October 28, 2011

Scala: 3 months in

So, I’ve been using scala in production for a few months now. Is it everything I expected it to be?

First, a little bit of background. The project I’m working on is a medium-sized app, with a Scala back-end and a WPF/C# fat client. The interface layer is a set of classes autogenerated from a rubyish DSL, communicating via a combination of client-initiated JSON-RPC over HTTP, and JSON-over-AMQP messages from server to client. It’s split into about half a dozen projects, each with it’s own team. (I should point out that I have little involvement in the architecture at this level. It’s a given; I just implement on top of it.)

The server side is probably a few thousand classes in all, but each team only builds 500 or so classes in its own project. It integrates with a bunch of COTS systems and other in-house legacy apps.

In functionality terms, it’s really fairly standard enterprisey stuff. Nothing is that complicated, but there’s a lot of stuff, and a lot of corner-cases and “oh yeah, we have to do it like that because they work differently in Tokyo” logic.

So, Scala. Language: Generally lovely. Lends itself to elegant solutions that are somehow satisfying to write in a way which java just isn’t. Everything else? Not so much. Here are my top-5 peeves:

  • The compiler is so slow. For a long time, the Fast(er) Scala Compiler simply wouldn’t compile our sources. It threw exceptions (no, it didn’t flag errors, it just died) on perfectly legitimate bits of code. This meant using the slow compiler, which meant that if you changed one line of code and wanted to re-run a test, you had to wait while your 8-core i7 churned for 2-3 minutes rebuilding the whole world. In a long-past life, I cut my teeth writing mainframe cobol with batch compilation. This was like that, all over again. TDD? Not so much. Recently the FSC has improved to the point where it can actually compile code, which is nice, but sometimes it gets confused and just forgets to recompile a class, which is hella confusing when you’re trying to work out why your fix doesn’t seem to be having any effect.
    I would gladly exchange a month of writing WPF automation code (that’s a big sacrifice, for those who haven’t endured it) for the ability to have eclipse’s hot-code-replace, unreliable as it is, working with scala.
  • The IDEs blow. Eclipse + ScalaIDE just plain doesn’t work – apart from choking on our maven config, even if you manually build up the project dependencies, it will randomly refuse to compile classes, highlight errors that don’t exist (highlighting errors on lines that contain no code is a personal favourite), and ignore actual errors. IDEA + Scala Plugin is better – this is now my day-to-day IDE, despite starting out as a dyed-in-the-wool Eclipse fanboy, but it’s slow and clunky, requires vast amounts of heap, and the code completion varies from arbitrary to capricious.
  • Debugging is a pain. Oh yes,
    data.head.zipWithIndex.map((t) => data.map((row: List[Int]) => row(t._2)).reverse)    

feels great when you write it, but just try stepping through that sucker to find out what’s not working. You end up splitting it up into so many temporary variables and function calls that you might as well have just written a big nested for-loop with couple of if-elses in it. Sure, you can take them all out again when you’ve finished, but are you really helping the next guy who’s going to have to maintain that line?

  • The java interop is a mixed blessing. Yes, it’s nice that you have access to all that great java code that’s out there, but the impedance mismatch is always lurking around causing trouble. Of recent note:
    – Scala private scope is not sufficiently similar to java private scope for hibernate field-level access to work reliably
    – Velocity can’t iterate scala collections, nor access scala maps.
    – Mokito can’t mock a method signature with default parameter values.

The list goes on. None of these are blockers in any sense – once you’ve learned about them, they’re all pretty easy to work around or avoid. But each one represents an hour or so’s worth of “WTF”-ing and hair-pulling, and these are just the ones that are fresh enough to remember. Scala-Java interop is a constant stream of these little papercuts.

  • Trivial parallelisation is of minimal value to me. Sure, there are loads of cases where being able to spread a workload out over multiple threads is useful. But, like a great many enterprise apps, this one is largely not CPU-bound, and where it is, it needs to operate on a bunch of persistent objects within the context of a JTA transaction. That means staying on one thread. Additionally, since we’re running on VMWare, we don’t have large numbers of cores to spread onto, so parallelisation wouldn’t buy us more than a factor of 2 or 3 in the best case. In a related vein, immutable classes are all well and good, but JPA is all about mutable entities. There have been a few bits of the design where baked-in immutability has led to a cleaner model, but they’re surprisingly few and far between.

Not that long after I started work on this project, there was a video doing the rounds, showing how Notch, the creator of Minecraft, used Eclipse to create a pretty functional Wolfenstein clone in java in a 48-hour hackathon. Whilst that kind of sustained awesomeness is out of my reach, it illustrates the degree to which the incredibly sophisticated tooling that’s available for Java can compensate for the language’s shortcomings. If you haven’t watched it, I recommend skimming though to get a sense of what it means to have a toolset that’s completely reliable, predictable, and capable of just getting out of the way and letting you create.

If I were starting this project again, I’d swap the elegance and power of the scala language for the flow of java with a good IDE. Even if it did mean having to put up with more FooManagerStrategyFactory classes


- One comment Not publicly viewable

  1. Nick Howes

    You can also add to that list

    • It’s impossible to create runtime-visible annotations

    Which is a pain when you want a custom constraint for Hibernate Validator. Fortunately annotations are by their nature pretty simple so it’s no big bother to add a Java class. Annoying though. I’m sticking with Scala for now – after an initial bunch of roadbumps, I seem to be mostly over them and am now grappling with Spring 3 roadbumps instead.

    On the topic of nice languages for the JVM, have you tried Mirah at all? Made by the JRuby guy and the language is extremely Ruby-like, but classes and things map directly to their JVM bytecode counterparts, unlike with JRuby. And it’s statically typed to a similar degree as Scala. I think with Java 7’s JVM it also supports dynamic typing. Doesn’t have its own runtime library, though, so as soon as you create a list of things you bump up against the regular old ArrayList API. Something to keep an eye on anyway.

    31 Oct 2011, 10:35


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