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Right.. Maybe this was not a good suggestion. The thought was that in 
small-scale deployments, requiring ZK might be a significant management 
overhead. While for large-scale ones it would be more acceptable. So, the 
question is how to make this work reasonably well on small scale without 
ZK, and enable flexible scale-up/out by adding ZK.
Maybe assume one conductor that would serialize (some of the) tasks and 
keep enough state in DB for failure recovery if there is no ZK, and do it 
in a more scalable & resilient manner if ZK is present.

Alex


Joshua Harlow <harlowja at yahoo-inc.com> wrote on 02/05/2013 08:50:29 PM:

> From: Joshua Harlow <harlowja at yahoo-inc.com>
> To: OpenStack Development Mailing List <openstack-
> dev at lists.openstack.org>, Alex Glikson/Haifa/IBM at IBMIL, 
> Date: 02/05/2013 08:50 PM
> Subject: Re: [openstack-dev] Nova workflow management update
> 
> So this brings up an interesting issue, the reason ZK exists or 
> partially exists is that something like ZK api's over DB weren't 
> really possible. Or that?s what I thought :-P
> 
> Or they weren't possible in a accurate and provable accurate manner.
> So spending time creating apis that 'sorta' work with DB's but 
> really don't (since ZK wouldn't exist if it was possible) does seem 
> sorta awkward. I thought most cloud providers are already using 
> zookeeper, for these exact same reasons, and deploying it now-adays 
> is pretty simple? 
> 
> I just worry about providing API's that really don't work correctly 
> with DB's that will cause more bugs (since certain problems just 
> can't be done with a DB, or at least any of the DBs that I have 
> used, maybe db2 can, idk) that we will have to say 'oh ya we know 
> that doesn't work with a DB'. But maybe that is a compromise that we
> have to make and is a evolutionary process where the amount of bugs 
> that will be caused by DB impls will eventually just cause people to
> move to the more attractive ZK backend? Its also sorta concerning 
> that those types of DB like bugs will be harder than heck to trace 
> down, but that might be a different issue that we can resolve.
> 
> From: Alex Glikson <GLIKSON at il.ibm.com>
> Reply-To: OpenStack Development Mailing List <openstack-
> dev at lists.openstack.org>
> Date: Thursday, May 2, 2013 7:53 AM
> To: OpenStack Development Mailing List 
<openstack-dev at lists.openstack.org>
> Subject: Re: [openstack-dev] Nova workflow management update
> 
> Changbin Liu <changbin.liu at gmail.com> wrote on 02/05/2013 05:32:05 PM:
> 
> > Subject: Re: [openstack-dev] Nova workflow management update 
> > 
> > Hi Joshua, 
> > 
> > Just to share some thoughts: 
> [...]
> 
> Using ZK makes a lot of sense.. The problem is that making ZooKeeper
> a mandatory component to support even basic workflow management 
> might be an issue. So, the approach which seems to make most sense 
> is to define abstract internal interfaces for the various 
> capabilities that ZK can provide (distributed locking, leader 
> election, etc), and then have one or more implementations (one of 
> which might be based on ZK). This is the approach that has been 
> taken for the membership service (service group monitoring APIs) -- 
> introducing the flexibility to use ZK backend, but keeping the 
> default to be DB-backed.
> 
> Regards, 
> Alex 
> 
> P.S. thinking about this.. would it be possible to implement ZK APIs
> over a DB? with some limitations, perhaps..
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