> On Feb 13, 2015, at 6:51 AM, Endre Varga <[email protected]> wrote: > > Hi Reid, > > On Fri, Feb 13, 2015 at 11:59 AM, Reid Spencer <[email protected] > <mailto:[email protected]>> wrote: > Hi Eric, > > Thanks for your response. It is a little comforting to know that I’m not the > only one on this planet also struggling with how to do something real with > akka-stream. I’m sure this thread will eventually lead to the clarity we both > seek. > > I probably need to add that one of the beginner issues is to try to do > *everything* with streams. Currently this is doomed to fail, and even in the > future it will be not possible in all cases. Plain old actors are always > there to use, so use streams where they really shine already, right now: for > example a chain of decoding/encoding steps, simple transformations that can > be expressed as an ask to an actor (you can use it together with mapAsync and > have your actor magically backpressured!). Also, graphs are almost always > needed for protocols.
Yes, this is exactly the case I wanted to use it for. I have higher level actors to deal with all manner of things but wanted to replace my Akka IO implementation of the socket encode/decode cycle with TcpStream. I thought it would be easy but got myself a bit confused. Your last email went a long way to pointing me in the right direction. > > > Yes, I’ve seen this before but it led to my grumbling about the lack of an > internal design document. That Flow.apply method creates a GraphBackedFlow. > > These will be gone in M4. A Flow is just a collection of processing steps > (any graph layout) with exactly one input and output port that is not wired > to anything. Okay, just to be clear, GraphBackedFlow will be gone as will the Flow.apply method that creates them ? > > When I first saw that I was encouraged because I realized I would just need > to understand GraphBackedFlow to understand what > Flow.apply(Sink[I],Source[O]) was giving me. But, the documentation for that > object only says “INTERNAL API”. I’m fine with that lack of documentation for > things that I don’t use directly, but this GraphBackedFlow is something > returned by the Flow companion and would be an instance of something I use in > my program. It needs user level documentation. Either that or the Flow.apply > method should be much more explicit about what is returned. All it says is > "Create a [[Flow]] from a seemingly disconnected [[Source]] and [[Sink]] > pair.” It would be very much appreciated if there was at least a line or two > of what this GraphBackedFlow represented. I get that it backs the flow with > some sort of graph of objects but that really doesn’t give me much > information about its operation. > > These things will go away with M4, see explanation below. > > >> This seems to allow you to "handle" a TCP connection with a Flow using >> independent Sink and Source. I am still in the process of trying to >> construct a FlowGraph using ActorSubscriber and ActorPublisher to integrate >> the TCP connection with application logic using actors, but finding it a >> challenge to do so in a simple way and avoiding circular references. So I'm >> still working on "grokking" it myself, but perhaps this discovery might help. > > I will have to dive into the Working With Graphs > <http://doc.akka.io/docs/akka-stream-and-http-experimental/1.0-M3/scala/stream-graphs.html> > page in more detail. I’ve been giving it less attention because of this > sentence: > > Graphs are needed whenever you want to perform any kind of fan-in ("multiple > inputs") or fan-out ("multiple outputs") operations. > > Well, I think that sentence really does not convey the importance of graphs. > It is very likely that you will need graphs for more complex application. I’m getting how important they are. Perhaps a documentation change that indicates their importance up front would help clue readers like me into the fact that I needed to pay more attention to that portion of the documentation. > > So, just to give you a taste of M4: > > - Internally, everything is just a "Module" which has input and output ports > - An atomic Module is directly translated to an executable entity > - A composite module consists of atomic modules and possible other composite > modues, with some ouput ports connected to some input ports > - you can hierarchically nest these modules > - A Flow is *really* nothing more than just a DSL shim over a module that > has exactly one input and output port, but has arbitrarily