Legacy Code Preservation

Rule One: Writing Software is Capturing Knowledge.

Consequence: Converting Software is Preserving Knowledge.

When software is revised for a new framework or operating system or database or when an algorithm is converted to a new language, then we're "converting" (or "migrating") software. We're preserving code, and preserving the knowledge encoded.

For the next few months, I'm going to post some examples of preserving legacy code and how this ties to the knowledge capture issue.

Once we've looked at some examples of business software, we can turn to something a little less concrete: HamCalc.

These examples are presented in historical order. Each example raises questions and outlines elements of a strategy for legacy code preservation.

• What's the Story? Late 1970's. What user story was encoded in the software?
• Are There Quirks? Late 1970's. Is the encoded knowledge really a useful feature? Or is it a bug? What if we can't be sure?
• What's the Cost? Early 1980's. What if the legacy code is complex and expensive? How can we be sure it doesn't encode some valuable knowledge?
• Paving the Cowpaths. Throughout the 80's. When converting from flat-file to database, how can we distinguish between encoded user stories and encoded technical details? Isn't all code equally valuable? There are several examples; I've combined them into one.
• Data Warehouse and Legacy Operations. This is a digression on how data warehouse implementation tends to preserve a great deal of legacy functionality. Some of that legacy functionality exists in stored procedures, a programming nightmare.
• The Bugs are the Features. Can you do software preservation when user doesn't seem to understand their own use cases?
• Why Preserve An Abomination? How do we preserve shabby code? How can we separating the user stories from the quirks and bugs? There are several instances, I've used one as an example.
• How Do We Manage This? The legacy code base was so old that no one could summarize it. It had devolved to a morass of details. With no intellectual handles, how can we talk about the process of converting and what needs to be preserved?
• Why Preserve the DSL? describes a modern instance of "Test-Driven Reverse Engineering" where the unit test cases were created from the user stories and the legacy code use merely as supporting details. An entirely new application was written which preserved very little of the legacy code, but met all the user's requirements.

These nine examples include some duplicates. It's really more like a dozen individual case studies. Some are simple duplicates; the name of the customer is changed, but little else.