Akka - Unable to send Discriminated Unions as messages in F# - akka.net

Question

Akka - Discriminated Unions as messages in F#
I am unable to use discriminated unions as messages to akka actors. If anyone can point me at an example that does this, it would be much appreciated.
My own attempt at this is at git#github.com:Tweega/AkkaMessageIssue.git. (snippets below). It is a cutdown version of a sample found at https://github.com/rikace/AkkaActorModel.git (Chat project)
Problem
The DU message never finds its target on the server actor, but is sent to the deadletter box. If I send Objects, instead, they do arrive.
If I send a DU, but set my server actor to listen for generic Objects, the message does arrive, but its type is
seq [seq [seq []]
and I can't get at underlying DU.
The DU I am trying to send as message
type PrinterJob =
| PrintThis of string
| Teardown
The client code
let system = System.create "MyClient" config
let chatClientActor =
spawn system "ChatClient" <| fun mailbox ->
let server = mailbox.Context.ActorSelection("akka.tcp://MyServer#localhost:8081/user/ChatServer")
let rec loop nick = actor {
let! (msg:PrinterJob) = mailbox.Receive()
server.Tell(msg)
return! loop nick
}
loop ""
while true do
let input = Console.ReadLine()
chatClientActor.Tell(PrintThis(input))
Messages are forwarded to the client from console input
while true do
let input = Console.ReadLine()
chatClientActor.Tell(PrintThis(input))
The server code
let system = System.create "MyServer" config
let chatServerActor =
spawn system "ChatServer" <| fun (mailbox:Actor<_>) ->
let rec loop (clients:Akka.Actor.IActorRef list) = actor {
let! (msg:PrinterJob) = mailbox.Receive()
printfn "Received %A" msg //Received seq [seq [seq []]; seq [seq [seq []]]] ???
match msg with
| PrintThis str ->
Console.WriteLine("Printing: {0} Do we get this?", str)
return! loop clients
| Teardown ->
Console.WriteLine("Tearing down now")
return! loop clients
}
loop []
Dependencies
(I am not using paket here) - PM commands below:
Install-Package Akka -Version 1.4.23
Install-Package Akka.Remote -Version 1.4.23
Install-Package Akka.FSharp -Version 1.4.23
I am hosting the application in net5.0
Constructor argument names - oddity?
When passing in class instances as objects, akka seems to be sensitive to the name of constructor parameters. The message gets handled, but the data is not copied across from client to server. If you have a property called Username, the constructor parameter cannot be, for example, uName, otherwise its value is null when it reaches the server. Code for this is in branch params.
type DoesWork(montelimar: string) =
member x.Montelimar = montelimar
type DoesNotWork(montelimaro: string) =
member x.Montelimar = montelimaro

Answer 1

I opened an issue in the Akka.NET repository: https://github.com/akkadotnet/akka.net/issues/5194
And added a detailed reproduction for this: https://github.com/akkadotnet/akka.net/pull/5196
But it looks like Newtonsoft.Json really can't perform this deserialization without being given a type hint, which Akka.NET's network serialization does not do by default for JSON:
type TestUnion =
| A of string
| B of int * string
type TestUnion2 =
| C of string * TestUnion
| D of int
[<Fact(Skip="JSON.NET really does not support even basic DU serialization")>]
member _.``JSON.NET must serialize DUs`` () =
let du = C("a-11", B(11, "a-12"))
let settings = new JsonSerializerSettings()
settings.Converters.Add(new DiscriminatedUnionConverter())
let serialized = JsonConvert.SerializeObject(du, settings)
let deserialized = JsonConvert.DeserializeObject(serialized, settings)
Assert.Equal(du :> obj, deserialized)
That test will not pass and it doesn't use any of Akka.NET's infrastructure at all - so the default JSON serializer simply won't work for real-world F# use cases.
We can try changing the defaults of our serialization system to include a type hint, but that will take a lot of validation testing (for old Akka.Persistence data serialized without one).
A better solution, which my pull request validates, is to use Hyperion for polymorphic serialization instead - it will be similarly transparent to you but it has much more robust handling for complex types than Newtonsoft.Json and is actually faster: https://getakka.net/articles/networking/serialization.html#how-to-setup-hyperion-as-default-serializer