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From Jeff Potter <jpotter-zookee...@codepuppy.com>
Subject Re: Changing leader to follower?
Date Mon, 13 Oct 2014 15:25:22 GMT

We’re using zookeeper cross-DC to coordinate communication of data that’s served to our
iOS app via HTTP API calls — in this case, the hosts that the app should be connecting to
for chat. Chat nodes get added into the cluster, register themselves in zookeeper; meanwhile,
clients issue API calls to web servers that return a list of chat nodes that the client should
be connecting to. There’s a little bit of other global settings that we also coordinate
via zookeeper, but that stuff could, in theory, be manually applied to each of the DCs, since
changes to it are manual. (We also run cassandra cross-DC, so we already have dependencies
on talking cross-DC; hence two main DCs and a tie-breaker third DC that also serves as a back-up

I’ve seen SmartStack before, and it seems like a good potential solution at larger scales,
but in our current size / capacity, registering directly on top of zookeeper is lightweight
and simple enough. I haven’t seen the Pinterst writeup; thanks for sending it!

You’d asked about frequency of leader elections. We don’t see leader elections happening
that often — the only time they come up is when we do something to take down the current
leader, which is very, very rare — our deploys don’t need to restart that service. So
far, the only time it’s happened in a year+ is the XEN-108 bug that caused the node to reboot.

To be clear, we’re “okay” with the leader re-election time; I’m just surprised that
it’s as choppy as it is and we were surprised looking through the “service zookeeper stop”
target as to how it was implemented. I would think there’d be some benefit to having a leader
“step down”, in that one could announce to the other nodes that the leader is retiring,
so there’s no need to wait for failed heartbeat responses to realize that the leader is
no longer serving.


On Oct 11, 2014, at 2:09 PM, ralph tice <ralph.tice@gmail.com> wrote:

> I'm not an expert but I don't think there is a magic bullet here, leader
> election has to happen in this circumstance and that takes time.
> You may be better served by building better resilience to eliminate
> ZooKeeper's uptime from being a single point of failure in your services
> layer.  Pinterest and Airbnb both have some prior art here,
> http://engineering.pinterest.com/post/77933733851/zookeeper-resilience-at-pinterest
> and http://nerds.airbnb.com/smartstack-service-discovery-cloud/
> I'm curious why you chose a cross-DC ensemble versus localized same-region
> ensembles.  Don't you deal with a significant frequency of leader elections
> from being in 3 regions anyway?
> On Sat, Oct 11, 2014 at 11:21 AM, Jeff Potter <
> jpotter-zookeeper@codepuppy.com> wrote:
>> The reason I ask is that we’ve noticed, when running zookeeper cross-DC,
>> that restarting the node that’s currently the leader causes a brief but
>> real service interruption for 3 to 5 seconds while the rest of the cluster
>> elects a new leader and syncs. We’re on AWS, with 2 ZK nodes in US-East, 2
>> in US-West-2, and 1 in US-West (as a tie-breaker).
>> It would seem taking a leader to follower status would be useful; and
>> doing so without it actually being a stop / disconnect on all clients
>> connect to the node. (Especially for doing rolling restarts of all nodes,
>> e.g. XEN-108 bug.)
>> -Jeff
>> On Oct 10, 2014, at 10:16 AM, Ivan Kelly <ivank@apache.org> wrote:
>>> Or just pause the process until someone else takes over.
>>> 1. kill -STOP <zookeeper_pid>
>>> 2. // wait for election to happen
>>> 3. kill -CONT <zookeeper_pid>
>>> This wont top it from becoming leader again. Also, client may migrate to
>>> other servers.
>>> -Ivan
>>> Alexander Shraer writes:
>>>> Hi,
>>>> I don't think there's a direct way, although this seems a useful thing
>> to
>>>> add.
>>>> One think you could do is to issue a reconfig changing the leader's
>>>> leading/quorum port (through which
>>>> it talks with the followers). This will cause it to give up leadership
>>>> while keeping it in the cluster.
>>>> Cheers,
>>>> Alex
>>>> On Fri, Oct 10, 2014 at 5:57 AM, Jeff Potter <
>>>> jpotter-zookeeper@codepuppy.com> wrote:
>>>>> Hi,
>>>>> Is there a way to “retire” a leader while keeping it in the cluster?
>>>>> Thanks,
>>>>> Jeff

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