Writing applications with Scala actors in practice
actor, scala
Solution
To me, your `ReportTrades` message is mixing two different concepts. One is a Request, the order is a Response. They might be named `GetTradesReport(Date)` and `SendTradesReport(List[Trade])`, for example. Or, maybe, `ReportTradesByDate(Date)` and `GenerateTradesReport(List[Trade])`.
Problem
I've now written a few applications using scala actors and I'm interested in how people have approached or dealt with some of the problems I've encountered. A plethora of Message classes or !? I have an actor which reacts to a user operation and must cause something to happen. Let's say it `react`s to a message `UserRequestsX(id)`. A continuing problem I have is that, because I want to modularize my programs, a single actor on its own is unable to complete the action without involving other actors. For example, suppose I need to use the `id` parameter to retrieve a bunch of values and then these need to be deleted via some other actor. If I were writing a normal Java program, I might do something like: ``` public void reportTrades(Date date) { Set<Trade> trades = persistence.lookup(date); reportService.report(trades); } ``` Which is simple enough. However, using actors this becomes a bit of a pain because I want to avoid using `!?`. One actor reacts to the `ReportTrades(date)` message but it must ask a `PersistenceActor` for the trades and then a `ReportActor` to report them. The only way I've found of doing this is to do: ``` react { case ReportTrades(date) => persistenceActor ! GetTradesAndReport(date) } ``` So that in my `PersistenceActor` I have a react block: ``` react { case GetTradesAndReport(date) => val ts = trades.get(date) //from persietent store reportActor ! ReportTrades(ts) } ``` But now I have 2 problems: - I have to create extra message classes to represent the same request (i.e. "report trades"). In fact I have three in this scenario but I may have many more - it becomes a problem keeping track of these - What should I call the first and third message `ReportTrades`? It's confusing to call them both `ReportTrades` (or if I do, I must put them in separate packages). Essentially there is no such thing as `overloading` a class by `val` type. Is there something I'm missing? Can I avoid this? Should I just give up and use `!?` Do people use some organizational structure to clarify what is going on?