And just to clarify slightly, there aren't really "null values" in
prometheus. A query like "node_blah" returns a *vector* of results, that
is, a variable number of values. e.g.
[
node_blah{instance="foo"} 123
node_blah{instance="bar"} 456
node_blah{instance="baz"} 789
]
If node "baz" goes down, then a query at a later point in time may return
[
node_blah{instance="foo"} 124
node_blah{instance="bar"} 457
]
If you want to test for this specific condition, i.e. there is no
"node_blah" metric present for a specific instance "baz", then you can form
a rather awkward join query using absent() in conjunction with the "up"
metric as Stuart described.
But usually, you just want to query the "up" metric itself.
On Wednesday, 20 July 2022 at 09:38:58 UTC+1 Stuart Clark wrote:
> On 20/07/2022 08:49, BHARATH KUMAR wrote:
>
> Hello all,
>
> I installed node exporters on many servers (around 300). Few of the
> servers are unreachable. So because of that, we are unable to get the CPU,
> and memory values of those servers.
>
> Now I want to add a filter in the Grafana dashboard to check the least CPU
> used, most CPU used servers. But due to unreachability, we are not getting
> values for a few servers.
>
> My question is
> "*how to compare the output of the Prometheus query is NULL"*
>
> Generally, I am comparing the output of the prom query like
> I) if the CPU usage is less than 10% then I am comparing like
> query >=0<=10%
> ii) if the CPU usage is greater than 10% and less than 30% then I am
> comparing like
> query >10<=30
> *similarly how to check the null values using the Prometheus query.*
>
> For servers which can't be scraped there will be no metrics, so any
> queries won't have any data to query.
>
> However Prometheus itself creates certain metrics for all scrape targets,
> including one called "up" which is either 0 or 1 - where 0 means the scrape
> failed. You can therefore create dashboards and alerts that list the
> servers which aren't accessible (up == 0).
>
> --
> Stuart Clark
>
>
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