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 <hzhel...@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....@kudu.apache.org [mailto: > user-return-1102-hzhelifu=corp.netease....@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 <hzhel...@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....@kudu.apache.org [mailto: > user-return-1100-hzhelifu=corp.netease....@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
