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Date: Tue, 22 Aug 2017 13:56:00 +0000 (UTC)
From: "Dan Benediktson (JIRA)" <jira@apache.org>
To: dev@zookeeper.apache.org
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Subject: [jira] [Commented] (ZOOKEEPER-2879) Adding observers dynamically
 without server id
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    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16136823#comment-16136823 ] 

Dan Benediktson commented on ZOOKEEPER-2879:
--------------------------------------------

[~hanm] No, local sessions are not a sufficient mitigation for the problems introduced by this proposed feature:
- Local sessions can be upgraded to a global session at any time by creating an ephemeral node; basically, enabling local sessions across an ensemble does not guarantee that all sessions are local. If one of them upgrades, for instance, what happens to the other one?
- Moreover, I believe this proposed feature would let you accidentally reuse a local session across two members of the same ensemble. Imagine two sessions connect to different members and get the same session ID: now, one of them disconnects and tries to reconnect, and lands on the second server (this is possible: local sessions are purely a server-side concept, so in practice clients with a local session DO try to reconnect to another server, even though it is guaranteed to fail). I expect it would succeed, at which point there are would be two connections for the same local session, one of which actually originated from another node. I don't know precisely what this would entail, but it's probably also undesirable.

> Adding observers dynamically without server id
> ----------------------------------------------
>
>                 Key: ZOOKEEPER-2879
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2879
>             Project: ZooKeeper
>          Issue Type: Improvement
>          Components: quorum
>    Affects Versions: 3.6.0
>            Reporter: Fangmin Lv
>            Assignee: Fangmin Lv
>
> Dynamic config requires observer has unique server id, which means we cannot simply add observer with dynamic server id -1. For large observer cluster, it's much more easier to add observer without unique server id if it doesn't need to be promoted to participant. Also, it will make dynamic config more efficient, we don't need to store and send the long list of observers during re-config.


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