Problem Determination Fallacies: Migration
kgibm 0600027VAP Visits (1997)
A common aspect to a problem is that an application worked and then the environment (WAS, etc.) was upgraded and the application stopped working. Many customers then say, "therefore, the product is the root cause." It is easy to show that this is a fallacy (neither necessary nor sufficient) with a real world example: A recent customer upgraded from WAS 6.1 to WAS 7 without changing the application and it started to throw various exceptions. It turned out that the performance improvements in WAS 7 and Java 6 exposed existing concurrency bugs in the application.
It is not wrong to bring up the fact that a migration occurred. In fact, it's critical that you do. This is important information, and sometimes helps to quickly isolate a problem. However, I often hear people make the argument that the fact of the migration is the key aspect to the problem. This may or may not be true, but what it does do is elevate the technique of isolation above analysis, which is often a time-consuming mistake.
Analysis is the technique of creating hypotheses based on observed symptoms, such as exceptions, traces, or dumps. In the above example, the customer experienced java
Isolation is the technique of looking at the end-to-end system instead of particular symptoms, and simplifying or eliminating components until the problem is isolated, either by the process of elimination, or by finding the right symptoms to analyze. Saying that the migration is the key aspect to the problem is really saying that the first step is to understand what changed in the migration and then use that to isolate which changed component caused the problem. As the above example demonstrates, changes such as performance improvements may have unknown and unpredictable effects, so isolation may not help.
In general, I recommend you always start with analysis instead of isolation. You should certainly bring up any changes that occurred right before the problem (migration, etc.), but be careful where this leads everyone. If analysis leads to a dead end, that's when I start to use isolation, including comparing changes, but even in this case, comparing product versions is difficult; many things change.