Tuesday, December 08, 2009
A New Kind of Service

Why study new and different programming languages? To change your programming mindset. Not sure what I mean by that? Check this out.

Ever done one of these?

   1: public interface IService
   2: {
   3:   DateTime GetDate();
   4:   int CalculateSomethingInteresting(int lhs, int rhs);
   5: }
   7: public class OneServiceImpl : IService
   8: {
   9:   public DateTime GetDate()
  10:   { return DateTime.Now; }
  11:   public int CalculateSomethingInteresting(int lhs, int rhs)
  12:   { return lhs + rhs; }
  13: }
  15: public class AnotherServiceImpl : IService
  16: {
  17:   public DateTime GetDate()
  18:   { return new DateTime(); }
  19:   public int CalculateSomethingInteresting(int lhs, int rhs)
  20:   { return lhs * rhs; }
  21: }
  23: public class ServiceFactory
  24: {
  25:   public static IService GetInstance(string which)
  26:   {
  27:     if (which == "One") return new OneServiceImpl();
  28:     else if (which == "Another") return new AnotherServiceImpl();
  29:     else throw new ArgumentException();
  30:   }
  31: }
  33: public class App
  34: {
  35:   public static void Main(string[] args)
  36:   {
  37:     foreach (string s in args)
  38:     {
  39:       IService serv = ServiceFactory.GetInstance(s);
  40:       Console.WriteLine("serv calc = {0}", serv.CalculateSomethingInteresting(3, 3));
  41:     }
  42:   }
  43: }

So has my client this week. In fact, it's fair to say that they're infatuated with them—they've got services all over the place, including at their communication layer, where they use configuration files to decide which of the two service implementations to use, either a "native" .NET implementation or the "real" Web services implementation that they're supposed to be using. (They end up going back to the native implementation because sometimes—which is to say, apparently a lot of times—the Web services implementation is broken in some fundamental way. Go figure.)

The problem is, very bluntly, that the interfaces they're defining (the IService definition above) are ever-so-slightly different from the communications-based proxy interfaces that they use to communicate outside of this process, so some poor schmuck ends up having to write the service implementation (OneServiceImpl) that simply takes the parameters passed in, translates them into a call through the communications-based interface, then takes the response and hands it back. Tedious, mind-numbing coding, particularly painful when there are dozens of interfaces with (in some cases) hundreds of methods per interface. Ouch.

There had to be a better way.

Based on some of the work/research/play I've been doing with both dynamic and functional programming languages, it occurred to me that what they really wanted was some kind of "forwarding" or "delegating" behavior that certain languages have baked in as a feature. In those languages, it's possible to nominate a "delegate" object to which method calls are automatically forwarded if no such method is implemented on this object; in this particular case, what I'd do to replace all of the above is simply create an IService object instance that has either a OneServiceImpl or a AnotherServiceImpl instance (depending on the value in the configuration file) set up as the "delegate" object. That way the method calls remain statically type-checked, but none of this service interface/service implementation/service factory nonsense has to be created just to switch between the two.

(By the way, all of this pain goes away completely in a language that supports deferred checking of signatures until runtime. In other words, if the client had been programming in IronPython or IronRuby or even Visual Basic, we could get away with not having to do any of the above, and just use Reflection to access the appropriate method on whichever of the two service implementations they want to use at the time. Fan would let us do it if we used "->" instead of "." to invoke the method; Cobra would switch between the two automatically; and so on.)

Now, this is C# 2.0 that they're using, and they're pretty entrenched on that point, so I can't simply suggest that they use a new language, but if we take the basic idea and adapt it to C#, we can get pretty much the same behavior without having to force the poor schmuck on the bottom of the totem pole to write all those service implementations by hand.

