[ClusterLabs] Ansible role to configure Pacemaker

Jan Pokorný jpokorny at redhat.com
Wed Jun 6 20:19:16 EDT 2018

On 06/06/18 15:51 -0400, Styopa Semenukha wrote:
> We wrote a role to configure Pacemaker clusters, and I'd like to share
> it with the community. Any questions or comments welcome.

Hello and thanks for the announcement.

And now, something I've meant to write down regarding configuration
management SW vs. cluster for quite some time; it just happens to be
provoked by the topic, so there's nothing to be taken personally
(good piece of work has been done with project if I can judge)

* * *

While I see why Ansible is compelling, I feel it's important to
challenge this trend of trying to bend/rebrand _machine-local
configuration management tool_ as _distributed system management tool_
(pacemaker is distributed application/framework of sorts), which Ansible
alone is _not_, as far as I know, hence the effort doesn't seem to be
100% sound (which really matters if reliability is the goal).

Once more, this has nothing to do with the announced project, it's
just the trending fuss on this topic that indicates me that people
independently, as they keenly invent their own wheel (here: Ansible
roles), get blind to the fallacy everything must work nicely with
multi machine shared-state scenarios like they are used to with
single host bootstrapping, without any shortcomings.

But there are, and precisely because not the optimal tool for the
task gets selected!  Just imagine what would happen if a single
machine got configured independently with multiple Ansible actors
(there may be mechanisms -- relatively easy within the same host --
that would prevent such interferences, but assume now they are not
strong enough).  What will happen?  Likely some mess-ups will occur as
glorified idempotence is hard to achieve atomically.  Voila, inflicted
race conditions, one by one, get exercised, until there's enough of
bad luck that the rule of idempotence gets broken, just because of
these processes emulating a schizophrenic (at the same time
multitasking) admin.  Ouch!

Now, reflect This to the situation with possibly concurrent
cluster configuration.  One cannot really expect the cluster
stack to be bullet-proof against these sorts of mishandling.
Single cluster administrator operating at a time?  Ideal!
Few administrators presumably with separate areas of
configuration interest?  Pacemaker is quite ready.
Cluster configuration randomly touched from random node
at random time (equivalent of said schizophrenic multitasking
administrator with a single host)?  Chances are off in
sufficiently long perioud when this happens.

The solution here is to break that randomness, configuration
is modified either:
1. from a single node at a time in the cluster (plus preferrably
   batching all required changes into a single request)
2, mutual time-critical exclusion of triggering the changes
   across the nodes
3. mutual locality-critical exclusion in the subject of the
   changes initiated from particular nodes

Putting 1. and 3. aside as not very interesting (1. means
a degenerate case with single point of failure, and 3. kills
the universality), what we get is really a dependency on some
kind of distributed lock and/or transactional system.
Well, we have just discovered that what we need to automate our
predestined configuration in the cluster reliably and without
hurting universality (like "breaking the node symmetry") is
said distributed system management ("orchestration") tool.
Has Ansible these capabilities?

Now, one idea there might be to make the tools like pcs compensate
for these shortcomings of machine-local configuration management ones.
Sounds good, right?  Absolutely not, more like a bad joke!
Because what else can it be, the development of orchestration-like
features (with all the complexities solved once in corosync/DLM
already; relaxing non-dependency on the very subject of management
may not be wise) on top of regular high-level cluster management tool
only[*] to bridge the gap in something that is simply subpar fit
in distributed environments to begin with?

As Czech proverb puts it: think twice, act once.

[*] non-automated/human-triggered usage is generally fine as it's
    highly unlikely none of 1.-3. would be satisfied, so there
    would be next to no gain for these workflows

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