On Tue, 21 Apr 2015 16:52:39 -0700 Avery Payne <avery.p.pa...@gmail.com> wrote:
> Dependency handling "with guarantee" means there is some means by > which the child service itself signals "I'm fully up and running", > vs. "I'm started but not ready". Because there is no polling going > on, this allows the start-up of the parent daemon to sleep until it > either is notified or times out. And you get a "clean" start-up of > the parent because the children have directly signaled that "we're > all ready". > > Dependency handling "without guarantee" is what my project does as an > optional feature - it brings up the child process and then calls the > child's ./check script to see if everything is OK, which is polling > the child (and wasting CPU cycles). This is fine for "light" use > because most child processes will start quickly and the parent won't > time out while waiting. I really like the ./check method. I don't need to make any assumptions about the startup behavior of the the dependency. I have all sorts of ./run scripts in my daemontools looking like: if ! ping -q -c1 8.8.8.8 > /dev/null; then echo failed ping test on $pingaddr sleep 3 echo "Bailing til next time" exit 1 fi I personally get to choose what I consider to be "operational" for a dependency. > There are trade-offs for using this > feature. First, ./check scripts may have unintended bugs, behaviors, > or issues that you can't see or resolve, unlike the child directly > signalling that it is ready for use. Second, the polling approach > adds to CPU overhead, making it less than ideal for mobile computing > - it will draw more power over time. Third, there are edge cases > where it can make a bad situation worse - picture a heavily loaded > system that takes 20+ minutes to start a child process, and the > result being the parent spawn-loops repeatedly, which just adds even > more load. I think the three tradeoffs you listed above are three variations on the same theme: One or more dependency services take too long to start. My first thought is whether you want a service like that on your computer at all. SteveT Steve Litt April 2015 featured book: Twenty Eight Tales of Troubleshooting http://www.troubleshooters.com/28
