Single Thread Continuation

[Alex Otenko]

Well, the code you shared only illustrates that it is not as simple as
claimed.

So, you don't use a queue. That's fine. Now, how will the generator stop?
You have the close() in finalize(), and a dance around hasNext() to totally
order the delivery of elements vs end of stream.

Basically, it's two channels.


On Tue, 4 Jul 2023, 15:07 robert engels, <rengels at ix.netcom.com> wrote:

> It is also straightforward to use a buffered queue to allow the producer
> to read ahead rather than the hand-off queue as implemented - which is
> often necessary when the producer is reading from a database, network
> calls, etc.
>
> There is no need to for specific generator support in Java when you have
> cheap virtual threads.
>
> On Jul 4, 2023, at 8:58 AM, robert engels <rengels at ix.netcom.com> wrote:
>
> I created a project to demonstrate. It uses finalizers for simplicity but
> it easily adaptable to weak reference queues. It only needs a single
> producer virtual thread per generator.
>
> see https://github.com/robaho/generators/tree/main
>
> On Jul 4, 2023, at 7:24 AM, Robert Engels <rengels at ix.netcom.com> wrote:
>
> You need 3 threads - the producer the consumer and the watcher (the
> watcher and producer might be able to be combined with async notifications
> from the weak reference queue) But it is all encapsulated and virtual
> threads makes this very cheap - as cheap as actual continuations would be.
>
> On Jul 4, 2023, at 7:19 AM, Alex Otenko <oleksandr.otenko at gmail.com>
> wrote:
>
> 
> So it's not really just the queue.
>
> Plus you need two of them. Otherwise you can't control the timing of when
> the next value is produced.
>
> On Tue, 4 Jul 2023, 13:13 Robert Engels, <rengels at ix.netcom.com> wrote:
>
>> To elaborate - the producer knows when no more values are coming and
>> marks the iterator as such. So the next calls to hasNext()/next() fail
>> appropriately. That is trivial.
>>
>> The only “complex” part of the design is that the returned iterator is
>> tracked with a weak reference by the producer. So if it goes out of scope
>> the producer can be woken from the blocked/waiting state and clean up.
>>
>> On Jul 4, 2023, at 7:01 AM, Robert Engels <rengels at ix.netcom.com> wrote:
>>
>> 
>> That is built into the Iterator interface.
>>
>> On Jul 4, 2023, at 4:05 AM, Alex Otenko <oleksandr.otenko at gmail.com>
>> wrote:
>>
>> 
>> How does the generator tell the consumer that no more values are
>> forthcoming?
>>
>> On Mon, 3 Jul 2023, 12:38 Robert Engels, <rengels at ix.netcom.com> wrote:
>>
>>> Believe me. Queues are all you need there is no memory leak and no need
>>> to “close”. The producer side uses a weak reference to the queue. When
>>> there are no more strong references the producer side can terminate.
>>>
>>> You can’t use a standard blocking queue for this - but the queue
>>> implementation is fairly trivial - with a wake-up thread that listens on
>>> the weak reference queue.
>>>
>>> On Jul 3, 2023, at 6:19 AM, Attila Kelemen <attila.kelemen85 at gmail.com>
>>> wrote:
>>>
>>> 
>>>
>>>>
>>>> 2. We need to synchronize access to mutable state to avoid memory
>>>>    hazards. This is a separate issue from synchronizing access to
>>>>    mutable state to avoid correctness issues. With virtual threads on
>>>>    a single platform thread, this goes away too (because it's always
>>>>    the same thread observing memory operations; no barriers needed).
>>>>
>>>>
>>> That still seems incorrect to me (in principle, in practice it most
>>> likely will end up to be fine, but I just wouldn't rely on it), because the
>>> barrier is needed to prevent instruction reordering by the compiler, and
>>> you are not safe from that by using the same platform thread.
>>>
>>>
>
>
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