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Date: Tue, 27 Jan 2015 12:23:22 -0500
From: Jordan Zimmerman <jordan@jordanzimmerman.com>
To: Ricardo Ferreira <ricardojsfer@gmail.com>, user@curator.apache.org
Message-ID: <etPan.54c7c98a.6b8b4567.13d@Jordans-MacBook-Pro.local>
In-Reply-To: 
 <CAFu-U0CvqKa5HYXWBBELaLS5DozYAi__OtaTqCFTbN4pgx2QJA@mail.gmail.com>
References: 
 <CAFu-U0CvqKa5HYXWBBELaLS5DozYAi__OtaTqCFTbN4pgx2QJA@mail.gmail.com>
Subject: Re: Regarding leadership status
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The process of leadership loss due to network issues is this:

* The current leader's ZooKeeper client will try to send its heartbeat
* The send will fail due to network failure
* The ZK client will pass=C2=A0KeeperState.Disconnected and cause Curator=
 to go into SUSPENDED mode. Any users of leader selectors, etc. should se=
e this and consider themselves no longer leader
* Because the heartbeat is 2/3 of a session, there will be no other leade=
r at this point
* After session expiration, the ZK server will delete the ephemeral node =
and cause the process of leader selection, etc.

So, as you can see, if you properly monitor the connection state you neve=
r have to worry about proper leadership.

-Jordan


On January 27, 2015 at 4:21:40 AM, Ricardo =46erreira (ricardojsfer=40gma=
il.com) wrote:

Hello all,


I'm using Zookeeper (through Curator) for leadership election on an appli=
cation I'm currently developing.

Some operations can only be written by one of the members of the cluster,=
 so I'm using a node's leadership
status to this effect.=C2=A0

=46or example, let's say a message arrives to all nodes but only one shou=
ld do something about it. What I do
is to check if the node is leader (through LeaderSelector=23hasLeadership=
()), and if it is then it does whatever
it needs to do with the message. If it's not, it simply ignores it.

My question is this: How can I be sure a node really is=C2=A0the leader=3F=
 What if the leader node is actually down but
this hasn't been detected by Zookeeper yet=3F Then the non-leader nodes w=
ill ignore the message and it will=C2=A0
never be processed.

How is this usually dealt with=3F


Regards
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<html><head><style>body=7Bfont-family:Helvetica,Arial;font-size:13px=7D</=
style></head><body style=3D=22word-wrap: break-word; -webkit-nbsp-mode: s=
pace; -webkit-line-break: after-white-space;=22><div id=3D=22bloop=5Fcust=
omfont=22 style=3D=22font-family:Helvetica,Arial;font-size:13px; color: r=
gba(0,0,0,1.0); margin: 0px; line-height: auto;=22>The process of leaders=
hip loss due to network issues is this:</div><div id=3D=22bloop=5Fcustomf=
ont=22 style=3D=22font-family:Helvetica,Arial;font-size:13px; color: rgba=
(0,0,0,1.0); margin: 0px; line-height: auto;=22><br></div><div id=3D=22bl=
oop=5Fcustomfont=22 style=3D=22font-family:Helvetica,Arial;font-size:13px=
; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;=22>* The curren=
t leader's ZooKeeper client will try to send its heartbeat</div><div id=3D=
=22bloop=5Fcustomfont=22 style=3D=22font-family:Helvetica,Arial;font-size=
:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;=22>* The s=
end will fail due to network failure</div><div id=3D=22bloop=5Fcustomfont=
=22 style=3D=22font-family:Helvetica,Arial;font-size:13px; color: rgba(0,=
0,0,1.0); margin: 0px; line-height: auto;=22>* The ZK client will pass&nb=
sp;KeeperState.Disconnected and cause Curator to go into SUSPENDED mode. =
Any users of leader selectors, etc. should see this and consider themselv=
es no longer leader</div><div id=3D=22bloop=5Fcustomfont=22 style=3D=22fo=
nt-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin:=
 0px; line-height: auto;=22>* Because the heartbeat is 2/3 of a session, =
there will be no other leader at this point</div><div id=3D=22bloop=5Fcus=
tomfont=22 style=3D=22font-family:Helvetica,Arial;font-size:13px; color: =
rgba(0,0,0,1.0); margin: 0px; line-height: auto;=22>* After session expir=
ation, the ZK server will delete the ephemeral node and cause the process=
 of leader selection, etc.</div><div id=3D=22bloop=5Fcustomfont=22 style=3D=
=22font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); ma=
rgin: 0px; line-height: auto;=22><br></div><div id=3D=22bloop=5Fcustomfon=
t=22 style=3D=22font-family:Helvetica,Arial;font-size:13px; color: rgba(0=
,0,0,1.0); margin: 0px; line-height: auto;=22>So, as you can see, if you =
properly monitor the connection state you never have to worry about prope=
r leadership.</div><div id=3D=22bloop=5Fcustomfont=22 style=3D=22font-fam=
ily:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; =
line-height: auto;=22><br></div><div id=3D=22bloop=5Fcustomfont=22 style=3D=
=22font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); ma=
rgin: 0px; line-height: auto;=22>-Jordan</div> <div id=3D=22bloop=5Fsign=5F=
1422379169930556928=22 class=3D=22bloop=5Fsign=22><div style=3D=22font-fa=
mily:helvetica,arial;font-size:13px=22><br></div></div> <br><p style=3D=22=
color:=23000;=22>On January 27, 2015 at 4:21:40 AM, Ricardo =46erreira (<=
a href=3D=22mailto:ricardojsfer=40gmail.com=22>ricardojsfer=40gmail.com</=
a>) wrote:</p> <blockquote type=3D=22cite=22 class=3D=22clean=5Fbq=22><sp=
an><div><div></div><div>


<title></title>


<div dir=3D=22ltr=22>Hello all,
<div><br></div>
<div><br></div>
<div>I'm using Zookeeper (through Curator) for leadership election
on an application I'm currently developing.</div>
<div><br></div>
<div>Some operations can only be written by one of the members of
the cluster, so I'm using a node's leadership</div>
<div>status to this effect.&nbsp;</div>
<div><br></div>
<div>=46or example, let's say a message arrives to all nodes but only
one should do something about it. What I do</div>
<div>is to check if the node is leader (through
LeaderSelector=23hasLeadership()), and if it is then it does
whatever</div>
<div>it needs to do with the message. If it's not, it simply
ignores it.</div>
<div><br></div>
<div>My question is this: How can I be sure a node <i>really
is</i><b style=3D=22font-style:italic=22>&nbsp;</b>the leader=3F What if
the leader node is actually down but</div>
<div>this hasn't been detected by Zookeeper yet=3F Then the
non-leader nodes will ignore the message and it will&nbsp;</div>
<div>never be processed.</div>
<div><br></div>
<div>How is this usually dealt with=3F</div>
<div><br></div>
<div><br></div>
<div>Regards</div>
</div>


</div></div></span></blockquote></body></html>
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