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From Todd Lipcon <t...@cloudera.com>
Subject Re: 答复: 答复: How kudu synchronize real-time records?
Date Thu, 26 Oct 2017 19:09:31 GMT
What Helifu said is correct that writes are funneled through the leader.

Reads can either be through the leader (which can perform immediately with
full consistency) or at a follower. On a follower, the client can choose
between the following:

a) low consistency: read whatever the follower happens to have. Currently
this mode is called READ_LATEST in the source but it should probably be
called READ_ANYTHING or READ_INCONSISTENT. It reads "the latest thing that
this replica has".
b) snapshot consistency at current time: this may cause the follower to
wait until it has heard from the leader and knows that it is up-to-date as
of the time that the scan started. This gives the same guarantee as reading
from the leader but can add some latency
c) snapshot consistency in the past: given a timestamp, the follower can
know whether it is up-to-date as of that timestamp. If so, it can do a
consistent read immediately. Otherwise, it will have to wait, as above.

You can learn more about this in the recent blog post authored by David
Alves at: https://kudu.apache.org/2017/09/18/kudu-consistency-pt1.html
Also please check out the docs at:
https://kudu.apache.org/docs/transaction_semantics.html


Hope that helps
-Todd

On Thu, Oct 26, 2017 at 3:18 AM, helifu <hzhelifu@corp.netease.com> wrote:

> Sorry for my mistake.
> The copy replica could be read by clients with below API in client.h:
>
>         Status SetSelection(KuduClient::ReplicaSelection selection)
>         WARN_UNUSED_RESULT;
>
>         enum ReplicaSelection {
>         LEADER_ONLY,      ///< Select the LEADER replica.
>
>             CLOSEST_REPLICA,  ///< Select the closest replica to the
> client,
>                           ///< or a random one if all replicas are
> equidistant.
>
>             FIRST_REPLICA     ///< Select the first replica in the list.
>         };
>
>
> 何李夫
> 2017-04-10 16:06:24
>
> -----邮件原件-----
> 发件人: user-return-1102-hzhelifu=corp.netease.com@kudu.apache.org [mailto:
> user-return-1102-hzhelifu=corp.netease.com@kudu.apache.org] 代表 ??
> 发送时间: 2017年10月26日 13:50
> 收件人: user@kudu.apache.org
> 主题: Re: 答复: How kudu synchronize real-time records?
>
> Thanks for replying me.
>
> It helps a lot.
>
> 2017-10-26 12:29 GMT+09:00 helifu <hzhelifu@corp.netease.com>:
> > Hi,
> >
> > Now the read/write operations are limited to the master replica(record1
> on node1), and the copy replica(record1 on node2/node3) can't be read/write
> by clients directly.
> >
> >
> > 何李夫
> > 2017-04-10 11:24:24
> >
> > -----邮件原件-----
> > 发件人: user-return-1100-hzhelifu=corp.netease.com@kudu.apache.org [mailto:
> user-return-1100-hzhelifu=corp.netease.com@kudu.apache.org] 代表 ??
> > 发送时间: 2017年10月26日 10:43
> > 收件人: user@kudu.apache.org
> > 主题: How kudu synchronize real-time records?
> >
> > Hi!
> >
> > I read from documents saying 'once kudu receives records from client it
> write those records into WAL (also does replica)'
> >
> > And i wonder it can be different time when load those records from WAL
> in each node.
> > So let's say node1 load record1 from WAL at t1, node2 t2, node3 t3 (t1 <
> t2 < t3) then reading client attached node1 can see record but other
> reading clients attached not node1(node2, node3) have possibilities missing
> record1.
> >
> > I think that does not happens in kudu, and i wonder how kudu synchronize
> real time data.
> >
> > Thanks!
> >
>
>


-- 
Todd Lipcon
Software Engineer, Cloudera

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