that enables the programmer to disregard those exceptions.
> Should we be porting Trio to Haskell? Or does Haskell offer enough
> abstraction already?
> There are some points in the blog post that I disagree with. If any
> function can fork a concurrent process, not all is lost. Because of
> immutable values we pretty much know what can be touched by that
> concurrent process. And because of that, often we can ignore how long
> the concurrent process runs because it can not affect our linear code.
> I should have read the Async documentation more carefully.
> startThread :: MyThread -> IO ()
> startThread (action,var) = withAsync action (putMVar var)
> immediately cancels the thread because putMVar returns.
> Hence one must use async instead of withAsync if the main thread can
> not be passed to withAsync. The pattern similar to your race would be:
> foldr (\a b -> withAsync a (const b)) mainThread daemons
> which, unlike race, does not kill the mainThread if one of the daemons
> crashes, but cancels the daemons if the mainThread terminates.
> Olaf
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