Hi! The Java vs Scala discussion is orthogonal to the actors discussion. We can use Akka actors in Java. And I think that makes a lot of sense, for the reasons that Till mentioned, plus the following reasons:
- Akka has made a lot of effort to combine message throughput (multiple actor calls in one message) with low message latency. I don't think we could do much better with something else - I am currently working on the ExecutionGraph and Scheduler to unify lazy computation / recovery / dynamic resource assignment. The Actor paradigm (order of calls, queuing invocations in the actor mailboxes) makes it much simpler to get concurrent situations right (such as certain calls overtaking each other, like for example deploy/cancel, etc) - Actors work with thread pools be themselves, so we can get rid of all the inner runnables sent to executor services. makes the code much more readable Stephan On Sun, Aug 31, 2014 at 1:31 PM, Till Rohrmann <[email protected]> wrote: > Hi Daniel, > > the RPC rework is discussed in > https://issues.apache.org/jira/browse/FLINK-1019. Jira is currently down > due to maintenance reasons. > > The ideas to use akka are the following. Akka allows us to reduce the code > base which has to be maintained. Especially, we get rid of all the > multi-threading programming of the rpc service which is always hard to work > with. With Akka we would get the heartbeat signal for free, because Akka > can detect dead actors. Akka uses supervision to handle fault tolerance as > well as recovery and it allows an easy forwarding of remote exceptions. At > the same time it offers a nice rpc abstraction which easily allows to > implement asynchronous services. Furthermore, it scales rather well to > large numbers of nodes and hopefully we get the latencies of Flink a little > bit down. > > Bests, > > Till > > > On Sun, Aug 31, 2014 at 11:35 AM, Daniel Warneke <[email protected]> > wrote: > > > Hi, > > > > will akka just be used for RPC or are there any plans to expand the > > actor-based model to further parts of the runtime system? If so, could > you > > please point me to the discussion thread? > > > > Spontaneously, I would say that adding a hard dependency on Scala just > for > > the sake of having a hip RPC service sounds like a pretty dodgy deal. > > Therefore, I would like understand how much value akka could bring to > Flink > > in the long run. The discussion whether to reimplement core components of > > the system in Scala should be the second step in my opinion. > > > > Bests, > > > > Daniel > > > > > > Am 29.08.2014 11:33, schrieb Asterios Katsifodimos: > > > > I agree that using Akka's actors from Java results in very ugly code. > >> Hiding the internals of Akka behind Java reflection looks better but > >> breaks > >> the principles of actors. For me it is kind of a deal breaker for using > >> Akka from Java. I think that Till has more reasons to believe that > Scala > >> would be a more appropriate for building a new Job/Task Manager. > >> > >> I think that this discussion should focus on 4 main aspects: > >> 1. Performance > >> 2. Implementability > >> 3. Maintainability > >> 4. Available Tools > >> > >> 1. Performance: Since that the job of the JobManager and the TaskManager > >> is > >> to 1) exchange messages in order to maintain a distributed state machine > >> and 2) setup connections between task managers, 3) detect failures etc.. > >> In > >> these basic operations, performance should not be an issue. Akka was > >> proven > >> to scale quite well with very low latency. I guess that the low level > >> "plumbing" (serialization, connections, etc.) will continue in Java in > >> order to guarantee high performance. I have no clue on what's happening > >> with memory management and whether this will be implemented in Java or > >> Scala and the respective consequences. Please comment. > >> > >> 2. Since the Job/Task Manager is going to be essentially implemented > from > >> scratch, given the power of Akka, it seems to me that the implementation > >> will be easier, shorter and less verbose in Scala, given that Till is > >> comfortable enough with Scala. > >> > >> 3. Given #2, maintaining the code and trying out new ideas in Scala > would > >> take less time and effort. But maintaining low level plumbing in Java > and > >> high level logic in Scala scares me. Anyone that has done this before > >> could > >> comment on this? > >> > >> 4. Tools: Robert has raised some issues already but I think that tools > >> will > >> get better with time. > >> > >> Given the above, I would focus on #3 to be honest. Apart from this, > going > >> the Scala way sounds like a great idea. I really second Kostas' opinion > >> that if large changes are going to happen, this is the best moment. > >> > >> Cheers, > >> Asterios > >> > >> > >> > >> On Fri, Aug 29, 2014 at 1:02 AM, Till Rohrmann <[email protected] > > > >> wrote: > >> > >> I also agree with Robert and Kostas that it has to be a community > >>> decision. > >>> I understand the problems with Eclipse and the Scala IDE which is a > pain > >>> in > >>> the ass. But eventually these things will be fixed. Maybe we could also > >>> talk to the typesafe guy and tell him that this problem bothers us a > lot. > >>> > >>> I also believe that the project is not about a specific programming > >>> language but a problem we want to tackle with Flink. From time to time > it > >>> might be necessary to adapt the tools in order to reach the goal. In > >>> fact, > >>> I don't believe that Scala parts would drive people away from the > >>> project. > >>> Instead, Scala enthusiasts would be motivated to join us. > >>> > >>> Actually I stumbled across a quote of Leibniz which put's my point of > >>> view > >>> quite accurately in a nutshell: > >>> > >>> In symbols one observes an