[concurrency-interest] Upcoming jdk9 j.u.c JEP

Oleksandr Otenko oleksandr.otenko at oracle.com
Mon Aug 10 08:13:17 EDT 2015

The initiator of the next data transfer. Push-stream - the producer 
initiates the transfer of the next portion of the data. Pull-stream - 
the consumer initiates the transfer of the next portion of the data. 
There are mixes of these, too (parser: token detected - producer pushes; 
now the token handling code pulls the rest of expression, but only the 
expression - the producer can't do that, because it doesn't know what 
the expression is).


On 10/08/2015 12:38, Viktor Klang wrote:
> #define initiator
> On Mon, Aug 10, 2015 at 12:00 PM, Oleksandr Otenko 
> <oleksandr.otenko at oracle.com <mailto:oleksandr.otenko at oracle.com>> wrote:
>     The direction of the data flow is not important, so upstream vs
>     downstream are irrelevant. The important aspect is the control flow.
>     There are push-streams and there are pull-streams. The error
>     should propagate to the initiator.
>     Alex
>     On 24/07/2015 08:12, Roland Kuhn wrote:
>>     Hi Greg,
>>     my reply has obviously opened two different discussions (namely
>>     “why are things as they are?” and “what is the suggested change
>>     all about”), I think it would be most fruitful if we stash the
>>     first one for now and come back to it after the second one has
>>     been understood better—at least by myself. That will put us into
>>     a better situation for judging the big picture.
>>     Considering the flow of data from the DB via application and
>>     framework processors into the Servlet container, at any point
>>     along this line failures can happen. The component emitting the
>>     failure will use whatever means it has outside of Reactive
>>     Streams to log/audit/monitor and provide metrics, I assume that
>>     that is just part of all reasonable code; the database will do
>>     that, the application will do it, the framework will probably
>>     allow the application to configure how to do that, and the
>>     application server will be configured how to do that. This means
>>     that everyone can debug their own failures.
>>     Data are flowing towards the Servlet (destined for whichever
>>     client made the request) and it is important to signal abnormal
>>     termination differently from normal termination, hence the
>>     onError propagation in this direction. This also allows
>>     downstream components to see failures coming from upstream, but
>>     this is a byproduct of needing to generate the right kind of
>>     final response to the external client. Now the interesting
>>     question is: why would the database need to know that some
>>     downstream component choked on the data it emitted? How exactly
>>     would this information be used by the database or its
>>     operators/programmers? Arguably the data exist and are “correct”
>>     by definition, guarded by Java types, and any validation errors
>>     that occur are not stream failures (cf. this definition
>>     <http://www.reactivemanifesto.org/glossary#Failure>) and should
>>     be treated as normal data elements and sent downstream (or
>>     filtered out, depending on the requirements & protocol).
>>     I am deliberately painting with high contrast colors here in
>>     order to better understand what exactly it is that you want to
>>     achieve instead of just discussing the proposed solution, thanks
>>     for your patience!
>>     Regards,
>>     Roland
>>>     24 jul 2015 kl. 08:31 skrev Greg Wilkins <gregw at webtide.com
>>>     <mailto:gregw at webtide.com>>:
>>>     Roland,
>>>     thanks for the response.
>>>     But I don't understand why you consider a terminal exception
>>>     being notified upstream as a data flow?   It is data, but it is
>>>     not a flow because it is terminal and cannot be used as a back
>>>     channel.
>>>     Implementations of the API are already required to send data
>>>     upstream: Cancellation is a terminal boolean data state that
>>>     must be sent upstream, and request(int) is a flow of integers
>>>     that must be sent upstream [and as an aside, it is not beyond
>>>     imagination that request(int) will be misused as a back channel
>>>     for data - hey it might even get used to send an error code
>>>     immediately prior/post to a cancel! ]
>>>     Thus I don't see that there is any significant additional
>>>     complexity with that cancellation having a reason associated
>>>     with it.   Implementations must already support upward bound
>>>     data and any sequencing and/or race conditions that exist with
>>>     cancel(Throwable) also exist with just cancel().
>>>     I also dispute that a Subscriber will be under the control of
>>>     the Publisher.     In the example cited and application is
>>>     providing a Processor, that is using a Publisher provided by a
>>>     3rd party database and an Subscriber provided by the Servlet
>>>     container, with perhaps some framework provided Processors for
>>>     serialization.   In this example there is the possibility of
>>>     components from at least 4 difference code sources being
>>>     combined in a chain that crosses deployment administration
>>>     boundaries of: database, application and server.     The log &
>>>     cancel handling of errors is going to be very difficult because
>>>     many different log mechanism may be in use and access may not be
>>>     easily achieved.  ie applications developers may not have full
>>>     viability of database logs or servlet container logs.
