Dependent service checks don't fail when depended-on service check fails

[Andreas Ericsson]

Jarrod Moore wrote:
> On Thu, Mar 26, 2009 at 7:57 PM, Andreas Ericsson <ae at op5.se> wrote:
>> Jarrod Moore wrote:
>>> Hello everyone,
>>>
>>> I have a couple of related questions regarding service dependencies in
>>> Nagios and their limitations. I have two service checks (let's call
>>> them A and B) and service A depends on service B to function
>>> correctly. I want to set Nagios up so that if service B crashes then
>>> both services A and B are put into the critical state in Nagios. I've
>>> tried using service dependencies in Nagios to represent this behaviour
>>> but have yet to be successful. I can only get it to suppress
>>> notifications of service A if both services go down.
>>>
>> This is expected behaviour. If A is truly dependant on B, then A will
>> turn into a non-ok state of its own volition rather than as a result
>> of any dependency magic. Dependencies are designed as a means of
>> suppressing notifications. Otherwise, you would *always* get a
>> notification for B first, and a minute or so later from A (actually,
>> without the dependency you could get from A first).
>>
>>> Is there a way to do what I'm trying to do here? I'd have thought it
>>> would be logical that if a service depends on another service and the
>>> service depended on dies then all services depending on it would fail
>>> their checks as well, but there;s probably some scenario where it
>>> doesn't work so well. I've had a look through the mailing list
>>> archives and found someone had asked a similar question to the
>>> nagios-devel list about 2.5 years ago and didn't end up getting an
>>> answer, so I thought I might ask whether solutions to this type of
>>> problem had been developed since then.
>>>
>> They haven't. You're using dependencies the wrong way, really. If
>> A is truly dependent on B and doesn't go into a non-ok state after
>> B has crashed, then your check isn't doing what it's supposed to do,
>> or you've misunderstood the relationship somehow.
>>
>> If you were to explain what the two services actually are, it would
>> be easier to point you to a solution that works.
>>
>> --
>> Andreas Ericsson                   andreas.ericsson at op5.se
>> OP5 AB                             www.op5.se
>> Tel: +46 8-230225                  Fax: +46 8-230231
>>
>> Considering the successes of the wars on alcohol, poverty, drugs and
>> terror, I think we should give some serious thought to declaring war
>> on peace.
>>
> 
> Well basically I have a map (similar to Google Maps) embedded in a
> website, which hits a URL to retrieve maps. So I have one check using
> check_http to check that the website itself is up and another check on
> that URL to make sure that the map service is available. Now if the
> map service goes down, the website is still up but the maps won't
> appear, which means the website's functionality is significantly
> affected. However, it is still up and viewable so doing a check on the
> website URL still passes.
> 

It sounds to me like you'd want to make the map-check dependent on
the webserver-check. That would suppress notifications from the
map-check when it's the webserver that's bombing out. Do you really
need two notifications when the map-service goes offline?

> Now of course I could just write a script or something to check both
> URLs and set that as the check command. There is a problem for me with
> this approach, however, because I have some other instances where a
> web service depends on other web services.

Define "depend". As I understand the definition, coal-based lifeforms
on our fine planet depend on water and sunlight; Life cannot function
properly without them.
It sounds like you want to make sunlight depend on coal-based lifeforms,
because without the life, the sun is rather pointless.

Instead of trying to coerce dependencies to work backwards, I'd sit
down and think what you want your Nagios installation to do for you,
and why you would want two services to go critical when one of them
does. Isn't one notification and one red blob in the UI enough? If
it isn't, what do you hope to gain from having two notifications and
two red blobs?

> When I want to use these
> services in websites, I'd then have to write a check for each script,
> each containing every service in the chain that is needed to display
> the website correctly. This way of doing things just seems a bit
> repetitive to me, especially when I have a check for these web
> services already.

I'm sorry, but I still fail to see the point. Perhaps you'd be better
off defining each website as a servicegroup with all of the services
that make up the entire visitor-experience parts of a particular
servicegroup. That would make it possible for you to get some sort of
visualization of what (Nagios-)services affect which customer-services,
while at the same time keeping configuration work to a minimum.

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

Considering the successes of the wars on alcohol, poverty, drugs and
terror, I think we should give some serious thought to declaring war
on peace.

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