GraphvizTutorial

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1 Creating Domain Models With PEAK: GraphViz Example

This tutorial will show you how to use the peak.model package to create "domain model" objects for some simple applications. Our chosen subject matter for the domain model will be GraphViz, the open-source graph visualization toolkit from AT&T. The GraphViz dot tool takes input files that look like this:

 
digraph finite_state_machine { 
        rankdir=LR; 
        size="8,5" 
        orientation=land; 
        node [shape = doublecircle]; LR_0 LR_3 LR_4 LR_8; 
        node [shape = circle]; 
        LR_0 -> LR_2 [ label = "SS(B)" ]; 
        LR_0 -> LR_1 [ label = "SS(S)" ]; 
        LR_1 -> LR_3 [ label = "S($end)" ]; 
        LR_2 -> LR_6 [ label = "SS(b)" ]; 
        LR_2 -> LR_5 [ label = "SS(a)" ]; 
        LR_2 -> LR_4 [ label = "S(A)" ]; 
        LR_5 -> LR_7 [ label = "S(b)" ]; 
        LR_5 -> LR_5 [ label = "S(a)" ]; 
        LR_6 -> LR_6 [ label = "S(b)" ]; 
        LR_6 -> LR_5 [ label = "S(a)" ]; 
        LR_7 -> LR_8 [ label = "S(b)" ]; 
        LR_7 -> LR_5 [ label = "S(a)" ]; 
        LR_8 -> LR_6 [ label = "S(b)" ]; 
        LR_8 -> LR_5 [ label = "S(a)" ]; 
} 
 

Many people have created interesting tools that use GraphViz to draw diagrams of workflow processes, clickpaths through a website, profiler call charts, even UML class diagrams. Most such programs use hardcoded templates for their .dot files and can be difficult to change because you have to know both the dot language and the application area of the software. In this tutorial, we'll develop a sophisticated framework for creating .dot files from application data that will give you flexible, object-oriented control over the visual objects to be rendered by GraphViz. We'll begin with a very simple structure using only three classes, and then, step by step, we'll show you more advanced ways to use the peak.model tools as we gradually enhance the framework's power and flexibility. Along the way, we'll share tips and techniques for ways to make the best design decisions for your own applications, in similar situations.

1.1 The Basics

Before beginning any software development effort, it's important to know your scope and requirements, so let's lay out ours for this project. We want to develop a framework that makes creating GraphViz diagrams easy, for a variety of application areas. We are only interested in dot graphs, which are always directed graphs (i.e. every line has a direction to it, even if no arrows are drawn). Also, we don't need or want to be able to read or modify GraphViz files, just write them. To ensure that we have a useful framework, we'll want to develop a few simple applications that create useful output. And for our "grand finale", we'd like to be able to create a script that uses the framework to create class diagrams of the framework itself, or perhaps generate UML diagrams from an XMI file. Last, but not least, we'd like to use a minimalistic, evolutionary approach, where we "do the simplest thing that could possibly work" at each stage.

Alright, so let's get started. We're creating a domain model, so we need to know what kind of objects are in our problem domain. GraphViz graphs include the ideas of nodes and edges. An edge is directed, and connects two nodes. Nodes have names, by which they can be referenced. So here is the simplest possible PEAK domain model for such a thing:

    1 from peak.api import model
    2 
    3 class Node(model.Element):
    4 
    5     class name(model.Attribute):
    6         referencedType = model.String
    7 
    8 class Edge(model.Element):
    9 
   10     class fromNode(model.Attribute):
   11         referencedType = Node
   12 
   13     class toNode(model.Attribute):
   14         referencedType = Node

Let's go through the above, piece by piece. Obviously, we have our Node and Edge classes to represent nodes and edges. What's not so obvious is why they inherit from model.Element, or what all those nested model.Attribute classes are for.