>>>     The type of error I'm concerned about are all terminal style
>>>     errors and not intended to be a back flow of data, nor
>>>     acknowledgement of messages sent.   It is probably that the
>>>     implementers of cancel(Throwable) would just log, cancel
>>>     themselves and pass on the cancel(Throwable) to any of their
>>>     Subscripions.   However the point being that would allow the
>>>     reason for the failure to cross the administrative boundaries so
>>>     that it can be known to all.
>>>     I think that any argument that can be made for not sending a
>>>     Throwable upstream can equally be made for not sending one
>>>     downstream (or for not having any exceptions in the java
>>>     language).   Exceptions are very rarely handled in any
>>>     meaningful way, but are extremely useful for passing details of
>>>     a failure so that they may be known to all who may need to know.
>>>     Without exceptions  I'm imagining many many  stack over flow
>>>     questions like "Why was my Subscription cancelled?" followed by
>>>     obligatory "RTFLog Stupid!" responses!
>>>     cheers
>>>     On 24 July 2015 at 15:55, Roland Kuhn <rk at rkuhn.info
>>>     <mailto:rk at rkuhn.info>> wrote:
>>>         Hi Greg,
>>>         the reasoning behind the asymmetric RS design is that this
>>>         communication primitive targets unidirectional
>>>         communication, bidirectional conversations would utilize two
>>>         such streams running in opposite directions. This means that
>>>         for a single stream data elements (of which onError is but a
>>>         special one) flow downstream and only demand flows upstream.
>>>         Publishers only need to know about when and if to produce
>>>         the next element(s), hence we didn’t see a use-case for
>>>         propagating more information than “N elements needed” and
>>>         “no more elements needed”.
>>>         If a single Reactive Stream could transport data upstream
>>>         then we would need to implement back-pressure on that back
>>>         channel as well, leading to the same complexity as having
>>>         two RS running in opposite directions. Another reason why we
>>>         made this separation lies in not burdening the API designers
>>>         of conforming implementations with an impossible task: the
>>>         combinators offered on stream transformation APIs flow with
>>>         the (English) language from left to right and describe
>>>         sequences of transformation stages but with data flowing
>>>         upstream there would be the need for also describing how to
>>>         handle that—even if it is “only” an error channel—and since
>>>         these data flow in the opposite direction there would be no
>>>         natural way to write this down.
>>>         Learning about the reason behind cancellation seems geared
>>>         towards recovery in the sense that the Publisher would then
>>>         construct and attach a different Subscriber
>>>         afterwards—please let me know if you have something else in
>>>         mind—and if you want to do that then the Subscriber will in
>>>         any case be under the Publisher’s control and can use a
>>>         different channel to communicate the onError signal back to
>>>         the data source. Since that channel would transport data it
>>>         would be a separate one flowing in the opposite direction as
>>>         mentioned above, at least conceptually; with a single
>>>         element like you describe it could well be a simpler
>>>         callback mechanism and might not need full back-pressure.
>>>         I hope this clarifies some of the background behind the RS
>>>         design. Please share more of your intended use of an error
>>>         back-channel so that we can understand what exactly the
>>>         upstream components would do with that data in the example
>>>         case you mention.
>>>         Regards,
>>>         Roland
>>>>         24 jul 2015 kl. 00:35 skrev Greg Wilkins <gregw at webtide.com
>>>>         <mailto:gregw at webtide.com>>:
>>>>         On 24 July 2015 at 00:23, Doug Lea <dl at cs.oswego.edu
>>>>         <mailto:dl at cs.oswego.edu>> wrote:
>>>>             * Reactive-stream users may be disappointed that we do
>>>>             not include any
>>>>             net/IO-based Flow.Publisher/Subscriber classes,
>>>>             considering that
>>>>             reactive-streams are mainly motivated by net-based
>>>>             frameworks.  The
>>>>             reasons for triaging these out are that (1) IO
>>>>             generally falls outside
>>>>             of java.util.concurrent (2) Most net-based frameworks
>>>>             seem to use
>>>>             custom data representation etc (e.g., JSON) that are
>>>>             even further out
>>>>             of scope. However class SubmissionPublisher can be used
>>>>             as an adaptor
>>>>             to turn just about any kind of source into a Publisher,
>>>>             so provides a
>>>>             nearly universal way of constructing a good non-custom
>>>>             Publisher even
>>>>             from IO-based sources.  (Also notice that
>>>>             SubmissionPublisher can
>>>>             serve as the basis of other actor-like frameworks,
>>>>             including those
>>>>             turning off back-pressure by calling
>>>>             subscription.request(Long.MAX_VALUE) in onSubscribe).
