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Clickhouse shows duplicates data in distributed table

I have 3 nodes with 3 shards and 2 replicas on each:

CLickhouse cluster settings: CLickhouse cluster settings

Added also the XML config for the sharding and replicas

<default_cluster>
   <shard>
      <internal_replication>true</internal_replication>
      <replica>
         <default_database>shard</default_database>
         <host>clickhouse-0</host>
         <port>9000</port>
         <user>default</user>
         <password>default</password>
      </replica>
      <replica>
         <default_database>replica</default_database>
         <host>clickhouse-2</host>
         <port>9000</port>
         <user>default</user>
         <password>default</password>
      </replica>
   </shard>
   <shard>
      <internal_replication>true</internal_replication>
      <replica>
         <default_database>shard</default_database>
         <host>clickhouse-1</host>
         <port>9000</port>
         <user>default</user>
         <password>default</password>
      </replica>
      <replica>
         <default_database>replica</default_database>
         <host>clickhouse-0</host>
         <port>9000</port>
         <user>default</user>
         <password>default</password>
      </replica>
   </shard>
   <shard>
      <internal_replication>true</internal_replication>
      <replica>
         <default_database>shard</default_database>
         <host>clickhouse-2</host>
         <port>9000</port>
         <user>default</user>
         <password>default</password>
      </replica>
      <replica>
         <default_database>replica</default_database>
         <host>clickhouse-1</host>
         <port>9000</port>
         <user>default</user>
         <password>default</password>
      </replica>
   </shard>
</default_cluster>

I am doing the following example:

create database test on cluster default_cluster;

CREATE TABLE  test.test_distributed_order_local on cluster default_cluster
(
id integer,
test_column String
) 
ENGINE = ReplicatedMergeTree('/default_cluster/test/tables/test_distributed_order_local/{shard}', '{replica}') 
PRIMARY KEY id
ORDER BY id;

CREATE TABLE  test.test_distributed_order on cluster default_cluster as test.test_distributed_order_local
ENGINE = Distributed(default_cluster, test, test_distributed_order_local, id);

insert into test.test_distributed_order values (1, 'test1');
insert into test.test_distributed_order values (2, 'test2');
insert into test.test_distributed_order values (3, 'test3');

The results are not the same, and they contain duplications: Eg

Result 1:

enter image description here

Result 2:

enter image description here

​

What am I missing?

I expect to not have duplicated rows in the select

Solution

  • I think this post probably sums up what you're trying to achieve - https://altinity.com/blog/2018/5/10/circular-replication-cluster-topology-in-clickhouse It's a little old but the principle applies - For Clickhouse not a topology that's recommended.

    Consider this simplified example:

    <shard>
    // These two are replicas of each other
    <replica>
        <host>cluster_node_0</host>
    </replica>
    **<replica>
        <host>cluster_node_2</host>
    </replica>**
</shard>
<shard>
    <replica>
        <host>cluster_node_1</host>
    </replica>
    <replica>
        <host>cluster_node_0</host>
    </replica>
</shard>
<shard>
    **<replica>
        <host>cluster_node_2</host>
    </replica>**
    <replica>
        <host>cluster_node_1</host>
    </replica>
</shard>

    Let's suppose data is written into the first shard on node cluster_node_0. It will then be replicated to the shard on cluster_node_2 - as the zookeeper path is the same.

    Now for the issue. You have also defined the 3rd shard on cluster_node_2. When you create this table, it will physically contain data from 2 shards - the 1st and 3rd - I've attempted to highlight with **.

    When a query comes in, it will be sent to each shard. The challenge is each local table will respond with results from both shards - hence you get duplicates.

    Generally, avoid more than one shard on a host - the blog explains how you can achieve more than one buts its not recommended or ever need.