[Pacemaker] Querying resource status programatically

[Brian Campbell]

Hi all!

I'm writing a domain-specific frontend for Pacemaker, which can set up
a few different pre-configured "stacks" of resources, and provide
simplified monitoring and administration of those stacks.

One thing I'm wondering about is the correct way to query for the
current status of a particular resource. It appears that using
"crm_mon --as-xml" should give me the information I need in the
<resources> section:

    <resources>
        <resource
id="editshare.stack.d11f9ebd-9f1b-4547-a7f2-ab92caab1450.ip"
resource_agent="ocf::heartbeat:IPaddr2" role="Started" active="true"
orphaned="false" managed="true" failed="false" failure_ignored="false"
nodes_running_on="1" >
            <node name="lambda-es7-cluster1" id="1" cached="false"/>
        </resource>
    </resources>

However, I could not find any specification for the schema supported
by crm_mon, so I'm wondering if this format can be considered stable.

It looks like it may be possible to parse this information out of the
CIB, but that involves the "Here be dragons" status section, and would
involve walking through the history to track the state of the resource
up to the present time. It seems like that would be possible by
walking through the history, checking that the last operations on a
node were a "start" or "promote" with the appropriate exit code,
followed by "monitor" operations with the appropriate exit code. But
that seems like a bit of a roundabout way of determining the status.

There is also the crm_resource command, but that seems to provide
human-readable output, even if I use the XML output options, not
parseable output. I could do custom parsing of the output of that, but
that seems fragile.

Taking a look at what crm_resource and crm_mon actually do to query
the state, they seem to be connecting via a libqb RPC connection,
which is not a terribly convenient thing or safe thing to do from my
code.

Taking a look at pcs, it looks like that does query both crm_mon for
status, and cibadmin for the configuration state, so that seems to
heavily imply that's the right way to query this information.

So is querying crm_mon the most authoritative way to query a resource
state? Tracking the state in the status section of the CIB as
retrieved from cibadmin? Parsing crm_resource? Or some other way that
I'm missing?

-- Brian