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From Raúl Gutiérrez Segalés <...@itevenworks.net>
Subject Re: SyncLimit and client notifications
Date Fri, 08 Jul 2016 04:34:12 GMT
Hi Paul,

On 7 July 2016 at 14:05, Paul K. Harter, Jr. <paul.harter@oracle.com> wrote:

> Camillle, thanks for your response!   If I could burrow a little deeper:
>
>
>
> I had seen about read-only clients, who can tolerate arbitrarily stale
> data and that servers can respond to them when they are isolated from the
> quorum.  I was actually wondered about an intermediate case, where, e.g.
> the leader crashes and a server who is still connected to the other servers
> initiates leader election.  Question:
>
>
>
> These servers can’t do updates until they’ve settled on a new leader, but
> can they accept connections or continue to respond to reads, based on their
> current data??
>

Participants (servers that can be either a follower or a leader) will
accept connections whilst in LOOKING mode, but immediately close them.
Confront with:

When the follower gets disconnected from the leader this happens:

https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/quorum/QuorumPeer.java#L1115

and the call to Learner.shutdown() happens:

https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/quorum/Learner.java#L616

which means that new connections will be established, but immediately
rejected:

https://github.com/apache/zookeeper/blob/trunk/src/java/main/org/apache/zookeeper/server/NIOServerCnxn.java#L429



>
> You said that a follower who loses quorum (can no longer see the majority
> of servers) drops connections to clients so they know to start trying to
> reconnect (perhaps except for read-only clients, which we can ignore  for
> the moment).  Question:
>
>
>
> It would seem to make sense that a follower who just loses contact with
> the leader DOES NOT drop clients during leader election because and leader
> election is usually successful and short enough that it would be too
> disruptive.  Is this true??
>
>
I wish it were, but it isn't. If it was, it would reduce the pressure on
participants that need to deal with closing & recreating all the client
sockets when the leader goes away and an election is called. But it would
also make the state machine referenced above much more complicated.

About read-only clients, you might be interested in:

https://issues.apache.org/jira/browse/ZOOKEEPER-1607

which we use in production at Pinterest.


-rgs

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