Scala, Java, Unix, MacOS tutorials (page 67)

Update, May 3, 2020: I added the single-page HTML version of the book to the Downloads section below.

Back in the summer of 2019, a book publisher approached me and asked if they could buy the rights to an “introduction to Scala” book I had independently published and titled, “Hello, Scala.” The book is about 225 pages long and provides a brief-but-thorough introduction to the Scala language.

I came extremely close to signing with that publisher, but as fate would have it, right at that time I saw some people on contributors.scala-lang.org say something to the effect, “If only we had an online book ...”

Scala/Akka problem: In some situations you can’t pass an ActorRef to another actor, so you want to see how to “look up” an Akka Typed actor so you can send a message to it.

Solution

There are at least two ways to find other Akka Typed actors, and both involve using the Akka Receptionist. The example shown here in the solution shows how to find an actor asynchronously, and the example shown in the Discussion shows how to use the ask method to find another actor synchronously.

In both examples I’ll use the idea of creating an actor system that works like Amazon Echo devices. The basic idea is that a device like this has ears to listen to you, a mouth to speak to you, and a brain to do all of its work. In these examples the Brain actor will need to discover the Mouth actor.

Scala/Akka Typed problem: You want to see the proper way to shut down Akka actors.

Solution

There are two main ways to shut down Akka Typed actors:

• Send them a “graceful shutdown” message, and let them shut themselves down by calling Behaviors.stopped

• As a parent, stop a child actor with actorContext.stop(actorRef)

Scala/Akka problem: You know that an actor that “blocks” is bad for your system, so you want to create Akka Typed actors that don’t block.

Solution

With Akka Typed, the mantra is always, “Delegate, delegate, delegate.” It’s important that high-level actors delegate work to lower-level actors, so the high-level actors can be free to receive new messages and respond to them immediately.

This is an old article I wrote back in 2009, but hopefully it’s still relevant in 2020:

Today I’m going to ask you to put yourself in the position of someone that needs to hire someone to do whatever it is you do. Completely imagine that you are this person. For instance, I design software systems, and I usually sell my services to other IT people, so I’ll put myself in the shoes of an IT Manager, or a Project Manager who needs an architect like myself.

Putting myself in their shoes, I have a good feel for all the technical skills I think an architect needs. After a lot of legwork, I finally finish interviews with two different people, and their technical skill sets are so good and so close I can’t tell the difference between them — they’re technical twins!