Temporal coupling in Fibers and Fibers vs State Machines.

Dávid Karnok akarnokd at gmail.com
Wed Feb 19 19:42:34 UTC 2020

Rafael's concerns are mostly related how the functional and declarative
nature of the reactive libraries are difficult to learn for some people.
Yet, these people happily code in SQL and use Java Streams. Maybe it's time
to go back to reading database files with InputStream and use for loops as

Martin's concerns are, if by composition he means what I think he means, if
you could set up pieces of computation and functionality and combine them
into larger components. If so, these components can have state and those
state could change in response to stimuli from other components. Reactive
is one of such approach that has individual pieces - operators chained up
on a dataflow have internal state tracking when and how their peers can
send data and commands to each other. Loom's, and Kotlin's Coroutines
instead say you compose via the source code itself, by writing larger and
larger methods encompassing a lot of imperative operations. If you have
such a method, but for some uses need some retry code, you may be out of
luck and have to code yet another set of methods to include that

Vaughn's concerns are that there aren't many helping hands with Fibers and
concurrency. In fact, you have to consider the Java Memory Model again the
moment two routines in separate fibers have to communicate: blocking
structures, atomics, locks and their effects on visibility. In reactive,
from an user's perspective, you don't have to worry about most of these as
the libraries can hide many of the intricacies of the JMM and provide you
with an API and callback surface. Want to create an item and hand it over
to another thread for processing? There are operators for that and you have
only to specify how to create the item, what thread it should end up and
what processing there should be for it. Want to generate many items?
Replace the front of the flow and the rest will still be the same.

Also note that Loom's sales pitch has shifted over the years. It started as
the solution that will outperform reactive and having users code in easy to
understand imperative fashion as the JVM will do the heavy lifting behind
the scenes. However, we have pointed out that a) suspension is more costly
and may dominate the overheads, b) composition (see above) is limited so
people, individually and over and over, may end up with DIY solutions and
c) you have to make sure locks/blocking happen in a virtual thread for
which you may have to inject virtual thread-based executors all over the

Ron Pressler <ron.pressler at oracle.com> ezt írta (időpont: 2020. febr. 19.,
Sze, 19:48):

> Hi.
> I don’t think I really understand what the concerns are, and believe they
> might
> be tied to a misunderstanding of Loom’s goals and design.
> Anyone who has concerns is welcome to explain them here, without
> character-count limits.
> Ron
> On 19 February 2020 at 18:37:53, Rahul Khandelwal (rahulnewai at gmail.com
> (mailto:rahulnewai at gmail.com)) wrote:
> > Hi All,
> >
> > I have been following Project Loom for quite some time now and I find it
> > really exciting.
> > This is my first post in the mailing list.
> >
> > I went through the following twitter discussion -
> > https://twitter.com/rafaelcodes/status/1176229314112741377
> > However I could not understand the final gist/conclusion of the
> discussion.
> >
> > 1. Here In the discussion Martin Thompson had the view that Fibers don't
> > compose and cannot escape temporal coupling.
> > He also advocated about the use of explicit state machines instead of
> > Fibers.
> > https://twitter.com/mjpt777/status/1176231250308325376
> >
> > 2. Vaughn Vernon had opinions around the Java memory model and the
> > complexity of the effects of predictive execution.
> > https://twitter.com/VaughnVernon/status/1176299015849136128
> >
> >
> > Could someone explain what were the concerns and suggestions they
> expressed
> > and how Fibers handle said concerns.
> >
> > It would be great if someone could suggest some resources to better
> > understand above twitter discussion.
> >
> >
> > Thanks,
> > Rahul

Best regards,
David Karnok

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