This chapter discusses the benefits of modularity in the design of software systems.

In summary, benefits of increased modularity include :-

  • Improved comprehension by individuals of individual entities and the system as a whole. A view of the architectural decomposition of the system is enabled by the explicit declaration of dependencies.

  • The elimination of architectural dependency 'cycles' which inhibit software maintenance.

  • Interchangeability of parts, thus improving adaptation to evolving environments and reducing the need for wholesale rewrites.

Improved comprehension

"Complexity is the enemy" (Fred Brooks)

citation needed

To understand why modularity is required in large software systems, we must first understand the nature of complexity and its effect on our software.

The reason complexity damages software is that it makes it harder for a person to understand how the software works. Without proper human comprehension of the whole system, maintenance becomes a risky endeavour as human mistakes lead to system instability and ultimately failure. In order to build yet larger systems, we need methods to control and manage the growth of complexity.

The role of consistency

When we attempt to comprehend a system we construct conceptual models in our brains as a mental aid to remembering how each part of the system works. A system that exhibits consistent abstractions and concepts is highly compatible with our mental cognitive mechanisms and we take on information about the system with ease. We begin to predict how other parts of the system will work, based on the understanding we already have. We build on these conceptual models, refining them where necessary until we have achieved a working understanding, adequate for confidence in making system improvements,

Systems that are inconsistent throughout require far more work on the part of the developer to comprehend. Our mental models are discordant, we are deluged in detail, which we soon forget.

Therefore we prefer designs for software that utilize patterns universal throughout the architecture.

Modular applications make dependency relationships explicit. Therefore it is usual to adopt a consistent pattern for an entity to resolve its dependencies. There are different approaches. For example, dependency resolution may be achieved indirectly via a component catalog or by dependency injection, a method for connecting a component with its dependencies at runtime.

To achieve consistency, we prefer that all dependency relationships are resolved in a similar manner.

Architectural visibility

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Reduction of dependency cycles

Despite great efforts to formalise software development, software is still usually written ad hoc. After a few years of development, software systems are described by their owners with illustrative metaphors such as 'a great ball of mud', or 'spaghetti code'. This is often perceived by the system owners as an unfortunate accidental consequence of a flawed approach to development.

But that is precisely what any non-trivial software system is.

balls of mud


the temporal aspect


ensure this is in the introduction sync this with modular's

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