[ClusterLabs] Ansible role to configure Pacemaker

Styopa Semenukha ssemenukha at developmentgateway.org
Thu Jun 7 11:08:13 EDT 2018

Thank you for your thoughts, Jan! I agree with the importance of the 
topics you raised, and I'd like to comment on them in the light of our 
project (and configuration management approach in general).

On 06/06/2018 08:26 PM, Jan Pokorný wrote:
> On 07/06/18 02:19 +0200, Jan Pokorný wrote:
>> 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.

I can't entirely agree on this. The solution we're suggesting is built 
specifically to address this concern. In the taxonomy you linked, it 
would probably be type 2B, and here's why.

>> 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!

This situation is actually altering the rules of the game as we play. 
Configuration management is a technical solution, it was never meant to 
solve administrative (i.e. human-centered) problems. No atomicity will 
safeguard us from another admin deciding to reboot the hypervisor with 
my host. Idempotence is a relative concept, and it's relative to one 
person/entity. If I run the same playbook again, any time, any number of 
times, the result will be the same.

However, if another actor is involved, unsurprisingly, their mileage 
will vary, and so will mine. What happens if two admins add the same 
host to two Kubernetes/Heat/Ansible environments? That's the same 
situation. And I'm not even trying to solve this type of situation.

>> 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 period 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?

Correct, all these capabilities are already there, let me explain.

Firstly, as you pointed out in #1, the CIB configuration section is run 
on a single node, Ansible's `run_once` makes sure of that. Additionally 
all required changes *are* in fact batched into a single reqest: as I 
mentioned, changes are made to an XML dump, which gets verified and 
pushed to the cluster using the vendor-approved method (cibadmin --replace).

Secondly, as you suggest in #2, CIB schema has this feature built in, 
it's `admin_epoch` property. The cluster will reject XML older than the 
one it runs. And our role makes sure it gets incremented whenever 
changes are made. Therefore, if other (valid) changes have been made, 
the playbook will fail until you rerun it without conflicts. Pretty much 
like Git requiring you to rebase/merge before you push.

>> 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?

In my understanding pcs was designed to make manual configuration more 
user-friendly, not as an orchestration tool.

Anyhow, I do appreciate your opinion and agree with the overall idea on 
orchestration/configuration problem. Thank you for the insights.
Best regards,
Styopa Semenukha,
Senior IT Analyst at Development Gateway.

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