Replication
- Configuration

Replication

Replication of a table in CrateDB means that each primary shard of a table is stored additionally on so called secondary shards. This can be useful for better read performance and high availability.

If not specified, CrateDB creates zero to one replica depending on the number of available nodes at the cluster. At a single-node cluster, replicas are set to zero to allow fast write operations with the default setting of write.wait_for_active_shards.

Table of Contents

Configuration

Configuration

Defining the number of replicas is done using the number_of_replicas property.

Example:

cr> create table my_table10 (
...   first_column int,
...   second_column string
... ) with (number_of_replicas = 0);
CREATE OK, 1 row affected (... sec)

The number_of_replicas property also accepts an string as parameter that contains a range.

A range is a definition of minimum number of replicas to maximum number of replicas depending on the number of nodes in the cluster. The table below shows some examples.

Range	Explanation
0-1	Will create no replicas if only one node. This will result in a yellow cluster health. One replica if more than one node.
2-4	Table requires at least two replicas to be fully replicated. Will create up to four replicas if nodes are added. If you have less than three nodes, one or more replica shards will be located on the same node as the primary shard. This will result in a yellow cluster health.
0-all	Will expand the number of replicas to the available number of nodes.

Range

Explanation

0-1

Will create no replicas if only one node. This will result in a yellow cluster health.

One replica if more than one node.

2-4

Table requires at least two replicas to be fully replicated.

Will create up to four replicas if nodes are added.

If you have less than three nodes, one or more replica shards will be located on the same node as the primary shard. This will result in a yellow cluster health.

0-all

Will expand the number of replicas to the available number of nodes.

For details of the range syntax refer to number_of_replicas.

Note

The number of replicas can be changed at any time.