advantage in discovery which is greatest > when > >>> they express the exact nature of a thing briefly and, as it were, > picture > >>> it; then indeed the labor of thought is wonderfully diminished -- > >>> Gottfried > >>> Wilhelm Leibniz > >>> > >>> > >>> On Thu, Aug 28, 2014 at 12:57 PM, Kostas Tzoumas <[email protected]> > >>> wrote: > >>> > >>> On Thu, Aug 28, 2014 at 11:49 AM, Robert Metzger <[email protected] > > > >>>> wrote: > >>>> > >>>> Changing the programming language of a very important system > component > >>>>> > >>>> is > >>> > >>>> something we should carefully discuss. > >>>>> > >>>>> Definitely agree, I think the community should discuss this very > >>>> > >>> carefully. > >>> > >>>> > >>>> I understand that Akka is written in Scala and that it will be much > >>>>> > >>>> more > >>> > >>>> natural to implement the actor based system using Scala. > >>>>> I see the following issues that we should consider: > >>>>> Until now, Flink is clearly a project implemented only in Java. The > >>>>> > >>>> Scala > >>> > >>>> API basically sits on top of the Java-based runtime. We do not really > >>>>> depend on Scala (we could easily remove the Scala API if we want to). > >>>>> Having code written in Scala in the main system will add a hard > >>>>> > >>>> dependency > >>>> > >>>>> on a scala version. > >>>>> Being a pure Java project has some advantages: I think its a fact > that > >>>>> there are more Java programmers than Scala programmers. So our > chances > >>>>> > >>>> of > >>> > >>>> attracting new contributors are higher when being a Java project. > >>>>> On the other hand, we could maybe attract Scala developers to our > >>>>> > >>>> project. > >>>> > >>>>> But that has not happened (for contributors, not users!) so far for > our > >>>>> Scala API, so I don't see any reason for that to happen. > >>>>> > >>>>> > >>>>> This is definitely an issue to consider. We need to carefully weight > >>>> how > >>>> important this issue is. If we want to break things, incubation is the > >>>> right time to do it. Below are some arguments in favor of breaking > >>>> > >>> things, > >>> > >>>> but do keep in mind that I am undecided, and I would really like to > see > >>>> > >>> the > >>> > >>>> community weighing in. > >>>> > >>>> First, I would dare say that the primary reason for someone to > >>>> contribute > >>>> to Flink so far has not been that the code is written in Java, but > more > >>>> > >>> the > >>> > >>>> content and nature of the project. Most contributors are Big Data > >>>> enthusiasts in some way or another. > >>>> > >>>> Second, Scala projects have attracted contributors in the past. > >>>> > >>>> Third, it should not be too hard for someone that does not know Scala > to > >>>> contribute to a different component if the interfaces are clear. > >>>> > >>>> > >>>> Another issue is tooling: There are a lot of problems with Scala and > >>>>> Eclipse: I've recently switched to Eclipse Luna. It seems to be > >>>>> > >>>> impossible > >>>> > >>>>> to compile Scala code with Luna because ScalaIDE does not properly > cope > >>>>> with it. > >>>>> Even with Eclipse versions that are supported by ScalaIDE, you have > to > >>>>> manually install 3 plugins, some of them are not available in the > >>>>> > >>>> Eclipse > >>> > >>>> Marketplace. So with a JobManager written in Scala, users can not just > >>>>> import our project as a Maven project into Eclipse and start > >>>>> > >>>> developing. > >>> > >>>> The support for Maven is probably also limited. For example, I don't > >>>>> > >>>> know > >>> > >>>> if there is a checkstyle plugin for Scala. > >>>>> > >>>>> I'm looking forward to hearing other opinions on this issue. As I > said > >>>>> > >>>> in > >>> > >>>> the beginning, we should exchange arguments on this and think about it > >>>>> > >>>> for > >>>> > >>>>> some time before we decide on this. > >>>>> > >>>>> Best, > >>>> > >>>>> Robert > >>>>> > >>>>> > >>>>> > >>>>> On Thu, Aug 28, 2014 at 1:08 AM, Till Rohrmann <[email protected] > > > >>>>> wrote: > >>>>> > >>>>> Hi guys, > >>>>>> > >>>>>> I currently working on replacing the old rpc infrastructure with an > >>>>>> > >>>>> akka > >>>> > >>>>> based actor system. In the wake of this change I will reimplement the > >>>>>> JobManager and TaskManager which will then be actors. Akka offers a > >>>>>> > >>>>> Java > >>>> > >>>>> API but the implementation turns out to be very verbose and > >>>>>> > >>>>> laborious, > >>> > >>>> because Java 6 and 7 do not support lambdas and pattern matching. > >>>>>> > >>>>> Using > >>> > >>>> Scala instead, would allow a far more succinct and clear > >>>>>> > >>>>> implementation > >>> > >>>> of > >>>>> > >>>>>> the JobManager and TaskManager. Instead of a lot of if statements > >>>>>> > >>>>> using > >>> > >>>> instanceof to figure out the message type, we could simply use > >>>>>> > >>>>> pattern > >>> > >>>> matching. Furthermore, the callback functions could simply be Scala's > >>>>>> anonymous functions. Therefore I would propose to use Scala for > these > >>>>>> > >>>>> two > >>>> > >>>>> systems. > >>>>>> > >>>>>> The Akka system uses the slf4j library as logging interface. > >>>>>> > >>>>> Therefore > >>> > >>>> I > >>>> > >>>>> would also propose to replace the jcl logging system with the slf4j > >>>>>> > >>>>> logging > >>>>> > >>>>>> system. Since we want to use Akka in many parts of the runtime > system > >>>>>> > >>>>> and > >>>> > >>>>> it recommends using logback as logging backend, I would also like to > >>>>>> replace log4j with logback. But this change should inflict only few > >>>>>> > >>>>> changes > >>>>> > >>>>>> once we established the slf4j logging interface everywhere. > >>>>>> > >>>>>> What do you guys think of that idea? > >>>>>> > >>>>>> Best regards, > >>>>>> > >>>>>> Till > >>>>>> > >>>>>> > > >
