system.clusters 

Contains information about clusters available in the config file and the servers in them.

Columns:

• cluster (String) — The cluster name.
• shard_num (UInt32) — The shard number in the cluster, starting from 1.
• shard_weight (UInt32) — The relative weight of the shard when writing data.
• replica_num (UInt32) — The replica number in the shard, starting from 1.
• host_name (String) — The host name, as specified in the config.
• host_address (String) — The host IP address obtained from DNS.
• port (UInt16) — The port to use for connecting to the server.
• user (String) — The name of the user for connecting to the server.
• errors_count (UInt32) - number of times this host failed to reach replica.
• estimated_recovery_time (UInt32) - seconds left until replica error count is zeroed and it is considered to be back to normal.

Please note that errors_count is updated once per query to the cluster, but estimated_recovery_time is recalculated on-demand. So there could be a case of non-zero errors_count and zero estimated_recovery_time, that next query will zero errors_count and try to use replica as if it has no errors.

See also

• Table engine Distributed
• distributed_replica_error_cap setting
• distributed_replica_error_half_life setting