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https://issues.apache.org/jira/browse/KUDU-3290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17447214#comment-17447214
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shenxingwuying edited comment on KUDU-3290 at 11/22/21, 11:20 AM:
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[~wangning]
1. The idea of duplication is by no means HTAP. The demand for real-time
duplication is always exists. Moreover, we do not want to build an HTAP system,
but to provide a data service. Just as HTAP is not all. This data service does
not include complex transactions. In addition to kudu's own services, it only
provides a single-row transaction (no transaction) scenario like traditional
NoSQL. This is easy to support. Kudu is supporting transactions in recently,
but this will not affect the subsequent scalability of duplication. Because
duplication keeps all Wals. However, the user's consumer program will become
more complex.
2. Kudu is downstream of Kafka, but Kafka is not the only source of kudu data.
For example, users can use impala's SQL to directly make updates (insert,
delete), so we cannot directly subscribe to upstream Kafka to duplicate data.
3. This function does not affect the original functions of kudu. We will set
its default compilation option and close duplication default. Only when
necessary users need it, open the compilation option and enable duplicating.
4. We provide a common interface for duplication and provide the implementation
of duplicating to Kafka. Users(developers) can directly implement its interface
through this example, or users can choose to enable synchronization to Kafka
and then subscribe to Kafka topic. The problem can be transformed into writing
a consumer program to subscribe to Kafka.
was (Author: shenxingwuying):
[~wangning]
1. The idea of duplication is by no means HTAP. The demand for real-time
duplication is always exists. Moreover, we do not want to build an HTAP system,
but to provide a data service. Just as HTAP is not all. This data service does
not include complex transactions. In addition to kudu's own services, it only
provides a single-row transaction (no transaction) scenario like traditional
NoSQL. This is easy to support. Kudu is supporting transactions in recently,
but this will not affect the subsequent scalability of duplication. Because
duplication keeps all Wals. However, the user's consumer program will become
more complex.
2. Kudu is downstream of Kafka, but Kafka is not the only source of kudu data.
For example, users can use impala's SQL to directly make updates (insert,
delete), so we cannot directly subscribe to upstream Kafka to duplicate data.
3. This function does not affect the original functions of kudu. We will set
its default compilation option and close duplication default. Only when
necessary users need it, open the compilation option and enable duplicating.
4. We provide a common interface for duplication and provide the implementation
of duplicating to Kafka. Users can directly implement its interface through
this example, or users can choose to enable synchronization to Kafka and then
subscribe to Kafka topic. The problem can be transformed into writing a
consumer program to subscribe to Kafka.
> Implement Replicate table's data to Kafka(or other Storage System)
> ------------------------------------------------------------------
>
> Key: KUDU-3290
> URL: https://issues.apache.org/jira/browse/KUDU-3290
> Project: Kudu
> Issue Type: New Feature
> Components: tserver
> Reporter: shenxingwuying
> Priority: Critical
>
> h1. background & problem
> We use kudu to store the user profile data, because business requirements,
> exchange and share data from multi-tenant users, which is reasonable in our
> application scene, we need replicate data from one system to another. The
> destination storage system we pick kafka, because of our company's
> architecture at now.
> At this time, we have two ideas to solve it.
> h1. two replication scheme
> Generally, Raft group has three replicas, one is leader and the other two are
> followers. We’ll add a replica, its role is Learner. Learner only receive all
> the data, but not pariticipart in ther leadership election.
> The learner replica, its state machine will be a plugin system, eg:
> # We can support KuduEngine, which just a data backup like mongodb’s hidden
> replica.
> # We can write to the thirdparty store system, like kafka or any other
> system we need. Then we can replicate data to another system use its client.
> At Paxos has a learner role, which only receive data. we need such a role for
> new membership.
> But it Kudu Learner has been used for the copying(recovering) tablet replica.
> Maybe we need a new role name, at this, we still use Learner to represent the
> new role. (We should think over new role name)
> In our application scene, we will replicate data to kafka, and I will explain
> the method.
> h2. Learner replication
> # Add a new replica role, maybe we call it learner, because Paxos has a
> learner role, which only receive data. We need such a role for new
> membership. But at Kudu Learner has been used for the copying(recovering)
> tablet replica. Maybe we need a new role name, at this, we still use Learner
> to represent the new role. (We should think over new role name)
> # The voters's safepoint of clean obsoleted wal is min(leader’ max wal
> sequence number, followers max wal sequence number, learner’ max wal sequence
> number)
> # The learner not voter, not partitipant in elections
> # Raft can replication data to the learner
> # The process of learner applydb, just like raft followers, the logs before
> committed index will replicate to kafka, kafka’s response ok. the apply index
> will increase.
> # We need kafka client, it will be added to kudu as an option, maybe as an
> compile option
> # When a kudu-tserver decomission or corrupted, the learner must move to new
> kudu-tserver. So the leader should save learner apply OpId, and replicate to
> followers, when learner's failover when leader down.
> # The leader must save the learners apply OpId and replicate it to
> followers, when learner's recovery can make sure no data loss when leader
> down. If leader no save the applyIndex, learner maybe loss data
> # Followers save the learners applyindex and term, coz followers maybe
> become leader.
> # When load balancer running，we shoud support move learner another
> kudu-tserver
> # Table should add a switch option to determine whether raft group has
> learner, can support setting it when creating table.
> # Support altering table to add learners maybe an idea, but need solve the
> base data migrate problem.
> # Base data migrate. The simple but heavy cost, when learner's max_OpId <
> committed_OpId (maybe data loss, maybe we alter table add learner replication
> for a existing table), we can trigger a full scan at the timestamp and
> replicate data to learner, and then recover the appendEntries flow.
> # Kudu not support split and merge, we not discuss it now. If KuduSupport
> split or merge, we can implement it use 12, of course we can use more better
> method.
> # If we need the funtion, our cluster should at least 4 tservers.
> If kafka fail or topic not exist, the learner will stop replicate wal, that
> will occupt more disk space. if learner loss or corrupted, it can recover
> from the leader. We need make sure the safepoint.
> h2. Leader replication
> We can replication data to kafka or any other storage system from leader
> directly.
> # We need not set a role, but the dest is kafka, PeerManager's one peer is
> different from the others, that will make something complex.
> # Reuse the leader’s wal, save a output network bandwidth compare to above
> method.
> # All replica should maintenance a point to save the apply OpId at kafka.
> # Safepoint of clean obsoleted wal is min( voters’ max wal sequence number,
> applyIndex at kafka), which leader save it.
> # Any leader transfer must recover the apply OpId at kafka.
> # We need kafka client, it will be added to kudu as an option, may be as an
> compile option
> # If kafka's topic or kafka failure, print errorlog. and wal holds
> # Some process the same as Learner replication. such as base data replicate.
> # If we need the funtion, our cluster should at least 3 tservers.
> If kafka fail or topic not exist, the leader will stop replicate wal to
> kafka, that will occupt more disk space. We need make sure the safepoint.
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