complex nested > graph layout inside > - Same goes for Source and Sink Okay, can I just suggest that “Module” might be a poor choice for a name as it is already highly overloaded in lot of other contexts. Node wouldn’t be much better but choosing good names is a significant aide to comprehension of these concepts. > > Now you can start imagining your application as a hieararchic setup of > "boxes" (modules) that represent services, and has input and output ports. > You compose simpler services to more complex ones and wrap them in another > box, until you have a nice nested hierarchy. > > And that's it :) Yes, that’s how I do think of my application and I’m looking forward to expressing it directly with akka-stream. > I believe my needs are two linear “roads” not a network of junctions that > fan-in and fan-out; but then again, I could be wrong. > > This is hard to know. I don't know the exact hassles you need. Your explanations have helped me realize that my thinking was too linear and based on keeping input and output separate. I’m going to try to drop that mental construct and look at the problem from the perspective of a graph that manages the request/response “loop” that StreamTcp provides. > Akka Team: Please don’t take any of this commentary as derogatory; I’m quite > enthusiastic about using akka-stream and StreamTcp. I understand you are > scrambling to get this released in a couple of months and there is much to do > and document. In the spirit of cooperation, I will suggest that if you can > point Eric and I in the right direction (e.g. how to write an asynchronous > Tcp protocol), then I will volunteer to add sample code and documentation to > Akka that explains this part (once I understand it, of course!) :) > > Well, just give a very dummy-sized example problem that you find enough to > start (please keep it minimal, we are short in resources) and I can attempt > to give some pointers. I’ll see what I can come up with in the next week or so. Hopefully my questions from here on out get more focused on specific issues. Thanks again, Endre. > > -Endre > > >> >> -- Eric >> >> >> On Thursday, February 12, 2015 at 11:56:03 AM UTC-5, Reid Spencer wrote: >> hAkkers, >> >> I've been unable to grok <http://en.wikipedia.org/wiki/Grok> how to >> communicate with a TCP socket using akka-stream and StreamTcp extension. At >> this point, I'm not sure the fault is entirely mine. :) I'm building a >> MongoDB driver <https://github.com/reactific/RxMongo> that uses Akka and I >> have it working well with akka.io <http://akka.io/>. Mongo requires >> asynchronous reading and writing on a TCP socket. You can write requests to >> it as they happen and you read responses as they can be satisfied by the >> server. Requests and responses are matched with an ID number (i.e. each >> response indicates the request ID to which it responds). This seems to be an >> ideal candidate for akka-streams, at least on the surface. I'm now trying to >> transition my design to use akka-streams and StreamTcp. After several days >> of fumbling around, I'm still not able to grasp how to connect all the >> pieces. So, I'm hoping the group can help and that this might be instructive >> for users of akka-stream, or at least shine some light on needed >> documentation or features. >> >> Just to address the obligatory: >> I've read the akka-stream (1.0-M3) documentation, many times, every page. >> I've looked at the akka-stream code and discovered that without some sort of >> internal design document, much of it will be unintelligible because I don't >> have a conceptual model for how the pieces fit together (essentially a >> forest/trees issue). >> I've read the (insufficient, IMO) API documentation. >> I've built, tried and studied the TcpEcho sample program. >> That sample TcpEcho program is the source of most of my misunderstanding as >> it is the only sample that relates to what I'm doing and I cannot >> extrapolate from it to do what I want to do. Here's the program, from the >> documentation >> <http://doc.akka.io/docs/akka-stream-and-http-experimental/1.0-M3/scala/stream-io.html#Connecting__REPL_Client>: >> >> val connection: OutgoingConnection = >> StreamTcp().outgoingConnection(localhost) >> >> val replParser = new PushStage[String, ByteString] { >> override def onPush(elem: String, ctx: Context[ByteString]): Directive = { >> elem match { >> case "q" ⇒ ctx.pushAndFinish(ByteString("BYE\n")) >> case _ ⇒ ctx.push(ByteString(s"$elem\n")) >> } >> } >> } >> >> val repl = Flow[ByteString] >> .transform(() => RecipeParseLines.parseLines("\n", maximumLineBytes = 256)) >> .map(text => println("Server: " + text)) >> .map(_ => readLine("> ")) >> .transform(() ⇒ replParser) >> >> connection.handleWith(repl) >> >> Here are the things that I find confusing: >> The repl value is defined by invoking Flow[ByteString]. Now, I know the API >> well enough to know that Flow requires two type parameters: Flow[+In,-Out] >> <http://doc.akka.io/api/akka-stream-and-http-experimental/1.0-M3/?