Commonality and Variability Management

Continuing the previous discussion on software product-lines ...

Central to the notion of product-lines and product-families are tracking and managing three different kinds of software assets:

• common/core assets that are shared by all the products in the product-line
• shared assets that are common to some products but not others, and ...
  
• product-specific assets (or custom-components) that are specific to a single product in the product-line.

Architecture for such product-lines is all about managing commonality and variability, and easing their evolution to achieve a diverse family of products to achieve economies of scale from reusing common assets. Change/Configuration Management for SPLs is a very challenging problem. And variability management techniques often come down to a matter of binding-times. There are also more advanced strategies (some involving mathematical models).

A few resources on the subject of Commonality and Variability are as follows:

• A good (albeit old) article on Variability Management in Software Product Lines has a nice overview of some basic variability enabling techniques and the use of binding time
• A dissertations on Variation Mechanisms and Multi-view Architecting in Platform-based Product Family Development
• Expressing Product Diversification -- Categorizing And Classifying Variability In Software Product Family Engineering
• Coplien, Weiss and Hoffman wrote a nice overview article on Commonality and Variability in Software Engineering
  
• Alan Shalloway has an excellent introductory article on Commonality and Variability Analysis in conjunction with Design Patterns.
  

In July 2006 I presented at the Dr Dobbs' Architecture & Design World conference about SCM Patterns for Agile Architectures, which included a section on managing variations. I summarized that portion of the presentation as follows:

Use Late-Binding instead of Branching:
• Build/Package Options
• Feature Configuration/Selection
• Business Rules

Think about which of the following needs to "vary" and what needs to stay the same:
• Interface vs. Implementation vs. Integration
• Container vs. Content vs. Context

Commonality & Variability analysis helps identify the core dimensions of variation for your project

Use a combination of strategies based on the different types of needed variation and the "dimension" in which each one operates