system.replicas

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.

Example:

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

Row 1:
──────
database:           merge
table:              visits
engine:             ReplicatedCollapsingMergeTree
is_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
log_max_index:      596273
log_pointer:        596274
total_replicas:     2
active_replicas:    2

Columns:

database:           database name
table:              table name
engine:             table engine name

is_leader:          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.

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.

is_session_expired: Whether the ZK session expired.
Basically, the same thing as is_readonly.

future_parts: The number of data parts that will appear as the result of INSERTs or merges that haven't been done yet. 

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.

zookeeper_path: The path to the table data in ZK. 
replica_name: Name of the replica in ZK. Different replicas of the same table have different names. 
replica_path: The path to the replica data in ZK. The same as concatenating zookeeper_path/replicas/replica_path.

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.

queue_size:         Size of the queue for operations waiting to be performed.
Operations include inserting blocks of data, merges, and certain other actions.
Normally coincides with future_parts.

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.

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.

The next 4 columns have a non-null value only if the ZK session is active.

log_max_index:     Maximum entry number in the log of general activity.
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.

total_replicas:     Total number of known replicas of this table.
active_replicas:    Number of replicas of this table that have a ZK session (the number of active replicas).

If you request all the columns, the table may work a bit slowly, since several reads from ZK 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.

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system.replicas

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