>>>>         Doug et al,
>>>>         The Jetty project has been experimenting with the reactive
>>>>         streams API:
>>>>         https://github.com/jetty-project/jetty-reactive albiet not
>>>>         with the JDK-9 version of it, but inspired by the proposed
>>>>         inclusion of it.
>>>>         We very much like the API and what it can bring to our
>>>>         space.  We don't see that it needs direct IO support and
>>>>         that it's power is actually bridging domains with a good
>>>>         asynchronous model that supports flow control.
>>>>         We've also begun some preliminary discussions about
>>>>         developing RS based proposal for the Servlet 4.0
>>>>         specification. Currently the Servlet API does well support
>>>>         asynchronous IO and behaviour, but the API is deceptively
>>>>         difficult to use correctly and gives no support for back
>>>>         pressure.   With RS's we can envisage solutions that look like:
>>>>           * A database provides a RS Producer that provides the
>>>>             large results of a query asynchronously from a remote
>>>>             database server
>>>>           * Some business logic is encapsulated as a RS Processor
>>>>             subscribed to the database producer
>>>>           * Some framework provided Porocessors subscribe to the
>>>>             business logic Processor to perform a chain of
>>>>             functions such as serialization, compression
>>>>           * A container provided Subscriber terminates the chain
>>>>             and sends the resulting byte out over HTTP/HTTP2 or
>>>>             Websocket. The flow control mechanisms of these
>>>>             protocols would be the basis of the RS back pressure.
>>>>         In such solutions, a full HTTP/2 flow control window would
>>>>         result in back pressure on the remote database server,
>>>>         allowing threadless waiting without unlimited queuing of data.
>>>>         However, we have a significant concern with the API in that
>>>>         we do not like it's error handling design. Specifically
>>>>         that it is asymmetric and an error in the middle of a chain
>>>>         of processors can be propagated downstream with
>>>>         onError(Throwable) but can only be propagated upstream with
>>>>         cancel().
>>>>         We believe that cancel without reason is an insufficient
>>>>         semantic to build a robust ecosystem of RS Processors that
>>>>         can be used to build applications. Consider the above
>>>>         example, it would be ideal if the object serialization was
>>>>         handled by a 3rd party Processor (let's say
>>>>         JSONEncodingProcessor). If the business logic erroneously
>>>>         sent an non-jsonable object, or if the JSON converter was
>>>>         incorrectly configured then the JSONEcondiingProcessor
>>>>         could encounter an error during its onNext(Object item)
>>>>         handling and it's only permitted handling of that is to
>>>>         cancel the stream, without explanation.
>>>>         I have raised this as an issue on the RS github and it the
>>>>         current recommendation is to log and cancel:
>>>>         https://github.com/reactive-streams/reactive-streams-jvm/issues/271#issuecomment-121974544
>>>>         However I believe that log and cancel is a insufficient
>>>>         semantic. Logging in assembled applications is often
>>>>         fraught as each component provider will fight over which
>>>>         logging framework is best.  RS chains may cross
>>>>         jurisdictional boundaries and logs may not even be readily
>>>>         available.
>>>>         The solution we see is to replace/augment cancel() with
>>>>         either cancel(Throwable reason) or an upstream
>>>>         onError(Throwable reason).  I acknowledge that the passed
>>>>         reason may not always be meaningful to the upstream
>>>>         processors and publishers, but it is better to ignore a
>>>>         meaningless reason than to be ignorant of a meaningful one.
>>>>         When considering this API, we have to look beyond usages
>>>>         that work well and consider usages that will fail well also!
>>>>         cheers
>>>>         -- 
>>>>         Greg Wilkins <gregw at webtide.com <mailto:gregw at webtide.com>>
>>>>         _______________________________________________
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>>>>         <mailto:Concurrency-interest at cs.oswego.edu>
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>>>         --
>>>         I'm a physicist: I have a basic working knowledge of the
>>>         universe and everything it contains!
>>>             - Sheldon Cooper (The Big Bang Theory)
>>     --
>>     I'm a physicist: I have a basic working knowledge of the universe
>>     and everything it contains!
>>         - Sheldon Cooper (The Big Bang Theory)
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