Akka is a very rich ecosystem, far too rich to do it justice in a single chapter. There are some important parts of the toolkit that you will need, but we have not covered them here. We will give brief descriptions, but you can refer to the Akka documentation for more details:
- The ask operator,
?, offers an alternative to the tell operator,!, that we have used to send messages to actors. Unlike "tell", which just fires a message to an actor, the ask operator expects a response. This is useful when we need to ask actors questions rather than just telling them what to do. The ask pattern is documented at http://doc.akka.io/docs/akka/snapshot/scala/actors.html#Ask__Send-And-Receive-Future. - Deathwatch allows actors to watch another actor and receive a message when it dies. This is useful for actors that might depend on another actor but not be its direct supervisor. This is documented at http://doc.akka.io/docs/akka/snapshot/scala/actors.html#Lifecycle_Monitoring_aka_DeathWatch.
- In our crawler, we passed references to actors explicitly through the constructor. We can also look up actors using the actor hierarchy with a syntax reminiscent of files in a filesystem at http://doc.akka.io/docs/akka/snapshot/scala/actors.html#Identifying_Actors_via_Actor_Selection.
- We briefly explored how to implement stateful actors with different receive methods and using
context.becometo switch between them. Akka offers a more powerful alternative, based on finite state machines, to encode a more complex set of states and transitions: http://doc.akka.io/docs/akka/snapshot/scala/fsm.html. - We have not discussed distributing actor systems across several nodes in this chapter. The message passing architecture works well with distributed setups: http://doc.akka.io/docs/akka/2.4.0/common/cluster.html.
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