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118 lines
4.3 KiB
Markdown
118 lines
4.3 KiB
Markdown
# system.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 = 'visits'
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FORMAT Vertical
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```
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```text
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Row 1:
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──────
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database: merge
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table: visits
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engine: ReplicatedCollapsingMergeTree
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is_leader: 1
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is_readonly: 0
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is_session_expired: 0
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future_parts: 1
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parts_to_check: 0
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zookeeper_path: /clickhouse/tables/01-06/visits
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replica_name: example01-06-1.yandex.ru
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replica_path: /clickhouse/tables/01-06/visits/replicas/example01-06-1.yandex.ru
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columns_version: 9
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queue_size: 1
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inserts_in_queue: 0
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merges_in_queue: 1
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log_max_index: 596273
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log_pointer: 596274
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total_replicas: 2
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active_replicas: 2
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```
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Columns:
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```text
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database: database name
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table: table name
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engine: table engine name
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is_leader: whether the replica is the leader
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Only one replica at a time can be the leader. The leader is responsible for selecting background merges to perform.
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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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is_readonly: Whether the replica is in read-only mode.This mode is turned on if the config doesn't have sections with ZK, if an unknown error occurred when reinitializing sessions in ZK, and during session reinitialization in ZK.
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is_session_expired: Whether the ZK session expired.
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Basically, the same thing as is_readonly.
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future_parts: 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: 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: The path to the table data in ZK.
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replica_name: Name of the replica in ZK. Different replicas of the same table have different names.
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replica_path: The path to the replica data in ZK. The same as concatenating zookeeper_path/replicas/replica_path.
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columns_version: 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: Size of the queue for operations waiting to be performed. Operations include inserting blocks of data, merges, and certain other actions.
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Normally coincides with future_parts.
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inserts_in_queue: Number of inserts of blocks of data that need to be made. Insertions are usually replicated fairly quickly. If the number is high, something is wrong.
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merges_in_queue: 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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The next 4 columns have a non-null value only if the ZK session is active.
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log_max_index: Maximum entry number in the log of general activity.
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log_pointer: 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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total_replicas: Total number of known replicas of this table.
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active_replicas: Number of replicas of this table that have a ZK session (the number of active replicas).
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```
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If you request all the columns, the table may work a bit slowly, since several reads from ZK are made for each row.
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If you don't 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 doesn't return anything, it means that everything is fine.
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