I am a fan of polylithic architectures. Such architectures have many advantages related to enhancing evolvability and maintainability. When you decide to create a system composed of small pieces how do you decide what functionality goes into which component?
Principles
The goal is to sub-divide the application into multiple highly cohesive components which are weakly connascence with each other. To achieve the desired cohesion it will be necessary to align the component boundaries with natural fissure points in the application.
The strategy should allow for the production of a arbitrary number of components. A component that was of a manageable size yesterday could easily become too large tomorrow. In that situation the over-sized component will need to be sub-divided. Applying the same strategy repeated will result in a system that is more easily understood.
We want to minimize redundancy in the components. Redundancy results in more code with must be understood and maintained. More importantly redundancy usually introduces connascence of algorithm, making changes more error prone and expensive. In a perfect world, any particular behavior would be implemented in exactly one component.
We want to isolate changes to the system. When implementing a new feature it is desirable to change as few components as possible. Each additional component that must be changed raise the complexity of the change. The componentization strategy should minimize the number of components involved in the average change to the system.
With those metrics in mind lets explore the two most common approaches and see how they compare with each other. Those two patterns of componentization are horizontal slicing and vertical slicing.
Horizontal slicing
In this approach the component boundaries are derived from that implementation domain. The implementation is divided into a set of stacked layers in such a way that a layer initiates communication with the layers below it. This results in a standard layered architectures. By implementing each layer in a separate component you can achieve the horizontal slicing. This style of componentization strategy results in the very common n-tier architecture pattern.
For example, an application that has a business logic and a presentation layer the application would be divided into two components. A business logic component and a presentation component.
Vertical slicing
In this approach the component boundaries are derived from the application domain. Related domain concepts are grouped together into components. Individual components communicate with any other components as needed.
This approach is also quite common but is usually thought of a lot less formally. It is more common for this type of segmentation to develop incidentally. For example, because separate teams developed the parts independently, and then integrated them later. Any time you integrate separate applications you have vertical componentization.
The Score
Against the metrics we laid out earlier, vertical slicing does much better than horizontal.
| Horizontal slicing | Vertical slicing | |
|---|---|---|
| Cohesion | high | high |
| Repeatability | low | high |
| DRYness | low | high |
| Change isolation | low | high |
Cohesion
Horizontal slicing has high cohesion. Each of the components can represent the a logically cohesive part of the implementation.
Vertical slicing also has high cohesion. Each component represents highly cohesive part of the application domain.
Repeatability
Vertical slicing provides a mechanism for reapply the subdivision pattern an arbitrary number of times. If any component gets too large to manage it can be divided into multiple components based on the application domain concepts. This same process can be repeated from the initial division of a monolithic application until components of the desired size have been achieved.
Horizontal slicing is less repeatable. The more tiers the harder it is to maintain cohesiveness. In practice it is very rare to see an tiered architecture with more than 4 tiers, and 3 tiers is much more common.
DRYness
Horizontal slicing tends to result in some repetition. Certain behaviors will have to be repeated a each layer. For example, data validation rules. You will need those in the presentation layer to provide good error messages and in the business logic layer to prevent bad data being persisted.
Vertical slicing allows you to reduce the connascence of algorithm because any single user activity is implemented in exactly one component. Components usually do end up communicating to each other, however, they do so in a way that does not require in the same algorithms be implemented in multiple components. For any one bit of data or behavior, one component will its authoritative source.
Change isolation
Vertical scaling tends to allow new features to be implemented by changing only one component. The component changed is the one which already contains features cohesive with the new one.
Horizontal slicing, on the other hand, tends to require changes in every layer. The new feature will require additions to the presentation layer, the business logic layer and the persistence layer. Having to work in every layer increase the cognitive load required to achieve the desired result.
Conclusion
Vertical slicing provides significant advantages. The high cohesion, dryness, and change isolation combine to drastically reduces the risks and cost of change. That is turn allow better/faster maintenance and evolution of the system. The repeatability allows you to retain these benefits even while adding functionality over time. Each time a component gets too large you can divide it until you have reach a application size that is human scaled.
Having a large number of components operate as a system does result in a good deal of communication between the components. It important to pay attention to the design of the APIs. Poor API design can introduce excessive coupling which will eat up most of the advantages described above. Hypermedia – or more precisely, following the REST architectural style – is the best way i know to reduce coupling between the components.