So I had a brief conversation on Twitter with Noah Sussman about testing a software system designed as a "Big Ball Of Mud" (BBOM).
We could talk about the technical definition of BBOM, but in practical terms a BBOM is a system where we understand and expect that changing one part of the system is likely to cause unknown and unexpected results in other, unrelated parts of the system. Such systems are notoriously difficult to test, but I have tested them long ago in my career, and I was surprised that Noah hadn't encountered this approach of using a "Golden Image" to accomplish that.
Let's assume that we're creating an automated system here. Every part of the procedure I describe can be automated.
First you need some tests. And you'll need a test environment. BBOM systems come in many different flavors, so I won't specify a test environment too closely. It might be a clone of the production system, or a version of prod with fewer data. It might be something different than that.
Then you need to be able to make a more-or-less exact copy of your test environment. This may mean putting your system on a VM or a Docker image, or it may be a matter of simply copying files. However you accomplish it, you need to be able to make faithful "Golden Image" copies of your test environment at a particular point in time.
Now you are ready to do some serious testing of a BBOM system using Golden Images:
Step One: Your test environment right now is your Golden Image. Make a copy of your Golden Image.
Step Two: Install the software to be tested on the copy of your Golden Image. Run your tests. If your tests pass, deploy the changes to production. Check to make sure that you don't have to roll back any of the production changes. If your tests fail or if your changes to production get rolled back, go back to Step One.
Step Three: the copy of your first Golden Image with the successful changes is your new Golden Image. You may or may not want to discard the now obsolete original Golden Image, see Step Five below.
Step Four: Add more tests for the system. Repeat the procedure at Step One.
Step Five (optional) You may want to be able to compare aspects of a current Golden Image test environment with previous versions of the Golden Image. Differences in things like test output behavior, file sizes, etc. may be useful information in your testing practice.