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133 lines
6.7 KiB
Markdown
133 lines
6.7 KiB
Markdown
# system.replicas {#system_tables-replicas}
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Contains information and status for replicated tables residing on the local server.
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This table can be used for monitoring. The table contains a row for every Replicated\* table.
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Example:
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``` sql
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SELECT *
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FROM system.replicas
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WHERE table = 'test_table'
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FORMAT Vertical
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```
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``` text
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Query id: dc6dcbcb-dc28-4df9-ae27-4354f5b3b13e
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Row 1:
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───────
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database: db
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table: test_table
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engine: ReplicatedMergeTree
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is_leader: 1
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can_become_leader: 1
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is_readonly: 0
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is_session_expired: 0
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future_parts: 0
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parts_to_check: 0
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zookeeper_path: /test/test_table
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replica_name: r1
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replica_path: /test/test_table/replicas/r1
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columns_version: -1
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queue_size: 27
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inserts_in_queue: 27
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merges_in_queue: 0
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part_mutations_in_queue: 0
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queue_oldest_time: 2021-10-12 14:48:48
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inserts_oldest_time: 2021-10-12 14:48:48
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merges_oldest_time: 1970-01-01 03:00:00
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part_mutations_oldest_time: 1970-01-01 03:00:00
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oldest_part_to_get: 1_17_17_0
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oldest_part_to_merge_to:
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oldest_part_to_mutate_to:
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log_max_index: 206
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log_pointer: 207
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last_queue_update: 2021-10-12 14:50:08
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absolute_delay: 99
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total_replicas: 5
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active_replicas: 5
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last_queue_update_exception:
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zookeeper_exception:
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replica_is_active: {'r1':1,'r2':1}
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```
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Columns:
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- `database` (`String`) - Database name
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- `table` (`String`) - Table name
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- `engine` (`String`) - Table engine name
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- `is_leader` (`UInt8`) - Whether the replica is the leader.
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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.
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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.
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- `can_become_leader` (`UInt8`) - Whether the replica can be a leader.
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- `is_readonly` (`UInt8`) - Whether the replica is in read-only mode.
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This mode is turned on if the config does not have sections with ZooKeeper, if an unknown error occurred when reinitializing sessions in ZooKeeper, and during session reinitialization in ZooKeeper.
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- `is_session_expired` (`UInt8`) - the session with ZooKeeper has expired. Basically the same as `is_readonly`.
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- `future_parts` (`UInt32`) - The number of data parts that will appear as the result of INSERTs or merges that haven’t been done yet.
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- `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.
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- `zookeeper_path` (`String`) - Path to table data in ZooKeeper.
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- `replica_name` (`String`) - Replica name in ZooKeeper. Different replicas of the same table have different names.
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- `replica_path` (`String`) - Path to replica data in ZooKeeper. The same as concatenating ‘zookeeper_path/replicas/replica_path’.
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- `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.
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- `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`.
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- `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.
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- `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.
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- `part_mutations_in_queue` (`UInt32`) - The number of mutations waiting to be made.
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- `queue_oldest_time` (`DateTime`) - If `queue_size` greater than 0, shows when the oldest operation was added to the queue.
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- `inserts_oldest_time` (`DateTime`) - See `queue_oldest_time`
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- `merges_oldest_time` (`DateTime`) - See `queue_oldest_time`
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- `part_mutations_oldest_time` (`DateTime`) - See `queue_oldest_time`
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The next 4 columns have a non-zero value only where there is an active session with ZK.
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- `log_max_index` (`UInt64`) - Maximum entry number in the log of general activity.
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- `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.
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- `last_queue_update` (`DateTime`) - When the queue was updated last time.
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- `absolute_delay` (`UInt64`) - How big lag in seconds the current replica has.
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- `total_replicas` (`UInt8`) - The total number of known replicas of this table.
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- `active_replicas` (`UInt8`) - The number of replicas of this table that have a session in ZooKeeper (i.e., the number of functioning replicas).
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- `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.
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- `zookeeper_exception` (`String`) - The last exception message, got if the error happened when fetching the info from ZooKeeper.
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- `replica_is_active` ([Map(String, UInt8)](../../sql-reference/data-types/map.md)) — Map between replica name and is replica active.
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If you request all the columns, the table may work a bit slowly, since several reads from ZooKeeper are made for each row.
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If you do not request the last 4 columns (log_max_index, log_pointer, total_replicas, active_replicas), the table works quickly.
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For example, you can check that everything is working correctly like this:
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``` sql
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SELECT
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database,
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table,
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is_leader,
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is_readonly,
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is_session_expired,
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future_parts,
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parts_to_check,
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columns_version,
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queue_size,
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inserts_in_queue,
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merges_in_queue,
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log_max_index,
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log_pointer,
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total_replicas,
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active_replicas
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FROM system.replicas
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WHERE
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is_readonly
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OR is_session_expired
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OR future_parts > 20
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OR parts_to_check > 10
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OR queue_size > 20
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OR inserts_in_queue > 10
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OR log_max_index - log_pointer > 10
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OR total_replicas < 2
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OR active_replicas < total_replicas
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```
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If this query does not return anything, it means that everything is fine.
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[Original article](https://clickhouse.com/docs/en/operations/system-tables/replicas) <!--hide-->
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