[concurrency-interest] draft Carrier API

Ghadi Shayban gshayban at gmail.com
Tue Mar 10 23:08:39 EDT 2020


As an API comparison point, Clojure's CSP library is called core.async. It
features channels and send/receive operations, and the CSP select operation
called "alts". The alts op is a plain function (no compiler magic) which
non-deterministically chooses the first operation that can proceed, whether
it is a channel put or a take. Channel buffers are polymorphic (fixed,
sliding and dropping; or no buffer, meaning rendezvous semantics).

Channel put returns whether the put succeeded (false if channel is
closed).  Critically, the channel isClosed predicate is not exposed
publicly, as it is hard to use without a TOCTOU bug.  Channel take receives
the item (which is any reference, including Boolean false!) or receives nil
when a channel is closed. There is only a single concrete implementation of
a channel, ManyToManyChannel, but the underlying interfaces are split into
read/write.

When is it useful to use the exception throwing variants of Carrier, rather
than trySend/tryReceive? Seems like there is a whole lot of API surface
area to deal with channels being closed or closing.

>From a receivers point of view, the distinction between shutdown & closed
seems arbitrary. You're not done receiving until a buffer (if present)
drains. Over the years, I have found from my usage of core.async that the
most useful ops around termination are: a sender signaling that they're
done, or a receiver abandoning the interaction early. Generally only one
side is in charge of closing a channel, but in the consumer abandonment
scenario, the producer will detect that the channel has closed during its
next put (then walks away, too.)

I don't understand the distinction between send+timeout and
sendSynchronously(). Isn't synchronicity more a property of whether a
buffer is present or not (rendezvous channel)?  Same question re:
tryReceiveEventually

I love that there is something CSP-like going into the JVM - it will be
killer when Loom drops.

On Tue, Mar 10, 2020 at 7:49 AM Alex Otenko via Concurrency-interest <
concurrency-interest at cs.oswego.edu> wrote:

> Thanks, that will probably work. Or some tryReceive version like poll with
> timeout. Passing max_value is essentially blocking until closed or a value
> arrives (or spurious wakeup-like condition).
>
> Alex
>
> On Tue, 10 Mar 2020, 11:30 Doug Lea via Concurrency-interest, <
> concurrency-interest at cs.oswego.edu> wrote:
>
>> On 3/9/20 3:29 PM, Alex Otenko wrote:
>> > IllegalStateException is ok if receiver should've known there are no
>> > more items to receive. This is a good idea in cases with definite length
>> > of stream, and the length being known to the receiver before entering
>> > receive(). This doesn't seem like a good idea for indefinite length
>> > cases - like, loop to read all items until eof.
>> >
>> This is the reason for:
>>     Stream<T> stream();             // destructive (consume-on-traverse)
>> But it is also sensible to provide a simpler forEach analog:
>>     long consumeEach(Consumer<? super T> proc); // return count
>>
>> For those who need stateful loops, we could add "eventually" forms of
>> tryReceive. With non-value-types, the preferable form that can co-exist
>> with value-types is usually to return a resultIfAbsent (that is almost
>> always chosen to be null), and for value types, Optional. To avoid
>> annoying people, we should probably have both.
>>
>>     T tryReceive(T resultIfAbsent); // resultIfAbsent if closed or empty
>>     Optional<T> tryReceive();       // Optional.empty if closed or empty
>>
>>     T tryReceiveEventually(T resultIfAbsent); // resultIfAbsent if closed
>>     Optional<T> tryReceiveEventually(); // Optional.empty if closed
>>
>> Maybe there is a better method name.
>>
>> (See updates at http://gee.cs.oswego.edu/dl/wwwtmp/Carrier.java)
>>
>> -Doug
>>
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