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

Example

:) SELECT * FROM system.clusters LIMIT 2 FORMAT Vertical;
Row 1:
──────
cluster:                 test_cluster
shard_num:               1
shard_weight:            1
replica_num:             1
host_name:               clickhouse01
host_address:            172.23.0.11
port:                    9000
is_local:                1
user:                    default
default_database:        
errors_count:            0
estimated_recovery_time: 0

Row 2:
──────
cluster:                 test_cluster
shard_num:               1
shard_weight:            1
replica_num:             2
host_name:               clickhouse02
host_address:            172.23.0.12
port:                    9000
is_local:                0
user:                    default
default_database:        
errors_count:            0
estimated_recovery_time: 0

2 rows in set. Elapsed: 0.002 sec. 

Original article

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