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.
SELECT * FROM system.replicas WHERE table = 'visits' FORMAT Vertical
Row 1: ────── database: merge table: visits engine: ReplicatedCollapsingMergeTree is_leader: 1 can_become_leader: 1 is_readonly: 0 is_session_expired: 0 future_parts: 1 parts_to_check: 0 zookeeper_path: /clickhouse/tables/01-06/visits replica_name: example01-06-1.yandex.ru replica_path: /clickhouse/tables/01-06/visits/replicas/example01-06-1.yandex.ru columns_version: 9 queue_size: 1 inserts_in_queue: 0 merges_in_queue: 1 part_mutations_in_queue: 0 queue_oldest_time: 2020-02-20 08:34:30 inserts_oldest_time: 0000-00-00 00:00:00 merges_oldest_time: 2020-02-20 08:34:30 part_mutations_oldest_time: 0000-00-00 00:00:00 oldest_part_to_get: oldest_part_to_merge_to: 20200220_20284_20840_7 oldest_part_to_mutate_to: log_max_index: 596273 log_pointer: 596274 last_queue_update: 2020-02-20 08:34:32 absolute_delay: 0 total_replicas: 2 active_replicas: 2
String) - Database name
String) - Table name
String) - Table engine name
UInt8) - Whether the replica is the leader.
Only one replica at a time can be the leader. The leader is responsible for selecting background merges to perform.
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.
UInt8) - Whether the replica can be elected as a leader.
UInt8) - Whether the replica is in read-only mode.
This mode is turned on if the config doesn’t have sections with ZooKeeper, if an unknown error occurred when reinitializing sessions in ZooKeeper, and during session reinitialization in ZooKeeper.
UInt8) - the session with ZooKeeper has expired. Basically the same as
UInt32) - The number of data parts that will appear as the result of INSERTs or merges that haven’t been done yet.
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.
String) - Path to table data in ZooKeeper.
String) - Replica name in ZooKeeper. Different replicas of the same table have different names.
String) - Path to replica data in ZooKeeper. The same as concatenating ‘zookeeper_path/replicas/replica_path’.
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.
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
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.
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.
UInt32) - The number of mutations waiting to be made.
DateTime) - If
queue_sizegreater than 0, shows when the oldest operation was added to the queue.
DateTime) - See
DateTime) - See
DateTime) - See
The next 4 columns have a non-zero value only where there is an active session with ZK.
UInt64) - Maximum entry number in the log of general activity.
UInt64) - Maximum entry number in the log of general activity that the replica copied to its execution queue, plus one. If
log_pointeris much smaller than
log_max_index, something is wrong.
DateTime) - When the queue was updated last time.
UInt64) - How big lag in seconds the current replica has.
UInt8) - The total number of known replicas of this table.
UInt8) - The number of replicas of this table that have a session in ZooKeeper (i.e., the number of functioning replicas).
If you request all the columns, the table may work a bit slowly, since several reads from ZooKeeper are made for each row.
If you don’t request the last 4 columns (log_max_index, log_pointer, total_replicas, active_replicas), the table works quickly.
For example, you can check that everything is working correctly like this:
SELECT database, table, is_leader, is_readonly, is_session_expired, future_parts, parts_to_check, columns_version, queue_size, inserts_in_queue, merges_in_queue, log_max_index, log_pointer, total_replicas, active_replicas FROM system.replicas WHERE is_readonly OR is_session_expired OR future_parts > 20 OR parts_to_check > 10 OR queue_size > 20 OR inserts_in_queue > 10 OR log_max_index - log_pointer > 10 OR total_replicas < 2 OR active_replicas < total_replicas
If this query doesn’t return anything, it means that everything is fine.