[ClusterLabs] Best way to create a floating identity file

[Ken Gaillot]

On Wed, 2020-12-16 at 04:46 -0500, Tony Stocker wrote:
> On Tue, Dec 15, 2020 at 12:29 PM Ken Gaillot <kgaillot at redhat.com>
> wrote:
> > 
> > On Tue, 2020-12-15 at 17:02 +0300, Andrei Borzenkov wrote:
> > > On Tue, Dec 15, 2020 at 4:58 PM Tony Stocker <
> > > akostocker at gmail.com>
> > > wrote:
> > > > 
> > 
> > Just for fun, some other possibilities:
> > 
> > You could write your script/cron as an OCF RA itself, with an
> > OCF_CHECK_LEVEL=20 monitor doing the actual work, scheduled to run
> > at
> > whatever interval you want (or using time-based rules, enabling it
> > to
> > run at a particular time). Then you can colocate it with the
> > workload
> > resources.
> > 
> > Or you could write a systemd timer unit to call your script when
> > desired, and colocate that with the workload as a systemd resource
> > in
> > the cluster.
> > 
> > Or similar to the crm_resource method, you could colocate an
> > ocf:pacemaker:attribute resource with the workload, and have your
> > script check the value of the node attribute (with attrd_updater
> > -Q) to
> > know whether to do stuff or not.
> > --
> 
> All three options look interesting, but the last one seems the
> simplest. Looking at the description I'm curious to know what happens
> with the 'inactive_value' string. Is that put in the 'state' file
> location whenever a node is not the active one? For example, when I
> first set up the attribute and it gets put on the active node
> currently running the resource group with the 'active_value' string,
> will the current backup node automatically get the same 'state' file
> created with the 'inactive_value'? Or does that only happen when the
> resource group is moved?
> 
> Secondly, does this actually create a file with a plaintext entry
> matching one of the *_value strings? Or is it simply an empty file
> with the information stored somewhere in the depths of the PM config?

The state file is just an empty file used to determine whether the
resource is "running" or not (since there's no actual daemon process
kept around for it).

> Finally (for the moment), what does the output of 'attrd_updater -Q'
> look like? I need to figure out how to utilize the output for a cron
> 'if' statement similar to the previous one:
> 
> if [ -f /var/local/project/cluster-node ] && [ `cat
> /var/local/project/cluster-node` = "distroserver" ]; then ...

First you need to know the node attribute name. By default this is
"opa-" plus the resource ID but you can configure it as a resource
parameter (name="whatever") if you want something more obvious.

Then you can query the value on the local node with:

    attrd_updater -Q -n <attribute-name>

It's possible the attribute has not been set at all (the node has never
run the resource). In that case there will be an error return and a
message on stderr.

If the attribute has been set, the output will look like

    name="attrname" host="nodename" value="1"

Looking at it now, I realize there should be a --quiet option to print
just the value by itself, but that doesn't exist currently. :) Also, we
are moving toward having the option of XML output for all tools, which
is more reliable for parsing by scripts than textual output that can at
least theoretically change from release to release, but attrd_updater
hasn't gained that capability yet.

That means a (somewhat uglier) one-liner test would be something like:

  [ "$(attrd_updater -Q -n attrname 2>/dev/null | sed -n -e 's/.* value="\(.*\)".*/\1/p')" = "1" ]

That relies on the fact that the value will be "1" (or whatever you set
as active_value) only if the attribute resource is currently active on
the local node. Otherwise it will be "0" (if the resource previously
ran on the local node but no longer is) or empty (if the resource never
ran on the local node).

> since the cron script is run on both nodes, I need to know how the
> output can be used to determine which node will run the necessary
> commands. If the return values are the same regardless of which node
> I
> run attrd_updater on, what do I use to differentiate?
> 
> Unfortunately right now I don't have a test cluster that I can play
> with things on, only a 'live' one that we had to rush into service
> with a bare minimum of testing, so I'm loath to play with things on
> it.
> 
> Thanks!
-- 
Ken Gaillot <kgaillot at redhat.com>