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system.replicas

Description​

Contains information and status for replicated tables residing on the local server. This table can be used for monitoring. The table contains a row for every Replicated* table.

Columns​

  • database (String) — Database name.
  • table (String) — Table name.
  • uuid (UUID) — Table UUID.
  • engine (String) — Table engine name.
  • is_leader (UInt8) — Whether the replica is the leader. Multiple replicas can be leaders at the same time. A replica can be prevented from becoming a leader using the merge_tree setting replicated_can_become_leader. The leaders are responsible for scheduling background merges. Note that writes can be performed to any replica that is available and has a session in ZK, regardless of whether it is a leader.
  • can_become_leader (UInt8) — Whether the replica can be a leader.
  • is_readonly (UInt8) — Whether the replica is in read-only mode. This mode is turned on if the config does not have sections with ClickHouse Keeper, if an unknown error occurred when reinitializing sessions in ClickHouse Keeper, and during session reinitialization in ClickHouse Keeper.
  • readonly_start_time (Nullable(DateTime)) — The timestamp when the replica transitioned into readonly mode. Null if the replica is not in readonly mode.
  • is_session_expired (UInt8) — Whether the session with ClickHouse Keeper has expired. Basically the same as is_readonly.
  • future_parts (UInt32) — The number of data parts that will appear as the result of INSERTs or merges that haven't been done yet.
  • parts_to_check (UInt32) — The number of data parts in the queue for verification. A part is put in the verification queue if there is suspicion that it might be damaged.
  • zookeeper_name (String) — The name of the [Zoo]Keeper cluster (possibly auxiliary one) where the table's metadata is stored
  • zookeeper_path (String) — Path to table data in ClickHouse Keeper.
  • replica_name (String) — Replica name in ClickHouse Keeper. Different replicas of the same table have different names.
  • replica_path (String) — Path to replica data in ClickHouse Keeper. The same as concatenating 'zookeeper_path/replicas/replica_path'.
  • columns_version (Int32) — Version number of the table structure. Indicates how many times ALTER was performed. If replicas have different versions, it means some replicas haven't made all of the ALTERs yet.
  • queue_size (UInt32) — Size of the queue for operations waiting to be performed. Operations include inserting blocks of data, merges, and certain other actions. It usually coincides with future_parts.
  • inserts_in_queue (UInt32) — Number of inserts of blocks of data that need to be made. Insertions are usually replicated fairly quickly. If this number is large, it means something is wrong.
  • merges_in_queue (UInt32) — The number of merges waiting to be made. Sometimes merges are lengthy, so this value may be greater than zero for a long time.
  • part_mutations_in_queue (UInt32) — The number of mutations waiting to be made.
  • queue_oldest_time (DateTime) — If queue_size greater than 0, shows when the oldest operation was added to the queue.
  • inserts_oldest_time (DateTime) — See queue_oldest_time.
  • merges_oldest_time (DateTime) — See queue_oldest_time.
  • part_mutations_oldest_time (DateTime) — See queue_oldest_time.
  • oldest_part_to_get (String) — The name of the part to fetch from other replicas obtained from the oldest GET_PARTS entry in the replication queue.
  • oldest_part_to_merge_to (String) — The result part name to merge to obtained from the oldest MERGE_PARTS entry in the replication queue.
  • oldest_part_to_mutate_to (String) — The result part name to mutate to obtained from the oldest MUTATE_PARTS entry in the replication queue.
  • log_max_index (UInt64) — Maximum entry number in the log of general activity. This column and the next three (log_pointer, total_replicas, active_replicas) have a non-zero value only where there is an active session with ClickHouse Keeper.
  • log_pointer (UInt64) — Maximum entry number in the log of general activity that the replica copied to its execution queue, plus one. If log_pointer is much smaller than log_max_index, something is wrong.
  • last_queue_update (DateTime) — When the queue was updated last time.
  • absolute_delay (UInt64) — How big lag in seconds the current replica has.
  • total_replicas (UInt32) — The total number of known replicas of this table.
  • active_replicas (UInt32) — The number of replicas of this table that have a session in ClickHouse Keeper (i.e., the number of functioning replicas).
  • lost_part_count (UInt64) — The number of data parts lost in the table by all replicas in total since table creation. Value is persisted in ClickHouse Keeper and can only increase.
  • last_queue_update_exception (String) — When the queue contains broken entries. Especially important when ClickHouse breaks backward compatibility between versions and log entries written by newer versions aren't parseable by old versions.
  • zookeeper_exception (String) — The last exception message, got if the error happened when fetching the info from ClickHouse Keeper.
  • replica_is_active (Map(String, UInt8)) — Map between replica name and is replica active.

Aliases:

  • readonly_duration — Alias for if(isNull(readonly_start_time), NULL, now() - readonly_start_time).

Example​

SELECT *
FROM system.replicas
WHERE table = 'test_table'
FORMAT Vertical

Query id: dc6dcbcb-dc28-4df9-ae27-4354f5b3b13e

Row 1:
───────
database:                    db
table:                       test_table
engine:                      ReplicatedMergeTree
is_leader:                   1
can_become_leader:           1
is_readonly:                 0
is_session_expired:          0
future_parts:                0
parts_to_check:              0
zookeeper_path:              /test/test_table
replica_name:                r1
replica_path:                /test/test_table/replicas/r1
columns_version:             -1
queue_size:                  27
inserts_in_queue:            27
merges_in_queue:             0
part_mutations_in_queue:     0
queue_oldest_time:           2021-10-12 14:48:48
inserts_oldest_time:         2021-10-12 14:48:48
merges_oldest_time:          1970-01-01 03:00:00
part_mutations_oldest_time:  1970-01-01 03:00:00
oldest_part_to_get:          1_17_17_0
oldest_part_to_merge_to:
oldest_part_to_mutate_to:
log_max_index:               206
log_pointer:                 207
last_queue_update:           2021-10-12 14:50:08
absolute_delay:              99
total_replicas:              5
active_replicas:             5
lost_part_count:             0
last_queue_update_exception:
zookeeper_exception:
replica_is_active:           {'r1':1,'r2':1}