Feature Architectures

How far can we go into the writing of an application without losing momentum?

As an application becomes richer, it typically becomes more complex. There are two aspects to this:

• Logical complexity increases as a result of new functionality binding to existing functionality.
• Physical complexity increases as a result of increased application size.

The result of increasing complexity is that developers find it increasingly challenging to add new features to an application. This complexity manifests itself in two ways:

1. Your workspace is increasingly cluttered with irrelevant entities – entities that do not contribute to the feature you are developing
2. Your code is increasingly dependant on existing elements of functionality.

Very large applications are typically broken down into modules. Modules reduce logical complexity by providing narrowing interfaces; physical complexity is reduced by keeping all the code for a given module separate from the code for other modules.

Feature architectures enforce micro-modularity:

• Each feature typically maps a single value increment or story.
• Features are abstracted from each other using channeled notifications.

Why design feature architectures?

Feature architectures are derived from story driven development. In agile, story driven development, developers collaborate towards the development of a given story cutting across the layers of a selected framework. Agile ensures that developers deliver value faster; in suitable contexts (assuming that value can be delivered incrementally), this maximises profitability while reducing the risk to produce large amounts of unused functionality.

Feature architectures take advantage of the separation between source spaces and runtime spaces to allow developing well formed (layered) runtime architectures while limiting linearly growing physical and logical complexity. This allows developers to focus on delivering atomic value increments without incurring mounting familiarization costs and splitting development across framework layers. Also, this may allow fairly large developer teams to collaborate without facing large communication costs.

In a nutshell, feature architectures and micro-modularity target value driven separation. In contrast, existing separation strategies and meta-patterns provide abstract roles that can be re-instantiated in a variety of contexts (the MVC meta-pattern is an example). This affords industry endorsement for high level design solutions. Beyond, classic separation models emphasize a component drive approach. While this means that component cores can be developed in parallel, this may result in glossing over communication interfaces between components.

Layered, component oriented architectures emphasize potential and structural integrity and are more suitable for long term investments and library development. Feature architectures emphasize value increments and may be ideal for writing application front ends:

• Reusable data and view components are already available
• Applications are feature rich, yet do not involve much complex code.
• Value increments typically involve cutting across layered architectures.

Working using a feature architecture

Feature architectures require developers to package features separately. Each feature is developed as an integrated micro-module – if you are writing an application, a feature will probably bundle view, model and controller functionality.

1. Each developer or pair are responsible for designing and implementing a feature derived from a user story.
2. The core functionality involved in developing a feature is developed independently from other features.
3. The source code related to a given feature is bundled within a separate package.
4. Developers integrate their feature by implementing handlers for channeled notifications.
5. Developers expose required resources by providing external notifications.
6. All source code for a given feature may be removed without causing the application to break at compile time.

Mmh… this may not be the best introduction to feature architectures I could write. I’ll try to improve on that and will also provide an application template very soon.

Feature Programming & Feature Annotations

In agile development, the emphasis is on delivering business value incrementally. While this explicits features within software cycle management, it may also reduce the developers’ visibility over software architecture; this may part in explain why agile methods rely heavily on refactoring.
In contrast, classic development approaches emphasize planning and upfront design. While this may promote architectural integrity, it has been largely demonstrated that  this reduces both customer satisfaction and software profitability, typically resulting in a large amount of unused functionality.
When debugging and processing change requests, development teams naturally adopt a feature oriented approach; this is because bugs and change requests are normally raised by end users.

Overall, software features, whether implicit or explicit to the development cycle as agile stories or bugs, remain largely implicit to code-bases because the high level structure of software applications follows a technical orientation – a paramount example in front end development being the unavoidable Model View Controller meta-pattern. Features typically cut across layered architectures. For agile programmers and maintenance teams, completing stories, fixing bugs and resolving change requests requires juggling between application layers. For traditional teams, reduced visibility over application features will result in a higher number of bugs and decreased customer satisfaction.

I define Feature Programming as a set of methods designed to reduce the cost of software development by revealing features within source code. This is mainly achieved in two ways:

• Feature Annotations using domain semantics allow software architects to retain a layered architecture while increasing feature visibility.
• Feature Architectures emphasize feature separation over layering when deciding the high level structure of an application.

Both approaches are expected to reduce costs in front end development. While feature annotations provides non invasive, lightweight techniques for teams that want to reduce liabilities in the software cycle, Feature architectures may be very suitable for decentralized and open source application projects and further afford the creation of reusable value increments.

In this article, I cover feature annotations; later I will cover feature architectures in another article and will also demonstrate a small framework guiding the development of feature centric front ends while affording elements of layering using a suitable interpretation of the MVC meta-pattern – stay posted!

Feature Annotations

Annotating features simply requires a fairly disciplined approach to tracing code. Feature annotations can be applied either during development or retrospectively, as a scaffolding technique used to reduce the cost of maintaining legacy code.

Feature annotations requires the following:

1. One or several feature classes define static functions expressing application features. Such feature traces must be easy to spot and suitable for automated discovery (in Java or ActionScript, you may use $ as a prefix).
2. Each function in a feature class encapsulates a trace statement. Traces may be logged, output to a console or even displayed in an application specific debug window.
3. Semantics for feature functions are typically provided either as story names or itemized requirements.
4. All code that relates to a specific feature must be annotated using trace function invocations (‘trace annotations’). Within an agile process, a developer writing a story will tag their code, and otherwise contributing code, using trace annotations (for each class involved in the developed story, at least one function or constructor should be annotated).
5. It is important that feature traces can be filtered easily either at runtime or at compile time (commenting out unwanted traces may be sufficient in simple cases).

Feature annotations constitute a non invasive technique. Adding feature annotations cannot break existing code and doesn’t require re-thinking your approach to software architecture. Developers working on agile stories need not be concerned with selecting trace annotations as the same annotation semantics are used over and over during story development.

In spite of this simplicity, feature modeling provides several benefits that will help in drastically increasing a development team’s reactivity to change requests while increasing software maintainability:

1. If you are doing code reviews, feature annotations reveal and delimit the code related to a given feature and afford validating the code from the point of effectiveness. Classical code reviews validate structural integrity, glossing over functionality scattered across application layers.
2. New starters observe feature traces at runtime and isolate features in the source; this complements a developer’s understanding of packaging and structure; developers also appreciate the software’s actual uses by reviewing feature classes. This results in decreased learning curves and reinforced consistency across the code-base given multiplied reuse opportunities.
3. Maintenance teams benefit orderly, channeled traces mapping business logic using domain language. Observing channeled traces and isolating defective features can save hours spent learning control flow and discovering all scattered elements potentially involved in a defect.
4. Liabilities associated with reuse are reduced given that concurrent, co-located traces provide tell-tale signs of reuse. A developer targeting a given code fragment will be able to determine which elements of functionality are affected, thereby correctly assessing the risk involved in modifying shared resources and identifying functionality that may require regression testing.