System Metaphor

System Metaphor

Metaphorical thinking is pervasive in software development, especially with models. But the Extreme Programming practice of "metaphor" has come to mean a particular way of using a metaphor to bring order to the development of a whole system.

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Just as a firewall can save a building from a fire raging through neighboring buildings, a software "firewall" protects the local network from the dangers of the larger networks outside. This metaphor has influenced network architectures and shaped a whole product category. Multiple competing firewalls—developed independently, understood to be somewhat interchangeable—are available for consumers. Novices to networking readily grasp the concept. This shared understanding throughout the industry and among customers is due in no small part to the metaphor.

Yet it is an inexact analogy, and its power cuts both ways. The use of the firewall metaphor has led to development of software barriers that are sometimes insufficiently selective and impede desirable exchanges, while offering no protection against threats originating within the wall. Wireless LANs, for example, are vulnerable. The clarity of the firewall has been a boon, but all metaphors carry baggage.[1]

[1] SYSTEM METAPHOR finally made sense to me when I heard Ward Cunningham use this firewall example in a workshop lecture.

Software designs tend to be very abstract and hard to grasp. Developers and users alike need tangible ways to understand the system and share a view of the system as a whole.

On one level, metaphor runs so deeply in the way we think that it pervades every design. Systems have "layers" that "lay on top" of each other. They have "kernels" at their "centers." But sometimes a metaphor comes along that can convey the central theme of a whole design and provide a shared understanding among all team members.

When this happens, the system is actually shaped by the metaphor. A developer will make design decisions consistent with the system metaphor. This consistency will enable other developers to interpret the many parts of a complex system in terms of the same metaphor. The developers and experts have a reference point in discussions that may be more concrete than the model itself.

A SYSTEM METAPHOR is a loose, easily understood, large-scale structure that it is harmonious with the object paradigm. Because the SYSTEM METAPHOR is only an analogy to the domain anyway, different models can map to it in an approximate way, which allows it to be applied in multiple BOUNDED CONTEXTS, helping to coordinate work between them.

SYSTEM METAPHOR has become a popular approach because it is one of the core practices of Extreme Programming (Beck 2000). Unfortunately, few projects have found really useful METAPHORS, and people have tried to push the idea into domains where it is counterproductive. A persuasive metaphor introduces the risk that the design will take on aspects of the analogy that are not desirable for the problem at hand, or that the analogy, while seductive, may not be apt.

That said, SYSTEM METAPHOR is a well-known form of large-scale structure that is useful on some projects, and it nicely illustrates the general concept of a structure.


When a concrete analogy to the system emerges that captures the imagination of team members and seems to lead thinking in a useful direction, adopt it as a large-scale structure. Organize the design around this metaphor and absorb it into the UBIQUITOUS LANGUAGE. The SYSTEM METAPHOR should both facilitate communication about the system and guide development of it. This increases consistency in different parts of the system, potentially even across different BOUNDED CONTEXTS. But because all metaphors are inexact, continually reexamine the metaphor for overextension or inaptness, and be ready to drop it if it gets in the way.

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The "Naive Metaphor" and Why We Don't Need It

Because a useful metaphor doesn't present itself on most projects, some in the XP community have come to talk of the naive metaphor, by which they mean the domain model itself.

One trouble with this term is that a mature domain model is anything but naive. In fact, "payroll processing is like an assembly line" is likely a much more naive view than a model that is the product of many iterations of knowledge crunching with domain experts, and that has been proven by being tightly woven into the implementation of a working application.

The term naive metaphor should be retired.

SYSTEM METAPHORS are not useful on all projects. Large-scale structure in general is not essential. In the 12 practices of Extreme Programming, the role of a SYSTEM METAPHOR could be fulfilled by a UBIQUITOUS LANGUAGE. Projects should augment that LANGUAGE with SYSTEM METAPHORS or other large-scale structures when they find one that fits well.

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