DI Composition Models: A Primer

To be able to achieve anything useful, your application code makes use of runtime data that comes in many shapes and forms. Providing access to that data can be accomplished in many ways. The way you provide object graphs with runtime data can affect the way you compose them using Dependency Injection. There are two competing models to choose from. This article introduces these two models: the Closure Composition Model and the Ambient Composition Model. It is the first of a five-part series on Dependency Injection composition models.

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Most of your application code uses runtime data in one form or another. Runtime data flows through the system in many forms. Your application will get its data from and send its data to browsers, databases, queues, services, the filesystem, and many other sources.

Data flowing through the system

The data flowing through your application can be categorized in many ways, but for the remainder of this article, I’ll divide it into two groups, as this serves us in the discussion that follows:

Each layer passes runtime data through the public API of the next layer

Note that the two groups are not strictly separated. Consider, for instance, how the user’s role can flow through the system with different use cases:

This means that data can hop from one group to the next and back, depending on the use case in which it participates. To simplify things, however, I will ignore this possible group-hopping behavior for the remainder of this article—it’s not that relevant.

In the next sections, I’ll discuss these two groups of data in more detail, starting with the first group. This will provide the necessary context for the remainder of the article, where I’ll describe their significance in the creation of your application components. Finally, building on that, I’ll introduce the two composition models that you can use to create an object graph.

Data passing through the public API

Let’s focus on the first group for a moment: consider web-request data, posted by a browser. If you’re building an ASP.NET (or ASP.NET Core) MVC application, a browser’s HTTP request is transformed by the framework into a view model object and passed to your MVC controller classes. The following sequence diagram visualizes this process:

Sequence diagram visualizing how a browser's HTTP request is transformed to public data

In this sequence diagram, the user’s request is handled by the framework. The framework then transforms and forwards the call to a controller’s action method—in this case, the AddItem method on ShoppingBasketController. When the controller finishes, it returns an action result that’s used to render HTML. The HTML is then sent back to the user.

ASP.NET will provide the AddItem action method with the runtime data coming from the user. The following code listing shows ShoppingBasketController with its AddItem method:

public class ShoppingBasketController : Controller
{
    [HttpPost]
    public IActionResult AddItem( The application’s public API
        AddShoppingBasketItem viewModel) Runtime data
    {
        ...
    }
}

The AddItem’s AddShoppingBasketItem argument captures the request data. It is runtime data, unique to the request, its data posted by the client, and supplied by the ASP.NET framework to ShoppingBasketController. The AddShoppingBasketItem runtime data is passed from the caller (the framework) to the callee (ShoppingBasketController) through the class’s public API (the AddItem method). This works great for request/response-related runtime data—such as AddShoppingBasketItem—but might not work well in other cases, which brings me to the second group of runtime data.

Contextual or internally oriented data

The AddShoppingBasketItem view model specifies the runtime data required by the AddItem API. But not all runtime data should be supplied to a class through its public API. Some data is an implementation detail—leaking its existence through the public API could complicate things for the clients, cause maintainability issues, or even raise security concerns. In many cases, this implementation-specific runtime data is more contextual in nature.

Take, for instance, the identity of the current user that is issuing the request. Part of the application needs to be aware of the user’s identity. Although the HTTP operation sends the identity, that information is not supplied to the controller’s public API. Making the identity part of AddShoppingBasketItem, for instance, could cause several problems—most likely, a security risk. It is not up to the user to supply an unverified identity through this request’s POST information. The user’s identity has long been established, and a security token is typically sent using a different “channel” (a cookie). The user identity can, in the context of adding an item to a basket, be regarded as an implementation detail.

Another example of implementation-detail runtime data is Entity Framework’s DbContext. From the perspective of the application’s public API, it is an implementation detail. AddShoppingBasketItem, for example, should not have to change if you decide to change the application’s persistence layer.

DbContext is a glorified state bag with cached and mutated entities, ready to be persisted at some point. Each request gets its own local set of entities, cached for the duration of the web request, and reusing it across requests is a bad idea. You wouldn’t let the browser provide the controller with a DbContext—that would be a scary thought. But equally so, you wouldn’t pass on a DbContext through the public API of the individual layers.

The significance of contextual data in the context of Object Composition

When it comes to composing object graphs using DI, the difference between AddShoppingBasketItem and DbContext becomes significant. While both constitute runtime data, you design your classes differently around them. As explained, the application’s abstractions and external-facing APIs expose runtime data objects such as AddShoppingBasketItem. Runtime data objects such as DbContext, however, are instead hidden behind these same abstractions, making them mere implementation details of their direct consumers. This means that whereas AddShoppingBasketItem is passed to the public methods of an already composed object graph, DbContext is supplied using a different mechanism—one option being Constructor Injection.

The following simplified object graph shows this. The application’s ShoppingBasketDbContext is created and supplied to the controller’s constructor:

var controller =
    new ShoppingBasketController(
        new ShoppingBasketDbContext()); Constructor injection

Later, when a web request comes in, the deserialized view model is passed along to the controller’s AddItem method. At that point, however, the controller’s object graph has long since been created.

This way of supplying ShoppingBasketDbContext to the object graph during construction is one model you can use to compose your object graphs, called the Closure Composition Model.

DEFINITION

The Closure Composition Model composes object graphs that capture runtime data in variables of the graph’s components.

An alternative to letting your application components consume these contextual runtime data objects is the Ambient Composition Model. With that model, contextual runtime data is no longer captured in variables inside the object graph, but instead is managed by the Composition Root. Application components requiring such data request it through method calls on provided abstractions.

DEFINITION

The Ambient Composition Model composes object graphs that do not store runtime data inside captured variables. Instead, runtime data is kept outside the graph and stored as ambient data. This ambient data is managed by the Composition Root and is provided to application components on request, long after those components have been constructed.

This completes this primer on Object Composition models. At this point, much remains to be explained, such has how application components are designed in each model, what exactly ambient data is, and what the pros and cons are of both models. I will go into this in more detail in the following articles, starting with a description of the Closure Composition Model, and continuing with a description of the Ambient Composition Model.

Stay tuned.

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