Oct 23, 2013

Providing with Providers and Bread::Board

So when I started using Dependency Injection the following problem happened, how do I Inject this dependency when the container is not accessible at this point. Ok, that sentence even confused me a little bit, so what do I mean. Let's say I have a Repository for Products that is injected into my controller. Each Product stored has one or more ProductVariants that is part of it's aggregate, which itself has Nested Categories. Loading this entire graph at once would be relatively expensive, so we decide to do some lazy loading via DBI in the classes. One problem, how on earth do we Inject a Database Handle all the way down to Categories. Most of these ways are against DI, but they are solutions to the problem, there are also ways to combine these. Also, your model class having a database handle is probably bad design itself, but I'm not going to get into that. Sadly I've done every one of these


Well at least you aren't hard coding the way to read your config file, or your database driver. You're smart enough to rely on an Interface rather than an Implementation. This is fraught with so many problems. Firstly if your web server (assuming it's a web application) is getting any kind of traffic at all you'll end up creating tons of database connections, you'll also be reading that config file every time (ok I forget if Config::Merge caches to memory, it might, but often when I see people design this way, they are basically slurping the file every time). Someday 5 years from now, someone is going to hate you because now they need to support replicants... and the config needs to support more connection strings, which means modifying every place you've done this. Also, you've completely lost the ability to inject your dependencies for whatever reason you may want to.


Ok, this is a little bit better than before, at least now you have Inverted your dependencies, you could provide the config or the database handle to the class. You've also put the code in a centralized place so it's easy to change when you need to. You're still reading the file fairly often, though perhaps less because it now depends on how long Product variant is alive. So what happens if your connection is lost? We still have a connection for each class, a connection that may now be held much longer. Why does Product Variant need access to the config? this is a violation of the Law of Demeter.

Naive Service Locator

We need to get rid of knowledge of the config. We can do this by using a Service Locator, which is simply a well known service to retrieve other services, usually a global singleton. In our example we're at least smart enough to allow ourselves to change the class out via injection for testing. We no longer have tons of connections or config reads. However, we now have a new problems, what happens when our Application Server forks a process and we lose the database connection? What about when our locator gets more complex, like nested containers, that could change or access, specifically with replication. Also our class is now directly dependent on Bread::Board, and its interface. At least we've stopped caring how our database handle is built. Our locator is a global singleton, and we can't change our Container class for testing.

Robust Service Locator

Ok, so this is much better we can now configure which locator instance we use at runtime. We have removed the dependency on the Bread::board interface. There is no longer a problem with database connections being dropped. However, our container is still a global singleton, and our class still knows about it, which again, law of Demeter.

Dependency Injection and Pass it down

For now I've been basically ignoring other classes because with all of these other approaches they aren't really a concern because you would do the same thing in every class, fetch your service. Much of the code is required here anyways, we always would have to do the sql, the transforms the loops. Dependency inversion is the opposite, do not think of how to retrieve the dependency instead have the dependency provided. But this becomes tricky to think of when you're 3 or more levels deep in your hierarchy. One way to do it simply pass the reference. We create a specific problem here, our Repository lifecycle is a singleton so we need to ensure re-connection, thus we must inject the connector which means we are immediately dependent on the DBIx::Connector interface. This doesn't seem that tricky until you add more than one service, which still may not seem that bad, until you have to add one later, and oh my god, now you're modifying several classes.

Dependency Injection with Providers

This next and final sample show's one way of doing this with Providers. A little context on a Provider first, a Provider is simply an object that can be used to retrieve a an instance of an object you need. It's really just a kind of factory, but tends to be specific to dependency injection, in scenarios where you need a new instance of an object each time. It seems that it might also work well for other cases, such as objects with a longer lifespan than a new instance on every request from the injector, but shorter than a permanent singleton. In short a provider should be able to provide you with an instance on request, without requiring to to depend on retrieval.

The code that I'm demonstrating will not work currently practical scenario, meaning one where variant parameters are required. I've opened a bug about resolving the issue. In the mean time, the patch is simple and you could apply it yourself. You could use BUILDARGS to rename an alternate key to the primary hashkey, in your models. You could also just define each model service one at a time instead of looping them, and actually validating their parameters.

You may note that I've removed the config, this was simply so I could build the code out so it works in completion. It maybe advantageous not to put config processing code in the Dependency injector, but rather provide the config to Bread::Board::Declare at the constructor via required services. This way of doing things requires much more code, but is also much more flexible. Every piece of the model, even those hat could not normally be accessed by the injector, can now have it's dependencies injected to it.