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Big Ball of Mud (Software Design)

A "Big Ball of Mud" is a historically grown software system (or a part of it) often characterized through lack of modularization, missing contextual boundaries, redundant code and information leaking through various levels of abstractions.

It is also referred to as spaghetti code, a common phrase for an unstructured, opaque, tangled mess of seemingly unrelated functions and classes.

The assumption would be that the quality of the source code in a Big Ball of Mud is weighed down by this negative connotation. Obviously, a Big Ball of Mud seems to represent an Anti-Pattern at first sight, but Foote and Yoder propose the Big Ball of Mud as a valid architectural pattern: In some cases, a software might benefit from it when reasonably applied, e.g. cost must be kept low in the initial phase of the project, and a quick time-to-market is important.

In the conclusion of their paper, Foote and Yoder state that

"Periods of moderate disorder are a part of the ebb and flow of software evolution. As a master chef tolerates a messy kitchen, developers must not be afraid to get a little mud on their shoes as they explore new territory for the first time. Architectural insight is not the product of master plans, but of hard won experience."

Also, existing Big Ball of Muds can be demarcated with clear boundaries and a coarse-grained API allowing for exposing selected functionality of the malicious system, preventing further entanglement and leaking of mud into the integrating system:

"Draw a boundary around the entire mess and designate it a big ball of mud. Do not apply sophisticated modeling within this context. Be alert to the tendency for such systems to sprawl into other contexts." [📖DDDR, p. 38]


The term was most likely coined by J. Moses, as quoted by Steele and Sussman:

_"LISP, on the other had, is like a ball of mud. You can add any amount of mud to it and it still looks like a ball of mud."

then gradually found its way into the vocabulary of Software Architecture, as Foote and Yoder write in their Paper Big Ball of Mud:

"Brian Marick first suggested the name "BIG BALL OF MUD" as a name for these sort of architectures, and the observation that this was, perhaps, the dominant architecture currently deployed, during a meeting of the University of Illinois Software Architecture Group several years ago. We have been using the term ever since."