A System Is Not a Tree
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This document discusses the inherent difficulties in software development, distinguishing between essential challenges and accidental ones. It critiques the oversimplification of software structures, likening them to trees, and advocates for a more complex understanding through the concept of semilattices. The author emphasizes the importance of communication structures in design and the need for a more intricate organizational approach in software system design.
![Let us examine [software's]
difficulties. Following Aristotle, I
divide them into essence — the
difficulties inherent in the nature
of software — and accidents —
those difficulties that today
attend its production but that
are not inherent.](https://image.slidesharecdn.com/asystemisnotatree-150601071229-lva1-app6891/85/A-System-Is-Not-a-Tree-4-320.jpg)
![The basic thesis [...] is that
organizations which design
systems [...] are constrained
to produce designs which are
copies of the communication
structures of these
organizations.
Melvin Conway
How Do Committees Invent?](https://image.slidesharecdn.com/asystemisnotatree-150601071229-lva1-app6891/85/A-System-Is-Not-a-Tree-25-320.jpg)
![We have seen that this fact
has important implications
for the management of
system design. [...] A design
effort should be organized
according to the need for
communication.
Melvin Conway
How Do Committees Invent?](https://image.slidesharecdn.com/asystemisnotatree-150601071229-lva1-app6891/85/A-System-Is-Not-a-Tree-26-320.jpg)
A System Is Not a Tree
- 1. A System Is Not a Tree @KevlinHenney
- 4. Let us examine [software's] difficulties. Following Aristotle, I divide them into essence — the difficulties inherent in the nature of software — and accidents — those difficulties that today attend its production but that are not inherent.
- 5. How much of what software engineers now do is still devoted to the accidental, as opposed to the essential?
- 7. Arboricide is the murder of trees. The victims of arboricide are the descriptive tree structures that are so often found in software, holding together many individual elements in one coherent and immediately understandable harmony.
- 8. Software development should not be a trade of constructing difficulty from simplicity. Quite the contrary. So where there are trees to be shown you should show them, and refrain from turning the relationships they describe into a puzzle. It is, essentially, a matter of the span of description. Aboricide, then, is using a smaller description span when a larger one would be better.
- 9. Trees sprout up just about everywhere in computer science. Donald Knuth
- 11. main subroutine subroutine subroutine subroutine subroutine subroutine subroutine subroutine afferent branch transform branch efferent branch
- 12. main function function function function function function function function afferent branch transform branch efferent branch
- 14. Concept Hierarchies The construction principle involved is best called abstraction; we concentrate on features common to many phenomena, and we abstract away features too far removed from the conceptual level at which we are working. Ole-Johan Dahl and C A R Hoare "Hierarchical Program Structures"
- 17. A city is not a tree Christopher Alexander
- 19. A city is not a tree Christopher Alexander
- 20. The tree of my title is not a green tree with leaves. It is the name of an abstract structure. I shall contrast it with another, more complex abstract structure called a semilattice. Both the tree and the semilattice are ways of thinking about how a large collection of many small systems goes to make up a large and complex system.
- 22. The semilattice is potentially a much more complex and subtle structure than a tree. It is this lack of structural complexity, characteristic of trees, which is crippling our concepts of the city. The reality of today's social structure is thick with overlap — the systems of friends and acquaintances form a semilattice, not a tree.
- 25. The basic thesis [...] is that organizations which design systems [...] are constrained to produce designs which are copies of the communication structures of these organizations. Melvin Conway How Do Committees Invent?
- 26. We have seen that this fact has important implications for the management of system design. [...] A design effort should be organized according to the need for communication. Melvin Conway How Do Committees Invent?
- 27. EPISODES: A Pattern Language of Competitive Development Ward Cunningham
- 29. In simplicity of structure the tree is comparable to the compulsive desire for neatness and order that insists the candlesticks on a mantelpiece be perfectly straight and perfectly symmetrical about the centre.
- 39. In simplicity of structure the tree is comparable to the compulsive desire for neatness and order that insists the candlesticks on a mantelpiece be perfectly straight and perfectly symmetrical about the centre. The semilattice, by comparison, is the structure of a complex fabric; it is the structure of living things, of great paintings and symphonies.
- 40. A program which has such a structure in which there is no single "highest level"... is called a heterarchy (as distinguished from a hierarchy).