I asked vkmc about logging in OpenStack since she wrote a nice post about it: http://vmartinezdelacruz.com/logging-and-debugging-in-openstack/
It's a good question... logs are often used when things go wrong
For instance, I worked on Horizon, and it was useful to me to check the Horizon screen logs when I got HTTP or Django errors
Usually the error you get in the browser lacks of details needed for you to locate where the problem is
So, complementing it with the screen log (which is simply the output you get when you run the component from the console), you can get an idea of what is happening and try to fix it
It's also useful to check the interaction between components
For example, I launch an instance in Horizon, and I can see how Horizon makes a call to Nova and it creates the new instance
If something goes wrong in that chain, you can obtain the precise information from the logs
The Ceilometer logs are saved in /var/log/ceilometer-api/. In that directory,
there is a file /var/log/ceilometer-api/ceilomter-api.log. It appears to
contain configuration details about the Ceilometer server started at a certain
time. The last line tells you where the Ceilometer server is located on the
network.
Here's an example: https://gist.github.com/terriyu/6208350
In this case, the Ceilometer server is
serving on 0.0.0.0:8777, view at http://127.0.0.1:8777
vkmc noted that Horizon used Mox for testing and now they are moving to use Mock (which is similar to Mox and easier/more intuitive according to vkmc). I noted that Ceilometer and Nova use testr.
jpich later told me that the reason to move to testr is that it takes too much time to run the tests right now (especially for Nova). testr allows you to run tests in parallel, which will save a lot of time.
jd dropped in on the discussion and told me to check
/etc/ceilometer/ceilometer.conf for configuration options including logging
He said he doesn't use logging much, only when he's running the application in a real deployment. Most of the time, he runs unit tests for which logging isn't useful.