English translation of first and only chapter of last part 4 "Techniques, used for debugging of Production applications" just published.

Chapter 11. Techniques, used for debugging of Production applications.

Unfortunately is not always possible to find error while testing. Often they only happen when high load.


How do you know about such problems?


One of most important sources of information about problems is error
log file. There you can find information such as server crashes,
connection errors (if option log-warnings=2 turned on), about options
which were specified in the configuration file, but had not turned on
because error and some others. There is a rule for working with error
log file: if something unexpected happens check error log first. Error
log file also contains information about server errors which are not
accessible for clients. Therefore is better to have it always turned on
even if you do logging at application level.

...

Rest of the chapter is here.