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From Cameron McKenzie <mckenzie....@gmail.com>
Subject Re: How to obtain stable leader election over unstable ZK connections
Date Fri, 21 Aug 2015 02:09:26 GMT
I think that could be a good way forward.

It will require some careful thought about which situations it is OK for a
recipe to continue operating in the SUSPENDED state.

I have implemented something similar for stuff at work.

On Fri, Aug 21, 2015 at 3:14 AM, Jordan Zimmerman <
jordan@jordanzimmerman.com> wrote:

> So, should we change CURATOR-246 into a larger issue for supporting a true
> SESSION_LOST state as well as pluggable error handling? If so, it would be
> nice to have this for 3.0
>
> -Jordan
>
>
>
> On August 20, 2015 at 11:19:21 AM, Scott Blum (dragonsinth@gmail.com)
> wrote:
>
> Assuming that clocks are usually not too out of step, Curator should be
> able to infer when the server would have terminated the existing session
> based on the clock.  A little bit of thought would need to be put into
> resolving the race condition when you reconnect right as you were about to
> time out, in order to present a unified view of the state change, but that
> doesn't seem infeasible.  This seems like exactly the kind of problem
> Curator should be solving.
>
> On Thu, Aug 20, 2015 at 11:21 AM, Jordan Zimmerman <
> jordan@jordanzimmerman.com> wrote:
> Yeah, in hindsight LOST isn’t useful which is why all the recipes refer to
> SUSPENDED. Having a session-expired state is complicated in Curator as
> Curator sometimes re-creates the connection without a ZK generated event.
> So, the SESSION lost would have to be inferred.
>
> -Jordan
>
>
>
> On August 20, 2015 at 10:13:19 AM, Scott Blum (dragonsinth@gmail.com)
> wrote:
>
> Ahh... that is confusing, and seems dubiously useful.  I think 99% of the
> time I'd rather get an event that represents that the session is definitely
> lost.
>
> On Thu, Aug 20, 2015 at 10:53 AM, Jordan Zimmerman <
> jordan@jordanzimmerman.com> wrote:
> Maybe I'm confused, but I thought that's what ConnectionState SUSPENDED
> vs.
> LOST was all about?
> It’s a big source of confusion with Curator. LOST does _not_ mean the
> session was lost. It means Curator has given up after retries, etc. Because
> Curator re-creates ZK handles internally the notion of a “session” is more
> complicated than using raw ZooKeeper.
>
>
>
> -Jordan
>
>
>
>
>
> On August 20, 2015 at 9:50:56 AM, Scott Blum (dragonsinth@gmail.com)
> wrote:
>
> Maybe I'm confused, but I thought that's what ConnectionState SUSPENDED vs.
> LOST was all about?
>
> Maybe the recipes just need to be tweaked a bit?
>
> I always assumed emphemeral nodes would be gone on LOST but not gone if you
> get a SUSPENDED followed by RECONNECTED.
>
> The one question I've always wondered is what happens to Watchers on
> SUSPENDED, do they all need to be re-applied, or will they still fire later
> as long as you don't get LOST?
>
> On Thu, Aug 20, 2015 at 10:41 AM, Jordan Zimmerman <
> jordan@jordanzimmerman.com> wrote:
>
> > I wonder if we can add error handling policies to Curator. Currently, the
> > policy of all recipes is hard-coded to treat SUSPENDED as a type of lost
> > session. We could change this to be injected like the retry policy. To
> > solve this particular issue we’d also need to introduce a SESSION_LOST
> > state of some type. This is complicated as Curator re-creates connections
> > internally.
> >
> > Thoughts?
> >
> > -Jordan
> >
> >
> >
> > On August 20, 2015 at 2:10:52 AM, Dong Lei (donglei@microsoft.com)
> wrote:
> >
> > Hi curator-devs:
> >
> > We use Spark in standalone mode in which Spark leverage curator to manage
> > ZK connections and elect leader. Our Zookeeper may be not very stable and
> > we get "session suspended and reconnected" sometimes. The problem is that
> > this kind of disassociated and reconnected triggers leader election quite
> > often. And Spark's reaction to leadership switching can be very costly.
> >
> > So I'm thinking about whether it's possible to tolerate such failure
> cases
> > if we can reconnect soon and the session is actually kept after the
> > reconnection?
> > Or does such a requirement makes sense to you?
> >
> > Any advice will be appreciated.
> >
> >
> > Thanks
> > Dong Lei
> >
> >
>
>
>

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