On Sun, 23 Aug 2015, Hans van der Made wrote:
Yes, you should still be looking at your logs, but I believe that what's
more critical is that you monitor the service *from the user's point of
view*, and that monitoring should reflect the users' experiences as
closely as possible. If you do that, you'll know when things are wrong
without having to look at the logs and say something like "oh, we alsl
need to add <string1> and <string2> to the 'this is an error' list."
I have to disagree: when serving 1000 users per time frame, and one of them
has a problem, there's less than 1% probability your monitoring checks will
notice, and about 100% your logging will. Depending on the number of users,
1 in 1000 might be too many.
It's actually worse than that. The servers tend to cache connection information,
so if you keep hitting it from the same source, you are more ikely to succeed
in many overload situations than if you are hitting it from a random IP
I agree you need the outside view, but don't fall into the trap of thinking it
tells you everything.
The outside probe can't tall you if you have 100 servers behind your load
balancer, all happily serving your site, or if you have 1, with 99 crashed, but
are at a low enough point of usage that that one is able to keep up with the
traffic.
I've seen failures that showed up to external monitoring that didn't show up to
internal monitoring (DNS/BGP/ISP failures for example), and it's good for
measureing average response time type of things. So you do need it. You just
also need information from inside your load balancers, and log information.
A good example of why you need to look at logs. If Apache hits maxclients, it
logs it, The log is unambiguous and unmistakable. Trying to figure out that you
are hitting max clients by probing from outside is impossible (things don't
neccessarily slow down when you hit max clients, the clients who can connect may
get good response tiems, but other clients just get errors)
David Lang
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