Three quick Scala plugs

I’m still thinking about the dependency-injection and Akka stuff and will have at least one more longish post on the subject, but I’ve recently become distracted by refactoring some of my old JavaScript code into ClojureScript and figuring out Clojure’s new core.async library (ultimately I hope to form all of these distractions into a ring instead of a straight line, at which time I will be the acme of productivity, but that’s another story). But before I get too deep into Clojure land I wanted to plug a few Scala-related things I’ve come across recently.

Firstly, Derek Wyatt’s book Akka Concurrency: Building reliable software in a multi-core world is excellent, covering a great many Akka topics in an enjoyable style. In particular, his chapter on testing seems profoundly relevant to the mock injection topics I’ve been thinking about, but I haven’t quite absorbed it yet. The whole thing is refreshingly up to date, too (which makes sense since it was only published a month or two ago), and it doesn’t pad out its length with a lot of “learn Scala in 30 days” remedial material. Anyways, I recommend it thoroughly.

Secondly, John Sullivan’s long post about the cake pattern is by far the best treatment I’ve seen of it online, and is required reading for anyone interested in dependency injection in Scala. (It’s been up for several months now but somehow I missed it until recently.) John has written a dependency-injection framework, congeal, which makes instant intuitive sense for me as someone coming from a Java / Spring background; unfortunately it depends on some macro stuff which won’t make it into Scala’s mainline, so it isn’t ready for prime time and will need to be rewritten down the road once Scala’s macros reach their next stable state. There’s a video from ScalaDays 2013 describing the framework.

And finally, following up on the subject of the cake pattern, Daniel Spiewak’s keynote from NEScala, “The Bakery from the Black Lagoon,” is an excellent talk which made me think about the cake pattern in a new way (as more of a compiler-enforced module system than as a form of dependency injection). His implementation of the cake pattern is also interestingly different from most examples I’ve seen online – in particular, he mostly eschews self-types, with the exception of needing one for a virtual class.


More on Akka and dependency injection

Just as a quick follow-up to my previous post, I thought I’d note that the official Akka blog has published a post regarding Akka and dependency injection (kind of weirdly expressed as a mini white paper, as though the internet at large had issued an RFP for an actor-based concurrency system with dependency injection).

While, as I mentioned before, I should emphasize that I’m by no means an expert in Akka or actor best practices, I’m not convinced that this document addresses my particular concerns with the intersection of Akka and dependency injection. I’m still thinking about a larger post breaking down the approaches to this topic that I’ve seen online, but this one sort of falls into the “mock stuff outside of Akka” camp.

The document has two main points: firstly, if you have an existing dependency-injected service, you can pass along a factory which knows where to find it to the Props constructor of an actor, and there’s a way to attach a DI application context to an ActorSystem to support this, in what seems like a pretty convenient way. Secondly, if you need to expose an interface from actors to an existing system based on a DI framework, you can include an ActorSystem singleton in your DI object graph, and then expose a sort of regular-object facade over it which finds specific actors and returns either ActorRefs or futures resulting from sending ask messages to them.

That’s all well and good, but it seems more like a way to integrate between Akka and an existing synchronous DI-based system than anything that makes dependency injection useful or usable inside a purely Akka-based system. (In particular, the document’s unfortunate final section seems to be aimed squarely at recalcitrant middle managers who need to be convinced that a move to Akka will not result in a whole bunch of now-legacy code needing to be tossed out.) While I’m not incredibly interested in this topic myself, I thought Akka already had a talking point for this integration problem in the form of “typed actors“.

The bit that I still haven’t seen addressed is that if Akka likes actors to explicitly manage the lifecycles of other actors they supervise, there doesn’t seem to be any room for the inversion of control that is the hallmark of dependency injection frameworks in the first place. To put it in more concrete terms, if I’m running a partial integration test of my simple notification service and I want it to have a real database actor and mock REST web service actors, how can I tell the actor that supervises all the HTTP worker actors to create mock actors instead of live ones?

I have a half-formed idea of how this could work that involves having a sort of service locator / factory actor which is responsible for actor instantiation, but the idea in my head doesn’t particularly jibe with Akka’s supervision hierarchy, which as far as I can tell is coupled very tightly to actor instantiation.

Testing Akka: actors, dependency injection and mocks

I’ve been digging into what the expected way is to test my small Akka system, as described in my previous post on the subject. I think my problem partially arises from being unclear as to the proper mode of dependency injection in Akka. In other words, I don’t know what the proper way is for my Root actor to obtain a reference to its Database and HTTP sub-actors. Does it create them itself? Look them up from a service locator? And what if I need to inject mock actors into the system in some parts in order to test it?

Various bits of Akka documentation suggest different approaches to wiring actors together; for instance, this page in the official docs suggests either passing dependent actors as constructor arguments, creating them in a preStart() method, or passing them to their referring actors in a message and using become() to switch between initialization states. This example from the testkit docs takes the latter approach, but I can’t say I like the result:

class MyDoubleEcho extends Actor {
  var dest1: ActorRef = _
  var dest2: ActorRef = _
  def receive = {
    case (d1: ActorRef, d2: ActorRef) =>
      dest1 = d1
      dest2 = d2
    case x =>
      dest1 ! x
      dest2 ! x
/* ... */

val probe1 = TestProbe()
val probe2 = TestProbe()
val actor = system.actorOf(Props[MyDoubleEcho])
actor ! (probe1.ref, probe2.ref)
actor ! "hello"
probe1.expectMsg(500 millis, "hello")
probe2.expectMsg(500 millis, "hello")

This does seem to work, but it seems to me that it pollutes the actor with a bunch of test-related code that probably doesn’t belong in production (by which I mean the receive pattern which takes the two dest parameters).

