Careful Placement You might say that I’ve had issues with Dependency Injection in the past. Well, with all the things I’ve learned not to do, I thought I’d go into a case where I’m considering dependency injection because it looks like it might be a good fit.

Project Background

Like many (most?) developers, I’ve been involved for many years in a particular vertical market. Like most, my involvement in that vertical market includes acquiring a substantial amount of very specific domain knowledge. I’m sometimes reluctant to admit it, but I spent a good decade and more of my career writing software designed for Multi-level Marketing companies. Specifically, I wrote programs that calculate the commission payments to their distributors. If you are familiar with that domain, my condolences. If you are considering entering that domain, talk to me (it’s not all bad).

Now, many accounting software packages deal with paying commissions, but compared to even the simplest MLM, their capabilities are pretty primitive. I won’t go into the full complexity of walking a tree with money on the line, but here’s what’s important for this post:

  • The data you have to process is well contained, finite, and known in advance.
  • Data concurrency tends not to be an issue (you read distributor information at the beginning and you write payout and promotion information as you go or at the end).
  • Every company has their own, unique twist for qualifications, promotions, and payouts—indeed, commission plan differentiation is a key business differentiation.
  • The only thing absolutely common to every commission plan is that you walk a tree of distributor nodes, processing orders at those nodes, and calculating payouts and/or promotions on them.

Most programmers working in that domain spend time designing their dream "bonus engine". One that can encapsulate the common elements so that their programming only has to deal with the payout and promotion logic. This turns out to be harder than you’d think to do right (if only due to the temptation to put more in the engine than really belongs there).

My Bonus Engine

Since I’m still getting occasional contract work programming commission payouts (and because I’ve seen a bonus engine or two and know what works and what doesn’t), I went and created an OSS bonus engine. Those of you dying to critique my code-fu to bring me up for well-deserved mockery, here’s your chance. So far I’m the only dev on the project, but that’s only because I don’t know anybody else masochistic enough to join me (hey, if you’re that nuts, let me know).

Right now, this engine is still pretty raw, but it works. Mostly. There are INode and IVolume interfaces for distributors and orders respectively. There’s a NodeBase object that implements INode with nifty events already implemented for easy inheritance and feedback and there’s a BonusManager object used to kick off a bonus run.

The tricky bit for this post is the data access. You see, when programming commission payouts, I seriously don’t care where the data comes from or where it is going. My clients all have their own tricky little data schemas and I’d just as soon not deal with the ins and outs of their systems. Abstracting that part out is beneficial all round.

In my initial design, I figured that something like that is a classic call for an Asp.Net-like provider model. This works well because the client can implement a provider (or have me create one for him) that we can drop in and we’re golden. That means that my eventual Distributor objects can be ignorant of all data implementation details and make a call to MLMBonus.DataProvider.BonusData.Promote() to notify of a promotion (for example).

Making the method references static seems like a good call because the distributor object has no other contextual information available. The engine is walking a tree and calling interface methods so the distributor and volume objects don’t really know much more about the outside world than their relationships to each other.

Because this is my first ever implementation of a "Provider Framework", I went digging for some sense of what kinds of things to look out for. While I initially went with an interface structure, I eventually decided to use Joel Ross' well written guideline for creating your own .Net providers.

There’s Always One

Unfortunately, I got some push-back when talking with my favorite client. You see, he’s chafing at the static nature of my provider model. He wants to make a grab for the holy grail of bonus commissions—executing mini-commission runs on-demand (most likely for partial distributor trees)—and the static provider just isn’t cutting it for him. mini-commission runs allow you to provide some interesting features to your distributors and that’s never a bad thing.

To support his functionality, he would need to be able to alter the data provider "on the fly" and/or be able to initiate multiple runs in the same application space without allowing them to step on each other. There’s just no way that static providers are going to work for that functionality. Naturally, I’ve been thinking about how best to work out meeting his need (mainly because I can see this feature will be something others will like but also because he’s my favorite client). Fortunately, my engine isn’t widely disseminated so it’s a good time for wholesale architectural alterations if we want to make them.

I’ve considered a number of ways to tackle the problem, playing with things like anonymous methods and context managers. They all turned ugly fast so it’s time that I face the fact that this functionality is pretty much tailor made for Dependency Injection.

Where To Inject

So where’s the best place to inject this little dependency? There are four options as far as I can tell.

  • constructor injection
  • property (or setter method) injection
  • method parameter injection (since the number of methods affected will be very small).
  • framework injection (Spring.Net or Castle Windsor for example)

Constructor and property injection are both object level references carried by the dependant object and have little to distinguish them in effect. Conceptually, I’ve heard that constructor injection is best for dependencies that are crucial to an object—something the object simply cannot do without. I think I agree with that guideline but I also like the convenience of being able to do grunt-work things in constructors, so if I were in charge of the objects, I’d include both a base constructor and one that includes the dependency as well as a setter method of some kind.

Method parameter injection is tempting, though. With method parameter injection, a client can set the dependency on BonusManager and rely on BonusManager to send the dependency to the Pay and Promote methods on INode. That way the reference to the data provider is short lived inside INode and IVolume objects. Thinking it over, though, I’m not sure why I’m so worried about carrying a bunch of references when garbage collection isn’t going to happen mid-run anyway. Yeah, there’s some overhead involved in tracking the reference on each INode object, but it’s negligible (are pointers in a 64 bit environment 64 bits long? They’d pretty much have to be right? I’ve never bothered to find that out—it’s never been significant before).

Framework injection simply isn’t going to be helpful in this case, I don’t think. For one, I’m not familiar enough with any DI Frameworks to make it worthwhile. For another, I don’t want to introduce a framework dependency on the project. And finally, I don’t want to commit future clients to a DI Framework that may be an issue in their environment.

Injecting

So my next task for the engine will be to muck around with the EngineInterfaces library. I’m thinking I’ll leave the provider object as-is (the BonusDataProvider abstract class) and simply add a property to INode to inject that sucker. In addition, I’ll update NodeBase to have an appropriate property and a new constructor that allows you to set the provider property on object creation.

Since I have the provider stuff already in place and no changes are needed to make this work, I’ll likely leave it alone. That way I won’t break anything currently working.

The only thing left is to decide if I want the IVolume dependency injected via a field or a method parameter on Pay. I’m pretty sure I don’t need the fine-grained control of allowing different orders for a single node to pay down a different path, but do I really want to commit to that? My arguments above against method parameter injection seem pretty solid for the IVolume object as well, but I haven’t completely convinced myself that they apply as strongly.