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