I am brand new to Akka (Java lib, v2.3.9). I am trying to follow the supervisor hierarchy best practices, but since this is my first Akka app, am hitting a mental barrier somewhere.
In my first ever Akka app (really a library intended for reuse across multiple apps), input from the outside world manifests itself as a Process message that is passed to an actor. Developers using my app will provide a text-based config file that, ultimately, configures which actors get sent Process instances, and which do not. In other words, say these are my actor classes:
// Groovy pseudo-code
class Process {
private final Input input
Process(Input input) {
super()
this.input = deepClone(input)
}
Input getInput() {
deepClone(this.input)
}
}
class StormTrooper extends UntypedActor {
@Override
void onReceive(Object message) {
if(message instanceof Process) {
// Process the message like a Storm Trooper would.
}
}
}
class DarthVader extends UntypedActor {
@Override
void onReceive(Object message) {
if(message instanceof Process) {
// Process the message like Darth Vader would.
}
}
}
class Emperor extends UntypedActor {
@Override
void onReceive(Object message) {
if(message instanceof Process) {
// Process the message like the Emperor would.
}
}
}
// myapp-config.json -> where the actors are configured, along with other
// app-specific configs
{
"fizzbuzz": "true",
"isYosemite": "false",
"borderColor": "red",
"processors": [
"StormTrooper",
"Emperor"
]
}
As you can see in the config file, only StormTrooper and Emperor were selected to receive Process messages. This ultimately results with zero (0) DarthVader actors being created. It is also my intention that this would result with a Set<ActorRef> being made available to the application that is populated with StormTrooper and Emperor like so:
class SomeApp {
SomeAppConfig config
static void main(String[] args) {
String configFileUrl = args[0] // Nevermind this horrible code
// Pretend here that configFileUrl is a valid path to
// myapp-config.json.
SomeApp app = new SomeApp(configFileUrl)
app.run()
}
SomeApp(String url) {
super()
config = new SomeAppConfig(url)
}
void run() {
// Since the config file only specifies StormTrooper and
// Emperor as viable processors, the set only contains instances of
// these ActorRef types.
Set<ActorRef> processors = config.loadProcessors()
ActorSystem actorSystem = config.getActorSystem()
while(true) {
Input input = scanForInput()
Process process = new Process(input)
// Notify each config-driven processor about the
// new input we've received that they need to process.
processors.each {
it.tell(process, Props.self()) // This isn't correct btw
}
}
}
}
So, as you can (hopefully) see, we have all these actors (in reality, many dozens of UntypedActor impls) that handle Process messages (which, in turn, capture Input from some source). As to which Actors are even alive/online to handle these Process messages are entirely configuration-driven. Finally, every time the app receives an Input, it is injected into a Process message, and that Process message is sent to all configured/living actors.
With this as the given backstory/setup, I am unable to identify what the "actor/supervisor hierarchy" needs to be. It seems like in my use case, all actors are truly equals, with no supervisory structure between them. StormTrooper simply receives a Process message if that type of actor was configured to exist. Same for the other actor subclasses.
Am I completely missing something here? How do I define a supervisory hierarchy (for fault tolerance purposes) if all actors are equal and the hierarchy is intrinsically "flat"/horizontal?
