ClickHouse/docs/en/operations/system-tables/clusters.md

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/en/operations/system-tables/clusters

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.
• is_local (UInt8) — Flag that indicates whether the host is local.
• user (String) — The name of the user for connecting to the server.
• default_database (String) — The default database name.
• errors_count (UInt32) — The number of times this host failed to reach replica.
• slowdowns_count (UInt32) — The number of slowdowns that led to changing replica when establishing a connection with hedged requests.
• estimated_recovery_time (UInt32) — Seconds remaining until the replica error count is zeroed and it is considered to be back to normal.
• database_shard_name (String) — The name of the Replicated database shard (for clusters that belong to a Replicated database).
• database_replica_name (String) — The name of the Replicated database replica (for clusters that belong to a Replicated database).
• is_active (Nullable(UInt8)) — The status of the Replicated database replica (for clusters that belong to a Replicated database): 1 means "replica is online", 0 means "replica is offline", NULL means "unknown".
• name (String) - An alias to cluster.

Example

Query:

SELECT * FROM system.clusters LIMIT 2 FORMAT Vertical;

Result:

Row 1:
──────
cluster:                 test_cluster_two_shards
shard_num:               1
shard_weight:            1
replica_num:             1
host_name:               127.0.0.1
host_address:            127.0.0.1
port:                    9000
is_local:                1
user:                    default
default_database:
errors_count:            0
slowdowns_count:         0
estimated_recovery_time: 0
database_shard_name:
database_replica_name:
is_active:               NULL

Row 2:
──────
cluster:                 test_cluster_two_shards
shard_num:               2
shard_weight:            1
replica_num:             1
host_name:               127.0.0.2
host_address:            127.0.0.2
port:                    9000
is_local:                0
user:                    default
default_database:
errors_count:            0
slowdowns_count:         0
estimated_recovery_time: 0
database_shard_name:
database_replica_name:
is_active:               NULL

See Also

• Table engine Distributed
• distributed_replica_error_cap setting
• distributed_replica_error_half_life setting