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From Mohammad arshad <mohammad.ars...@huawei.com>
Subject RE: How to handle zookeeper data inconsistency
Date Thu, 21 Jan 2016 15:03:00 GMT
Thanks for Flavio Junqueira for your response.

assume C received the commit request but before committing it failed, When C will be synced?
What event will at leader or follower will synch it up.

Here is another scenario we faced.
Node got deleted successfully in leader node B. But due to network issue in Master node, the
delete could not sync up to follower A and C.  At this moment, Leader node also goes down
as faulty. 

Now one of the A and C becomes leader but it has inconsistent data. ( delete is not executed
here)

As I know, This behavior is fine as per current ZK design. But to solve above data inconsistency
issue, any suggestions ? I thought to commit the delete not only in leader but to at least
in N/2 nodes in the same client call and then only mark delete as successful

Any suggestions ?


Best Regards
Mohammad Arshad

-----Original Message-----
From: Flavio Junqueira [mailto:fpj@apache.org] 
Sent: 21 January 2016 19:11
To: user@zookeeper.apache.org
Cc: dev
Subject: Re: How to handle zookeeper data inconsistency

Hi Mohammad,

A delete operation only needs to reach a quorum to complete and A B form a quorum in your
3-server ensemble. If the delete operation never gets propagated to C and other write operations
that have been ordered later complete on C, then you have an issue. If C simply stops receiving
updates, then you have a problem with your C server and it could be a problem with ZK or just
the environment.

If there has been write operations ordered after the delete and server C has seen those but
not the delete, then I'd recommend that you have a look the txn logs with the log formatter.

> shall I check exists from leader only? but even leader can have some 
> node undeleted in the above scenario

There is no such a requirement, but you need to be aware that server C could definitely make
an update visible later compared to other servers. ZooKeeper doesn't guarantee that updates
are visible to all clients as soon as they are acknowledged.

I'd also search for jiras, especially if you're deleting an ephemeral. 

-Flavio

> On 21 Jan 2016, at 13:24, Mohammad arshad <mohammad.arshad@huawei.com> wrote:
> 
> Hi All,
> I came across a scenario where zookeeper was left in inconsistent 
> state(but that is valid as per the zookeeper theory) and because of 
> that dependent application's behaved wrongly The scenario is as follow
> 
> 1) I have three server Zookeeper cluster, let's say servers are A, B 
> and C. B is the leader
> 2) In one successful delete operation, a znode znode1 was deleted from A and B but somehow
not deleted from C. The reason for not deleted from C can be either proposal or commit failed.
> 3) Now for application, which is connected to C, ZooKeeper.exists  
> returns the znod1 and that is why application enters into node exists 
> flow which is wrong
> 
> shall I check exists from leader only? but even leader can have some 
> node undeleted in the above scenario Any guideline to handle the above said valid data
inconsistency ??
> 
> Any suggestion/help is highly appreciated.
> 
> Best Regards
> Mohammad Arshad
> HUAWEI TECHNOLOGIES CO.LTD.
> Huawei Tecnologies India Pvt. Ltd.
> Near EPIP Industrial Area, Kundalahalli Village Whitefield, 
> Bangalore-560066 www.huawei.com<http://www.huawei.com/>
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