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Jalal EL-SHAER
@jalchr
@Horusiath I'm thinking about using DData as a state holder for ensuring message delivery. So instead of scattering state in every actor, I have it centralized and cluster-ready. What's your thoughts on this ?
Aaron Stannard
@Aaronontheweb
@jalchr DData is intended for having multiple writable replicas of the same state distributed throughout the cluster
so it's like having N copies of the same actor
but the DData data structures (CRDTs) allow for the state of all N replicas to be merged together into a consistent view of the entity as a whole eventually
but there are tradeoffs in terms of the types of things you can do with your entity's state
for instance, some of the collection data structures may not allow you to delete items from the collection (you have to use a "tombstone" instead to indicate that something has been removed)
and some of the counter data structures are monotonic (don't allow decrements)
if you're working on reliable delivery type problems
I made a YouTube video about this last year, let me see if I can grab it
in general, I recommend not using exactly once if you can avoid it
but if you really need it, that video goes through how to implement it
Aaron Stannard
@Aaronontheweb
and the technique I use there is exactly once without strong ordering
meaning each message is delivered independently from every other message
if you need to do exactly once delivery AND preserve ordering for all messages to each individual recipient
you'd need to ditch using an AtLeastOnceDeliveryActor base class
and implement something custom where each send operation to each recipient is atomic
and any subsequent sends get queued up until the current send is ACKed
and then you can proceed from there
in a happy-path scenario the performance is probably OK
once you actually start dealing with prolonged trouble on the network though then that technique is going to result in back-ups quickly
especially if you end up with a poisoned message
I'm working on some finance applications at the moment; real-time price analysis, trading indicators, and eventually an execution engine
In the spec design I have currently we only use something stronger than at most once delivery inside the execution engine
if I miss some real-time order book events from the exchanges I'm connected to
Aaron Stannard
@Aaronontheweb
¯_(ツ)_/¯
however, if my ability to process those real-time order book events is slow
then my application can't really do its job properly
we will use the stronger delivery guarantee in the execution engine though
because my view of an account balance and the exchange's view of the account balance have to be identical
otherwise the trading algorithms are going to try to execute trades that can't be filled or might otherwise blow away the liquidity (cash-on-hand) parameters the users set... which would be bad
AnchishkinAlex
@AnchishkinAlex
hi! I have a problem: there are two independent projects and I am using Akka Remoting for connection. I am also using Autofac DI.
Two projects. Two actor systems. Two Autofac DI containers.
Actor of the first ActorSystem calls actor of the second ActorSystem which should have constructor injection.
But the first actor system doesn't know (and shouldn't know) about this dependencies.
Is it possible to call this actor after resolving dependency through the second actor system?
Aaron Stannard
@Aaronontheweb
@AnchishkinAlex are you remotely deploying those actors?
if not, then none of this stuff matters - both actors are started independently
and neither needs to know about each other's constructor arguments
they just need to acquire a remote actor reference to each other
which you can via an ActorSelection initially
AnchishkinAlex
@AnchishkinAlex
Ok, we tried to use ActorSelection but it needs hardcode address of actor.
Is it possible to get address of actor from HOCON?
Aaron Stannard
@Aaronontheweb
yep, you can create a custom HOCON field
to store the address a remote node
I do this inside the client-server app from our Akka.NET trainings
let me see if I can grab it...
akkatime{
             sessions{
              handshake-timeout = 4.0s #max time to complete the handshake timeout process
              verify-timeout = 1.0s #max time to authentication an end-user locally on the server
              verify-chat = on
             }
             server = "akka.tcp://akkatime@127.0.0.1:14445"
          }

          akka {
the akkatime section is custom
and I have a akkatime.server property that the client reads from configuration in order to figure out how to contact the server
var actorSystem = ActorSystemRefs.AkkaTimeSystem = ActorSystem.Create("akkatime");
            var serverAddress = ActorSystemRefs.ServerAddress = Address.Parse(actorSystem.Settings.Config.GetString("akkatime.server"));
            var serverStatus =ActorSystemRefs.ServerConnectivityActor =
                actorSystem.ActorOf(
                    Props.Create(() => new ServerConnectivityStatusActor((Address)serverAddress.Clone(), ServerConnectivityStatus.Disconnected)),
                    ActorPaths.ServerConnectivity.Name);
which I then read in here when I'm starting my ActorSystem
and that serverAddress gets passed to an actor that uses an ActorSelection to contact another actor on the server
AnchishkinAlex
@AnchishkinAlex
Thank you. Could you provide link to this example? I'll explain it