We start by transforming the IService interface into an IService "interface" (meaning it's not really an interface anymore, but it'll sure look like one to anybody who's not paying attention):

   1: public class IService
   2: {
   3:   public Func0<DateTime> GetDate;
   4:   public Func2<int, int, int> CalculateSomethingInteresting;
   5: }

IService is now a class with fields (not properties, though I suppose if you really wanted them to be properties you could make them such, not that I see much value to doing so), where each field corresponds in name to the method of the interface it wants to replace, and the type is a delegate type parameterized to match the return type and parameter types of that same method of the original interface. Func0 and Func2 are delegate types I had to create, since nothing like them existed until C# 3.0; their definitions are pretty simple:

   1: public delegate R Func0<R>();
   2: public delegate R Func1<R, P1>(P1 p1);
   3: public delegate R Func2<R, P1, P2>(P1 p1, P2 p2);

Now, assuming we have the implementation classes from before, we have two choices; one is to write a by-hand factory that fills out the fields to point to the appropriate method on the implementation class, like so:

   1: if (which == "One")
   2: {
   3:   servInstance.GetDate = delegate() { return DateTime.Now; };
   4:   servInstance.CalculateSomethingInteresting = delegate (int lhs, int rhs) { return lhs + rhs; };
   5: }
   6: else if (which == "Another")
   7: {
   8:   servInstance.GetDate = delegate() { return new DateTime(); };
   9:   servInstance.CalculateSomethingInteresting = delegate (int lhs, int rhs) { return lhs * rhs; };
  10: }
  11: else
  12:   throw new ArgumentException();

But, quite frankly, this defeats the point—the point was to avoid writing all this stuff by hand, not simply repeat it in a different form. So instead, we leverage Reflection, which depends on the basic assumption that the field name in the IService "interface" matches the method name on the implementation class we wish to invoke. Assuming that holds (which it does, in my client's case, anyway), we can reflect on the IService field, find the matching method name in the implementation, then construct a delegate instance around that method and assign the delegate instance to the field. Once complete, we hand back the completed service instance, and the client literally doesn't know that anything's different:

   1: public class ServiceFactory
   2: {
   3:   public static IService GetInstance(string which)
   4:   {
   5:     IService servInstance = new IService();
   7:     Type targetType = Assembly.GetExecutingAssembly().GetType(which + "ServiceImpl");
   9:     foreach (FieldInfo fi in servInstance.GetType().GetFields())
  10:     {
  11:       MethodInfo targetMethod = targetType.GetMethod(fi.Name);
  12:       //Console.WriteLine("Wiring up {0} against {1} with {2}", fi.Name, targetType, targetMethod);
  13:       Delegate d = Delegate.CreateDelegate(fi.FieldType, null, targetMethod);
  14:       //Console.WriteLine(d);
  15:       fi.SetValue(servInstance, d);
  16:     }
  18:     return servInstance;
  19:   }
  20: }

Remember, the client code still looks the same...

   1: public class App
   2: {
   3:   public static void Main(string[] args)
   4:   {
   5:     foreach (string s in args)
   6:     {
   7:       IService serv = ServiceFactory.GetInstance(s);
   8:       Console.WriteLine("serv calc = {0}", serv.CalculateSomethingInteresting(3, 3));
   9:     }
  10:   }
  11: }

... because what the client doesn't know is that he's accessing a field, then invoking the delegate that's being returned from that field dereference.

What this permits, aside from the automated wiring up of the IService "interface", is a greater degree of flexibility—rather than having to choose which implementation to use on an interface-by-interface basis, we can now configure to use different implementations on a method-by-method basis. But considering how many interfaces and implementations my client was looking at having to write by hand, the real win is in the automated ServiceFactory wiring.

By the way, the only reason we can get away with this sleight-of-hand is because delegates are deliberately designed to act like method calls; no explicit .Invoke() call is required, it's implied with the () after the delegate instance's name. If Java7 closures and/or method handles end up with support for that kind of syntax, then we can do the same thing in Java7 (more or less).

Make sense?

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Tuesday, December 08, 2009 11:25:18 AM (Pacific Standard Time, UTC-08:00)
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