Johnno's Nose: Jack and Jill rewrite a system and then go on a dig

A software system is like a bucket of water.

But other than just the code and the persistence of data there's knowledge too. Knowledge about the system that is sometimes stored in documents, sometimes stored in tests but always stored with people.

I've just been watching Scott Bellware's "Beyond Agile" talk and he talks about software development of simply being knowledge transfer. A product sponsor transfers the knowledge to an analyst, who transfers to the business analyst, who transfers to the develop to tester into a product.

When we rewrite systems we will leak this knowledge. Exactly if you tried to pour a full bucket of water into another bucket or water. Chances are you'll spill a bit. Chances are the documentation is not comprehensive. Chances are the tests don't convey the meaning of the application well enough. Chances are the developers have forgotten what they were doing when they wrote that feature. Chances are the product sponsor has forgotten exactly why they wanted it 'that' way.

When system stakeholders leave the project the bucket springs a leak. You have lost some of that knowledge. Even with comprehensive documentation, tests and a full handover. And your bucket is probably not a bucket any more. What once was a bucket is now probably a kettle that does credit card payments... and its rusty but only on a Thursday.

So how do I summarise this leaky abstraction? (go on groan)

A software system is is more than just code. It is the users, the stakeholders, the delivery team and the state of the system. When rewriting the system it is going to be impossible to relive the journey of knowledge accumulation that the team went through to get to the system that you are rewriting. Even though there will be a set of artefacts you will have discovered to reconstruct the system this will not be the full picture. That's ok because you only need to do what your stakeholder's need now. Reconstructing 100% of the old system when no one knows what 100% is wasteful (and impossible)

Rewriting software is like an archaeological dig.