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From Stevo Slavić <ssla...@gmail.com>
Subject Re: [jira] [Comment Edited] (ZOOKEEPER-2169) Enable creation of nodes with TTLs
Date Mon, 01 Aug 2016 20:41:10 GMT
Imagine a stateless app in front of Kafka/ZooKeeper cluster. It handles
request to create a temporary topic. It creates temporary topic, which
results in TTL node in ZK, signals success to the caller. And then app
crashes or gets restarted or gets relocated to run on different host.
Starting it again, with no state, will have no idea about TTL node it
created before, TTL node might have died in the meantime, so no
on-TTL-node-deletion trigger gets handled and as result ZK has junk content
wasting resources.

On Mon, Aug 1, 2016 at 9:58 PM, Jordan Zimmerman <jordan@jordanzimmerman.com
> wrote:

> I think it would be simpler and less disruptive to use the existing
> Watcher mechanism. Treat it as a deleted node watcher. When the
> TTL/container node gets deleted the watcher triggers. Actually, come to
> think of it, wouldn’t that work already? Like any other node, after
> creating it you can call exists() to set a watcher. Do we really need new
> behavior here?
>
> -Jordan
>
> > On Aug 1, 2016, at 11:02 AM, Stevo Slavić <sslavic@gmail.com> wrote:
> >
> > Yes, all server side. Client that successfully creates TTL node and
> > registers such "trigger", could disconnect - trigger should fire on TTL
> > node deletion and be handled on server side only. So server watching and
> > handling event. It shouldn't happen that TTL node gets deleted and event
> > does not get handled - both deletion of TTL node and handling of a
> trigger
> > both should either succeed or fail.
> >
> > On Mon, Aug 1, 2016 at 5:37 PM, Jordan Zimmerman <
> jordan@jordanzimmerman.com
> >> wrote:
> >
> >> That’s an interesting idea - so a watcher for container expirations?
> >>
> >> -Jordan
> >>
> >>> On Aug 1, 2016, at 2:20 AM, Stevo Slavić <sslavic@gmail.com> wrote:
> >>>
> >>> Hello Apache ZooKeeper developers,
> >>>
> >>> Thinking, for a use case like support temporary topics in Apache Kafka,
> >>> which could be based on ZooKeeper TTL feature, might be useful to be
> able
> >>> to register a ZooKeeper "trigger" once TTL expires for a node - e.g. in
> >>> same transaction that deletes temporary node, create another persistent
> >>> node (request to delete the topic). Of course one could workaround
> this,
> >> by
> >>> creating persistent and TTL node, and check if there is persistent node
> >>> without matching temporary node, but option with trigger would have
> been
> >>> better/easier from consistency point of view.
> >>>
> >>> What do you think about the idea?
> >>>
> >>> Kind regards,
> >>> Stevo Slavic.
> >>>
> >>>
> >>> On Mon, Aug 1, 2016 at 3:31 AM, Raul Gutierrez Segales (JIRA) <
> >>> jira@apache.org> wrote:
> >>>
> >>>>
> >>>>   [
> >>>>
> >>
> https://issues.apache.org/jira/browse/ZOOKEEPER-2169?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15401429#comment-15401429
> >>>> ]
> >>>>
> >>>> Raul Gutierrez Segales edited comment on ZOOKEEPER-2169 at 8/1/16 1:31
> >> AM:
> >>>>
> >>
> ---------------------------------------------------------------------------
> >>>>
> >>>> [~fpj]: it's here: https://reviews.apache.org/r/46983/.
> >>>>
> >>>> cc: [~randgalt]
> >>>>
> >>>>
> >>>> was (Author: rgs):
> >>>> [~fpj]: it's here.
> >>>>
> >>>> cc: [~randgalt]
> >>>>
> >>>>> Enable creation of nodes with TTLs
> >>>>> ----------------------------------
> >>>>>
> >>>>>               Key: ZOOKEEPER-2169
> >>>>>               URL:
> >>>> https://issues.apache.org/jira/browse/ZOOKEEPER-2169
> >>>>>           Project: ZooKeeper
> >>>>>        Issue Type: New Feature
> >>>>>        Components: c client, java client, jute, server
> >>>>>  Affects Versions: 3.6.0
> >>>>>          Reporter: Camille Fournier
> >>>>>          Assignee: Jordan Zimmerman
> >>>>>           Fix For: 3.6.0
> >>>>>
> >>>>>       Attachments: ZOOKEEPER-2169-2.patch, ZOOKEEPER-2169-3.patch,
> >>>> ZOOKEEPER-2169-4.patch, ZOOKEEPER-2169-5.patch, ZOOKEEPER-2169.patch
> >>>>>
> >>>>>
> >>>>> As a user, I would like to be able to create a node that is NOT
tied
> to
> >>>> a session but that WILL expire automatically if action is not taken
by
> >> some
> >>>> client within a time window.
> >>>>> I propose this to enable clients interacting with ZK via http or
> other
> >>>> "thin clients" to create ephemeral-like nodes.
> >>>>> Some ideas for the design, up for discussion:
> >>>>> The node should support all normal ZK node operations including
ACLs,
> >>>> sequential key generation, etc, however, it should not support the
> >>>> ephemeral flag. The node will be created with a TTL that is updated
> via
> >> a
> >>>> refresh operation.
> >>>>> The ZK quorum will watch this node similarly to the way that it
> watches
> >>>> for session liveness; if the node is not refreshed within the TTL, it
> >> will
> >>>> expire.
> >>>>> QUESTIONS:
> >>>>> 1) Should we let the refresh operation set the TTL to a different
> base
> >>>> value?
> >>>>> 2) If so, should the setting of the TTL to a new base value cause
a
> >>>> watch to fire?
> >>>>> 3) Do we want to allow these nodes to have children or prevent this
> >>>> similar to ephemeral nodes?
> >>>>
> >>>>
> >>>>
> >>>> --
> >>>> This message was sent by Atlassian JIRA
> >>>> (v6.3.4#6332)
> >>>>
> >>
> >>
>
>

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