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    <p><strong>How do you handle large changes?</strong></p> <p>As small as needed.</p> <p>Sometimes refactorings have a big surface but are trivial in detail. These can be done in quite big steps. Putting too much effort in trying to break them down will be waste.</p> <p>I would argue that a XML library change is in this category. You're putting XML in and get some representation out. As long as your representation does not change (from a graph representing the state to a event stream) the library switch is easy.</p> <p>Most of the time refactorings are not trivial and have to be broken down. The problem is when to do bigger steps and when the smaller ones. My observation is that I'm quite bad at estimating the impact of a change. Most software is complicated enough that you may think they change is easily manageable but then there is the whole fine print that has to work again. So I do start with an amount of change. But I'm prepared to rollback everything if it starts to get unpredictable. I would say this happens in one out of ten refactorings. But this one will be hard. You have to track down the part of the system that does not behave as you expect. The problem has to be split now in multiple smaller problems. I do solve one problem at a time and check in when it's done. (Multiple iterations of revert and splitting are not uncommon.)</p> <p>If you change the XML parser and representation in your code this should be definitely be at least two separate refactorings. </p> <p><strong>Mock testing</strong></p> <p>You're testing a communication protocol between objects/layers with mock objects.</p> <p>The whole mock approach can be though of as a communication model like the <a href="http://en.wikipedia.org/wiki/OSI_model" rel="nofollow noreferrer">OSI model</a>. When layer X gets as call with parameter x it will call layer Z with parameters a and b. Your test specifies this communication protocol.</p> <p>As useful as mock test can be, test as few functionality with them as possible. The best option are state-based tests: setup fixture, call system under test, check state of system under test and pure functional (as in functional programming) tests: call with x returns a.</p> <p>Try to design your system in a way that most of its functionality is loosely coupled. Some of the functionality has to be tested with mock tests (a fully decoupled system is useless).</p> <p>Integration tests are no option to test your system. They should only be used to test aspects of the system that can break with the integration of multiple units. If you try to test your system with integration tests you'll enter into the permutation casino.</p> <p>So your strategy for GUI testing should be clear. The parts of the GUI code that cannot be tested in isolation should be tested with a mock tests (when this button is pressed service X is called with parameter y).</p> <p>Databases muddy the water a bit. You cannot mock a database, unless you're going to reimplement the behavior of every database you would like to support. But this is not a unit test as you're integrating an external system. I've made peace with this conceptional problem and think of the DAO and database as one inseparable unit that can be tested with a state-based test approach. (Sadly, this unit behaves differently when it has its oracle day compared to its mysql day. And it may break in the middle and tell you that it cannot talk to itself.)</p>
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