Q:

TDD: possible to bootstrap when enhancing existing large app?

Chapter about TDD from Martin's "Clean Code" caught my imagination.   
However.
These days I am mostly expanding or fixing large existing apps.
TDD, on the other hand, seems to only be working only for writing from scratch.  
Talking about these large existing apps:
1. they were not writted with TDD (of course).
2. I cannot rewrite them.
3. writing comprehensive TDD-style tests for them is out of question in the timeframe.  
I have not seen any mention of TDD "bootstrap" into large monolite existing app.
The problem is that most classes of these apps, in principle, work only inside the app.
They are not separable. They are not generic. Just to fire them up, you need half of the whole app, at least. Everything is connected to everything.
So, where is the bootstrap ?
Or there is alternative technique with results of TDD
that'd work for expanding the existing apps that were not developed with TDD ?

A:

The bootstrap is to isolate the area you're working on and add tests for behavior you want to preserve and behavior you want to add.  The hard part of course is making it possible, as untested code tends to be tangled together in ways that make it difficult to isolate an area of code to make it testable.
Buy Working Effectively with Legacy Code, which gives plenty of guidance on how to do exactly what you're aiming for.
You might also want to look at the answers to this related question, Adding unit tests to legacy code.