I have found an interesting take on this question in this presentation by Roland Kuhn, introducing akka-testkit from Scala Days 2012—the entire presentation is worth watching, but the part I’m interested in starts at around 22:05 or so.  After a not terribly helpful note about how if you have difficulty injecting mocks into your code, then there is probably something wrong with your design (there may be something to that, but it’s not all that helpful to hear when you’re looking for a solution for injection), Mr. Kuhn mentions a third option for users of the (then-new) Akka 2.0: actors can use actor path naming to look up their dependent actors; the test ActorSystem can then supplant the real implementations with mocks at the same locations.

Of course, all of this sort of assumes that you have a way of separating out actor creation and lifecycle control from dependency injection itself. A lot of the other Akka literature I’ve read seems to posit the integrated lifecycle management bits of Akka as a feature, right down to the “Let It Crash” maxim on the public Akka blog, and all of these features seem to be in direct opposition to the inversion of control notions that most dependency injection systems are founded on. In the last part of Mr. Kuhn’s talk above, he suggests breaking up actor models into somewhat discrete trees, which then use service locators or similar things to find one another; this might be something I can look into.

There was also a talk at this year’s Scala Days about integrating Spring and Akka, which might have some merit for this purpose, and I recently ran across this promising post which describes an approach to autowiring actors with Spring and Akka 2.2 (in Java). Overall, though, this doesn’t seem to be a problem with a clear solution.

Adventures in Akka

My current technical interest, mercurial as ever, is in Akka. My present employer is mostly a java shop, but they are open-minded and I have a notion to prototype out a rewrite of a simple system there into Akka and Scala.  The system is probably one of the simpler ones we have, known as the “notification service.” It periodically checks for new rows in a particular database table.  If it finds any, it fires off a JSON-formatted request to a REST web service, the “delivery service”; if it gets a successful response from this service it will mark the message as delivered in the database. There are a few wrinkles related to locking, and there are actually a few different web services involved, but that’s pretty much the basics. Something possessed me to make a diagram of the existing flow: 



The purpose of this system is to deliver notifications to particular users, with the idea being that any subsystem which needs to send a notification to someone can put the right data in the database, where this system will pick up up and hand it off to an existing REST service which winds up doing most of the heavy lifting.  The existing service is implemented in Java and Spring, using Quartz as a cron job to kick off a polling method once every 30 minutes or so (we don’t need this service to run particularly swiftly).

It’s not really hard to see how this would translate into a message-based actor model in Scala.  You’d probably have one root actor coordinating things.  You’d have an actor talking to the database, maybe with a supervisor to restart it as needed, and you’d have another actor to handle the HTTP client calls.  Most likely the client actor would spawn off a new actor per individual row of data, and have each of these worker actors make a single HTTP request. On a success, the worker would send a message back to the database actor to update the database row as “completed”; on a failure the worker might just log an error and die.

A rough sketch of that might look like this (pardon my sub-par OmniGraffle skills):


Note that the single-line arrows here represent the actor supervision hierarchy, not message-passing.  I’m also not positive that the “DB Worker” actor needs to exist, versus just having the “Database” actor do the work, but it simplifies things to do it this way and I suppose there might be more than one of those (more on this later).

I’m been struggling a bit to come up with a good way to represent message passing in a diagram, but I think I’ve got the gist of the design in this one:


Everything is started by Tick messages which are sent to the root object every 5 seconds via Akka’s scheduler interface (this would be more like 15-30 minutes in production).  This causes the root actor to pass a PollDatabase message to the Database actor; the message includes a reference to the HTTP Client actor.  For each notification row the Database actor finds in the database, it sends a Notify message to the HttpClient actor.  This actor composes a MakeRequest message to one of a pool of Worker actors, including the data from the database and a reference to the Database actor.  The Worker performs the HTTP request; if it is successful it sends a RequestSucceeded message to the Database actor, which will ask a DB Worker thread to update the database to mark the relevant row as successfully delivered.  If the Worker gets an error, it sends a RequestFailed message to the HTTP Client actor, which at this point will just log the error and continue on.

I will have more to say about this, but this post is already decently long, so maybe I’ll leave this here so I can refer back to it later. There are a few things I’m struggling with:

  • Despite having read a lot of articles and blog posts on the subject, it’s not obvious to me what the correct way to instantiate and connect these actors is (constructor arguments, preStart() methods, dependency injection, etc).
  • Related to the above, it’s not clear to me how to test this system without mixing up test code and business logic.  In particular I’d like to replace the nodes in yellow above with mock objects and verify that the system still works properly.
  • I would like to have a reasonable interface to Oracle, without needing to include Spring or something in the project. The Typesafe, Inc solution is to use Slick, but I don’t have a burning desire to sell my co-workers on closed-source, commercial software in addition to a new language and framework. 

I’ll have more to say about all this in the days to come.