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Version: 2.4.x

Placement Policy


Only Enterprise Edition supports

For important online production, cross-regional level disaster recovery support is required.The multi-replica feature of CnosDB, combined with a reasonable data distribution policy, can meet this goal.

Two dimensions of data distribution

rack: Rack dimension, generally refers to the data nodes on the same rack in the same data center.

dc: Data center dimension, generally refers to a computer room, a data center.

Policy Type

Simple Policy

Replica data placement on the same rack Name: simple Parameters: dc, rack

All replicas are placed on machines in rack1 of dc1

create placement_policy policy1 WITH rule simple dc 'dc1' rack 'rack1';

Perceive Rack Policy

Replica data should be placed on different racks within the same dc as much as possible Name: rack_aware Parameters: dc, and list of racks

All replicas are placed on machines in different racks of rack1-4 in dc1

create placement_policy policy2 WITH rule rack_aware dc 'dc1' rack 'rack1,rack2,rack3,rack4';

Perceive DC Policy

Replica data should be placed on different dcs as much as possible Name: dc_aware Parameters: list of dc

All replicas should be placed on machines in dc1-4 as much as possible

create placement_policy policy3 WITH rule dc_aware dc 'dc1,dc2,dc3,dc4';


The placement policy belongs to the tenant, and within the same tenant, the policy name is unique.

Policy Associations

When creating/modifying a Database, you can associate it with a specified placement policy name. If no placement policy is specified, the existing logic will be followed, which means it is not aware of the data center and rack.

create database db with replica 2 placement_policy 'policy1';
alter database db set placement_policy 'policy2';

View Policy

You can view the policies and usage under the current tenant through the system tables

select * from information_schema.placement_policys;
| name | type | dc | rack | in_use_dbs |
| policy1 | SIMPLE | dc1 | rack1 | |
| policy2 | RACKAWARE | dc1 | rack1,rack2,rack3,rack4 | |
| policy3 | DCAWARE | dc1,dc2,dc3,dc4 | | |

Modification and deletion of policies

When there is no database usage policy, drop/alter can be performed

drop placement_policy policy1;
alter placement_policy policy2 SET rule dc_aware dc 'dc1,dc2,dc3,dc4';

Node Label

When a node joins the cluster, it specifies its belonging dc and rack through the configuration file, and can be viewed by using show datanodes.

location = "/dc1/rack1"
show datanodes;
| node_id | host | attribute | status | disk_free | disk_total | location | last_updated_time |
| 1001 | localhost | hot | healthy | 31.53 GB | 240.11 GB | /dc1/rack1 | 2023-11-07 06:20:01 |
| 2001 | localhost | hot | healthy | 73.64 GB | 128.34 GB | /dc1/rack2 | 2023-11-07 06:20:01 |