Documenting the design of software using patterns?

I have experimented an approach that considers every design pattern as the recursive composition of smaller patterns. This led to a prototype tool to illustrate its benefits by generating design-level documentation of annotated source code.

Eat your own dog food

The source code of this tool itself was used as the code base to apply the tool on, in order to generate its own design documentation. The Java Doclet API was used to retrieve the Javadoc tags in the class Javadoc comments, e.g. :

@pattern KnowledgeLevel OperationLevel=MyOtherClass some free text comment…

The pattern catalogue

A subset of design patterns and some other patterns such as the Knowledge Level (Fowler) has been defined in small definitions files then loaded at tool startup. We call this set of patterns definitions the pattern catalogue.

Here is an example of the definition file for the Builder pattern, notice the declaration of the member roles at the bottom, that also declares the expected pattern kind:

Encapsulates the creation of a complex object from a source object.
@extends DesignPattern
@category DesignPattern
@category Creational
@author GoF
@book [GoF 95]  E. Gamma, R. Helm. R. Johnson, and J. Vlissides. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley 1995.
@style ObjectOrientedProgramming
@reference [http://en.wikipedia.org/wiki/Builder_pattern ]
@defaultrole Builder
@role Director=TypeHierarchy
@role Builder=TypeHierarchy
@role Product=TypeSet
@role Factory=AbstractFactory

Parsing the annotations to build the pattern occurrences graph

Every time a Javadoc tag “@pattern Pattern” is met in the Javadoc part of a class, an occurrence of the corresponding pattern is created in memory, linking to the corresponding class in the Java programming language metamodel.

Whenever such pattern uses a Type Hierarchy (this is declared in the definition of the pattern), then the tool will look for every subtype of the declared (super) type in order to complete the pattern occurrence as well as possible.

Even patterns that are usually considered as atomic patterns can be explained as the composition of smaller “elemental” patterns. For instance, A Builder [GoF] usually defines an AbstractBuilder role and a ConcreteBuilder role that implements it. We can express this part of the Builder pattern as a TypeHierarchy elemental pattern in the role Builder; the supertype of the TypeHierarchy therefore represents the AbstractBuilder role, while every other concrete type of this hierarchy represents ConcreteBuilder roles.

By analogy with the members of a class, we call members patterns and member occurrences the patterns and occurrences (respectively) a pattern or occurrence is composed of. The tool totally ignore fields and methods, it only considers types (interfaces and classes).

The benefits of this approach are to reuse a bigger part of the pattern definitions through composition and to help address the variant problem.

Once the pattern occurrences graph is completely built, it is rendered into an UML class diagram using Graphviz dot. Here is an example of overview for the full project:

coremodulediagram_final
Example of overview diagram for the Core module of the prototype source code

In the above diagram we can see how the patterns naturally sit on top of the several type hierarchies while connecting them together. Also the relationships between patterns become very obvious, such as the Interpreter that references the Builder and the Composite sub-patterns. This suggests that such diagrams are very valuable to document the design of a code base.

Topological sort

The approach of representing every pattern as the composition of other patterns yields to directed graphes, at the pattern level (the “meta” level) and at the pattern occurrences level. One consequence is that we can apply a topological ordering on such graphs. In the current prototype we need to do a topological sort on the patterns definitions so that we can know in advance in what order to process pattern occurrences declarations: the patterns used must be built before the patterns using them.

The pattern occurrences graph can also be navigated using the usual Depth-First search or Breadth-First search. DFS is convenient to generate the Overview diagrams such as the diagram shown above, whereas BFS is convenient to drill down the design from the top to the bottom, one level at a time. This can therefore generate the following table of contents:

Patterns graph navigation index
Patterns graph navigation index

Natural language generation alternative

As an alternative to visual diagrams, a tool can also generate natural language text to communicate the exact same information. I have tried this approach using simple text templates in each pattern definitions. Each pattern therefore defines its own design description, but using variable names to be filled later with the data from the pattern occurrences graph.

The biggest work in this approach is to deal with the plural, the enumeration of collections (comma between each item but the last etc.), truncating collections that are too long, and the complete omission of a section if a pattern member is totally missing.

Here is a sample of text generation for the prototype tool itself. Notice how the structure is rigid for each pattern occurrence: Pattern name, short description, then template text evaluated with the actual types names, followed by the comment that was put after the @pattern declaration:

Design of the module Core
This module is essentially made of 9 pattern occurences: Strategy, Visitor, KnowledgeLevel, Interpreter, Builder and AbstractFactory.

Visitor
The visitor design pattern is a way of separating an algorithm from an object structure. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures
This module defines a Visitor OccurrenceVisitor. comment

Strategy
A Strategy object encapsulates an algorithms that can be selected on-the-fly at runtime.
This module defines the interface of a Strategy: Plugin. It enables plugins to manipulate the Patternity metamodel.

AbstractFactory
Provides one or more method(s) that encapsulate(s) the creation of object.
The type OccurrenceFactory defines the interface of an AbstractFactory that creates instances of Occurrence and TypeOccurrence.

Interpreter
Define a representation for a language along with an interpreter that uses the representation to interpret sentences in the language.
The type(s) Occurrence and TypeOccurrence define(s) an object representation of the considered language. The considered language describes a graph of patterns occurrences. It uses the Visitor Visitor to add new operations to the object representation without modifying this structure.

Strategy
A Strategy object encapsulates an algorithms that can be selected on-the-fly at runtime.
This module defines the interface of a Strategy: Loader. Encapsulates how to load every pattern definition into the pattern.repository.

[...]

KnowledgeLevel
A Knowledge Level is a group of objects that describes how another group of objects should behave.
This module defines a KnowledgeLevel (also known as metamodel). It is made of two levels of objects: objects in the knowledge level, i.e. the instances of Pattern define how objects in the operational level can behave, i.e. instances of Occurrence. Typically each type in the operation level maintains a reference to its corresponding type in the knowledge level in order to know how to behave.

Conclusion

Later, at work, when asked to produce a design documentation targeted at other developers, I have experimented doing a similar effort manually, using nothing but patterns. I am still impatient to receive some feedback from the people that will read the resulting documentation in order to validate or infirm this approach.

cyrille

Software development, Domain-Driven Design, patterns and agile principles enthusiast

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