Compatibility Notes for rsyslog v5
Written by Rainer Gerhards (2009-07-15)
The changes introduced in rsyslog v5 are numerous, but not very intrusive. This document describes things to keep in mind when moving from v4 to v5. It does not list enhancements nor does it talk about compatibility concerns introduced by earlier versions (for this, see their respective compatibility documents).
The $HUPisRestart directive is supported by some early v5 versions, but has been removed in 5.1.3 and above. That means that restart-type HUP processing is no longer available. This processing was redundant and had a lot a drawbacks. For details, please see the rsyslog v4 compatibility notes which elaborate on the reasons and the (few) things you may need to change.
Please note that starting with 5.8.11 HUP will also requery the local hostname.
Queue on-disk format
The queue information file format has been changed. When upgrading from v4 to v5, make sure that the queue is emptied and no on-disk structure present. We did not go great length in understanding the old format, as there was too little demand for that (and it being quite some effort if done right).
Queue Worker Thread Shutdown
Previous rsyslog versions had the capability to "run" on zero queue worker if no work was required. This was done to save a very limited number of resources. However, it came at the price of great complexity. In v5, we have decided to let a minium of one worker run all the time. The additional resource consumption is probably not noticable at all, however, this enabled us to do some important code cleanups, resulting in faster and more reliable code (complex code is hard to maintain and error-prone). From the regular user's point of view, this change should be barely noticable. I am including the note for expert users, who will notice it in rsyslog debug output and other analysis tools. So it is no error if each queue in non-direct mode now always runs at least one worker thread.