[ClusterLabs] Pacemaker with Zookeeper??

[Jan Friesse]

Ken Gaillot napsal(a):
> On 05/17/2016 09:54 AM, Digimer wrote:
>> On 16/05/16 04:35 AM, Bogdan Dobrelya wrote:
>>> On 05/16/2016 09:23 AM, Jan Friesse wrote:
>>>>> Hi,
>>>>>
>>>>> I have an idea: use Pacemaker with Zookeeper (instead of Corosync). Is
>>>>> it possible?
>>>>> Is there any examination about that?
>>>
>>> Indeed, would be *great* to have a Pacemaker based control plane on top
>>> of other "pluggable" distributed KVS & messaging systems, for example
>>> etcd as well :)
>>> I'm looking forward to joining any dev efforts around that, although I'm
>>> not a Java or Go developer.
>>
>> Part of open source is the freedom to do whatever you want, of course.
>>
>> Let me ask though; What problems would zookeeper, etcd or other systems
>> solve that can't be solved in corosync?
>>
>> I ask because the HA community just finished a multi-year effort to
>> merge different projects into one common HA stack. This has a lot of
>> benefits to the user base, not least of which is lack of confusion.
>>
>> Strikes me that the significant time investment in supporting a new
>> comms layer would be much more beneficially spent on improving the
>> existing stack.
>>
>> Again, anyone is free to do whatever they want... I just don't see the
>> motivator personally.
>>
>> digimer
>
> I see one big difference that is both a strength and a weakness: these
> other packages have a much wider user base beyond the HA cluster use
> case. The strength is that there will be many more developers working to
> fix bugs, add features, etc. The weakness is that most of those

This is exactly what I was thinking about during 2.x developement. If 
replacement of Corosync wouldn't make more sense than continue 
developing of Corosync. I was able to accept implementing some features. 
Sadly, there was exactly ONE project which would be able to replace 
corosync (Spread toolkit) which is even less widespread than Corosync.

 From my point of view, replacement of corosync must be (at least) able to:
- Work without quorum
- Support 2 node clusters
- Allow multiple links (something like RRP)
- Don't include SPOF (so nothing like configuration stored on one node 
only and/or different machine on network)
- Provide EVS/VS
- Provide something like qdevice

Both zookeeper and etcd builds on top of quite simple to understand 
membership mechanism (zookeeper = elected master, something like amoeba, 
etcd = raft), what's nice, because it means more contributors. Sadly 
bare metal HA must work even in situations where "simple" quorum is not 
enough.


> developers are ignorant of HA clustering and could easily cause more
> problems for the HA use case than they fix.
>
> Another potential benefit is the familiarity factor -- people are more
> comfortable with things they recognize from somewhere else. So it might
> help Pacemaker adoption, especially in the communities that already use
> these packages.
>
> I'm not aware of any technical advantages, and I wouldn't expect any,
> given corosync's long HA focus.
>
>>>>  From my point of view (and yes, I'm biased), biggest problem of Zookeper
>>>> is need to have quorum
>>>> (https://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html#sc_designing).
>>>> Direct consequence is inability to tolerate one node failure in 2 node
>>>> cluster -> no 2 node clusters (and such deployment is extremely
>>>> popular). Also Corosync can operate completely without quorum.
>>>>
>>>> Regards,
>>>>    Honza
>>>>
>>>>>
>>>>> Thanks for your help!
>>>>> Hai Nguyen
>
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