_ga=1.17864249.1697328887.1413587984#akka.stream.scaladsl.Flow$>. >> This is confusing because the Flow companion object's apply method takes >> only one type argument which it expands by duplicating. So Flow[ByteString] >> actually instantiates a Flow[ByteString,ByteString]. I only note this >> because it took some digging around in the API before I understood how this >> worked and while it is handy, it is also not straight forward. >> The documentation implies that a Flow is uni-directional >> <http://doc.akka.io/docs/akka-stream-and-http-experimental/1.0-M3/scala/stream-flows-and-basics.html#Defining_and_running_streams>. >> It says a Flow "connects its up- and downstreams by transforming the data >> elements flowing through it." That, to me, says "unidirectional". The use of >> a Flow[ByteString,ByteString] for the repl value indicates to me that a >> uni-directional "transformation" from ByteString to ByteString is occurring >> and yet this code implies that it is doing both reading and writing to the >> socket (i.e. it is bi-directional). How can that be? >> I see repl as a Flow that does this: takes a ByteString as input, chunks it >> into \n terminated lines up to 256 bytes, prints those lines out prefixed by >> "Server: " and then discards that input and replaces it with a line read >> from the console which is then output with a newline appended unless the >> input was "q" in which case it is replaced by "BYE\n" and a termination >> signal. Okay, that's all great and it is all unidirectional writing data to >> the socket. So, now the questions: >> Where is the reading from the server to get the original line(s) as input to >> the flow? I.e. where is the Source[ByteString]? >> Assuming that connection.handleWith(repl)does some magic to set up the >> Source, how does the conversation get started? Is it assumed the server will >> send some data upon connection? The echo server example >> <http://doc.akka.io/docs/akka-stream-and-http-experimental/1.0-M3/scala/stream-io.html#Accepting_connections__Echo_Server> >> seems to have the same issue! >> FYI: OutgoingConnection.handleWith's documentation >> <http://doc.akka.io/api/akka-stream-and-http-experimental/1.0-M3/?_ga=1.17864249.1697328887.1413587984#akka.stream.scaladsl.StreamTcp$$OutgoingConnection> >> says this: >> "Handles the connection using the given flow. This method can be called >> several times, every call will materialize the given flow exactly once >> thereby triggering a new connection attempt to the remoteAddress. If the >> connection cannot be established the materialized stream will immediately be >> terminated with a akka.stream.StreamTcpException >> <http://doc.akka.io/api/akka-stream-and-http-experimental/1.0-M3/akka/stream/StreamTcpException.html>." >> It would be nice if there was a little more description around "Handles the >> connection". Handles it how? Does it set up a Source and Sink? Are >> asynchronous bi-directional reading from the socket and writing to the >> socket implied? >> Am I to infer from all this that the StreamTcp.outgoingConnection creates a >> Source[ByteString] from the socket's input and a Sink[ByteString] for the >> socket's output and that the flow provided to handleWith is run between >> these two? In other words, the OutgoingConnection can really only transform >> its input to its output? If so, then: >> How is that generally applicable? >> This approach works fine for an echo client, but clearly there are protocols >> where the input and output can and should be processed independently, aren't >> there? >> How would one do what Mongo needs and have an asynch flow of requests that >> is independent of an asynch flow of responses? >> I noticed the Add Bi-directional Flow >> <https://github.com/akka/akka/issues/16416> issue that is slated for >> inclusion in 1.0-M4. Is this intended for solving this issue where two >> related flows are paired to do bi-directional input/output? >> Am I just trying to implement my mongo