A new v5 version of rsyslog will be released today. Originally, I did not plan to start the v5 version before the end of the year (2009). But then we received sponsorship to enhance queue performance. And then we saw that an audit-grade queue subsystem was needed (audit-grade means that no message is ever lost, not even in fatal failure cases like sudden power loss).
Especially the audit-grade queue subsystem resulted in very large design changes to the queue engine. Their magnitude is so large that I assume we need some time to stabilize it. Thus, I have decided to start a new v5 branch, which will feature the redesigned queue engine.
When we introduced the queue engine in early 2008 (in rsyslog v3), it took roughly three to five month until it got decently stable. With the magnitude of changes we have done now, it will probably take some time, again. It depends a bit on the actual feedback we receive from practice. Also, this time I have added lots of automated tests, so a lot of bugs should already have been caught. Also, during the next weeks I will focus on actual deployment scenarios, rather than things that theoretically may happen (the testbench covers many of those). So, all in all, I expect that the new queue engine will become production-ready faster than the v3 engine.
Still, I think it is desirable to create a new major version branch for this change. So here we are, at v5. I will continue to develop functionality that does not necessarily need the new queue engine inside the v4-devel. That way, we will have this functionality available both with the proven queue engine as well as with the new experimental one. Note that I can not do this with a stable branch: per definition, stable branches never receive enhancements (as that would potentially destabilize the branch). So, for the time being and probably a couple of month, we will have two development branches: the v4 as well as the v5 branch. With that v5 will focus on the new queue engine plus any other additions, which are done in v4.