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From: Bryan Thompson <bryan@systap.com>
To: Ted Dunning <ted.dunning@gmail.com>, Patrick Hunt <phunt@apache.org>
CC: "zookeeper-user@hadoop.apache.org" <zookeeper-user@hadoop.apache.org>
Date: Wed, 30 Jun 2010 18:48:54 -0500
Subject: RE: Guaranteed message delivery until session timeout?
Thread-Topic: Guaranteed message delivery until session timeout?
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Ted,

Yes, that is clear.  I was looking for this:

> On some failures (communication errors, timeouts, etc) the client will no=
t know if the update has applied or not. We take steps to minimize the fail=
ures, but the only guarantee is only present with successful return codes.

With regard to timeliness:

> The clients view of the system is guaranteed to be up-to-date within a ce=
rtain time bound. (On the order of tens of seconds.) Either system changes =
will be seen by a client within this bound, or the client will detect a ser=
vice outage.

This seems to imply that there are retries for transient communication fail=
ures.  Is that true?

For example, if a client registers a watch, and a state change which would =
trigger that watch occurs _after_ the client has successfuly registered the=
 watch with the zookeeper quorum, is it possible that the client would not =
observe the watch trigger due to communication failure, etc., even while th=
e clients session remains valid?  It sounds like the answer is "no" per the=
 timeliness guarantee.  Is that correct?

Thanks,
Bryan

________________________________
From: Ted Dunning [mailto:ted.dunning@gmail.com]
Sent: Wednesday, June 30, 2010 7:38 PM
To: Patrick Hunt
Cc: zookeeper-user@hadoop.apache.org; Bryan Thompson
Subject: Re: Guaranteed message delivery until session timeout?

Also this:

Once an update has been applied, it will persist from that time forward unt=
il a client overwrites the update. This guarantee has two corollaries:

If a client gets a successful return code, the update will have been applie=
d. On some failures (communication errors, timeouts, etc) the client will n=
ot know if the update has applied or not. We take steps to minimize the fai=
lures, but the only guarantee is only present with successful return codes.=
 (This is called the monotonicity condition in Paxos.)
Any updates that are seen by the client, through a read request or successf=
ul update, will never be rolled back when recovering from server failures.

I think that the clear implications here are:

a) if you get a successful return code and no session expiration, your ephe=
meral file is there

b) if the ephemeral files is created, you might not get the successful retu=
rn code (due to connection loss), but the ephemeral file might continue to =
exist (because connection loss !=3D session loss)

c) if you get a failure return code, your ephemeral file was not created

On Wed, Jun 30, 2010 at 4:33 PM, Patrick Hunt <phunt@apache.org<mailto:phun=
t@apache.org>> wrote:
in particular see "timeliness" http://hadoop.apache.org/zookeeper/docs/curr=
ent/zookeeperProgrammers.html#ch_zkGuarantees


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