driver before the required features >> are ready? >> The documentation says, about this code: >> >> A resilient REPL client would be more sophisticated than this, for example >> it should split out the input reading into a separate mapAsync step and have >> a way to let the server write more data than one ByteString chunk at any >> given time, these improvements however are left as exercise for the reader. >> >> I would like a "resilient client" and I think leaving this part as an >> "exercise for the reader" is asking a bit much from the audience. We need an >> example of how to do this as it is likely the typical case not the exception >> (nobody needs another echo server/client). I suspect that the answer to my >> confusion lies in the information intended but not stated by this sentence >> from the documentation. >> >> Specifically, I do not comprehend how mapAsync (or mapAsyncUnordered) help >> to split out the input reading because it is NOT obvious to me where this >> "input reading" is being done! If I used mapAsynch to obtain the request >> data from my driver's clients, it seems very obtuse to be setting up >> numerous Futures as opposed to just allowing them to give me a >> Source[Request] from which their requests are read and processed. >> >> Any help you can provide to prevent me from drowning in these waters would >> be much appreciated!! >> >> Thanks, >> >> Reid. >> >> -- >> >>>>>>>>>> Read the docs: http://akka.io/docs/ <http://akka.io/docs/> >> >>>>>>>>>> Check the FAQ: >> >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html >> >>>>>>>>>> <http://doc.akka.io/docs/akka/current/additional/faq.html> >> >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user >> >>>>>>>>>> <https://groups.google.com/group/akka-user> >> --- >> You received this message because you are subscribed to a topic in the >> Google Groups "Akka User List" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/akka-user/BQhnmseCyN0/unsubscribe >> <https://groups.google.com/d/topic/akka-user/BQhnmseCyN0/unsubscribe>. >> To unsubscribe from this group and all its topics, send an email to >> [email protected] >> <mailto:[email protected]>. >> To post to this group, send email to [email protected] >> <mailto:[email protected]>. >> Visit this group at http://groups.google.com/group/akka-user >> <http://groups.google.com/group/akka-user>. >> For more options, visit https://groups.google.com/d/optout >> <https://groups.google.com/d/optout>. > > > -- > >>>>>>>>>> Read the docs: http://akka.io/docs/ <http://akka.io/docs/> > >>>>>>>>>> Check the FAQ: > >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html > >>>>>>>>>> <http://doc.akka.io/docs/akka/current/additional/faq.html> > >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user > >>>>>>>>>> <https://groups.google.com/group/akka-user> > --- > You received this message because you are subscribed to the Google Groups > "Akka User List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > To post to this group, send email to [email protected] > <mailto:[email protected]>. > Visit this group at http://groups.google.com/group/akka-user > <http://groups.google.com/group/akka-user>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. > > > -- > >>>>>>>>>> Read the docs: http://akka.io/docs/ <http://akka.io/docs/> > >>>>>>>>>> Check the FAQ: > >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html > >>>>>>>>>> <http://doc.akka.io/docs/akka/current/additional/faq.html> > >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user > >>>>>>>>>> <https://groups.google.com/group/akka-user> > --- > You received this message because you are subscribed to a topic in the Google > Groups "Akka User List" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/akka-user/BQhnmseCyN0/unsubscribe > <https://groups.google.com/d/topic/akka-user/BQhnmseCyN0/unsubscribe>. > To unsubscribe from this group and all its topics, send an email to > [email protected] > <mailto:[email protected]>. > To post to this group, send email to [email protected] > <mailto:[email protected]>. > Visit this group at http://groups.google.com/group/akka-user > <http://groups.google.com/group/akka-user>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- >>>>>>>>>> Read the docs: http://akka.io/docs/ >>>>>>>>>> Check the FAQ: >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user --- You received this message because you are subscribed to the Google Groups "Akka User List" group. 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