Software Architecture

Aug 5, 2024

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Effective Software Architecture: Building Better Software Faster

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Their Relationships to the Environment

Systems never operate in a vacuum. In some cases, they may be the only software running on the hardware at hand, in which case that hardware is their primary environmental concern. However, most of the time, systems run on top of or as part of some other system.

For example, consider the relationship between a program (a system) and the operating system on which it runs (also a system). Operating systems impose a fundamental organization on the programs they host. This organization is inescapable: The programs are started by the operating system and, to a lesser or greater extent, the operating system monitors and controls the program during execution and through termination. Without some basic agreement between the two, these programs would never run.

Operating systems vary immensely in regard to how much of the fundamental organization of a program they define. In the Unix model, for example, the imposed organization is quite limited. Programs are started by invoking a function with a well-known name (“main”) and set of arguments, so those elements must exist. To be sure, the typical structure of Unix programs encompasses many conventions and APIs. But very little of that is truly required. As a result, Unix successfully and easily supports writing programs of many types, languages, and structures.

The iOS platform, by comparison, is much more opinionated. iOS applications do not have a single entry point but rather a whole set of functions they are expected to respond to. Much of this has to do with the life cycle of these applications. In the Unix model, programs start, run until they’re done with the task at hand, and then exit. On iOS, applications are started, brought to the foreground, moved to the background, stopped to preserve resources, restarted due to user interactions or notifications, and so on. It’s much more complicated!

On Unix, you can use the fundamental organization of an iOS program if you want to. Again, Unix doesn’t impose its organization; the program designer has significant discretion. But on iOS, your application largely needs to be fundamentally organized around a model dictated by iOS. This relationship of the program to the iOS environment becomes a major driver of the program’s architecture.

RELATIONSHIPS TO MULTIPLE ENVIRONMENTS

More opinionated environments create tension with code reuse. Building complex applications is expensive, and many software producers would like to write a system once and use it in multiple environments. From an architect’s perspective, the problem of managing the relationship to the systems’ environment becomes one of managing the relationship to multiple environments.

This can be difficult to accomplish when environments impose different—and in the worst case, conflicting—fundamental organizations. There are a few standard ways in which this issue is tackled:

Ignore the environment and organize the software in some other way. As a rule, it’s expensive to develop systems this way because a good deal of time and effort go into reproducing behaviors that you could get “for free” from the environment. Furthermore, those reproductions are never perfect, and differences tend to look like defects to your users.
Create an abstraction layer that adapts two or more environments to a single model. This strategy can work well for systems that don’t need deep integration with capabilities provided by the environment. For example, it often works well with games. The abstraction layer might be part of the system, or it might be externalized. These layers are sometimes products themselves.
Split the system into two subsystems: an “environment-specific” core that is written separately for each target, and an “environment-agnostic” edge that is shared across each. While this may sound superficially like the abstraction layer approach, it is a discrete strategy because the environment capabilities are not abstracted. In this case, the environment-specific layer becomes as deep as necessary (but, ideally, no more than that) to connect with the environment-agnostic logic.

As with engineering in general, there’s no universal correct answer to the challenge of managing these environmental relationships.

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