Galera Cluster System Tables

Starting with Galera 4 (used in MariaDB 10.4 and later), several system tables related to replication are available in the mysql database. These tables can be queried by administrators to get a real-time view of the cluster's layout, membership, and current operations.

You can view these tables with the following query:

SHOW TABLES FROM mysql LIKE 'wsrep%';

+---------------------------+
| Tables_in_mysql (wsrep%)  |
+---------------------------+
| wsrep_allowlist           |
| wsrep_cluster             |
| wsrep_cluster_members     |
| wsrep_streaming_log       |
+---------------------------+

mysql vs. mariadb

You'll see queries referencing the mysql database (e.g., FROM mysql.wsrep_cluster). This is intentional. MariaDB, a MySQL fork, retains the mysql name for its internal system schema to ensure historical and backward compatibility where it manages user permissions and system tables.

This is different from the command-line client, which should always be invoked as mariadb.

These tables are managed by the cluster itself and should not be modified by users, with the exception of wsrep_allowlist.

wsrep_allowlist

This table stores a list of allowed IP addresses that can join the cluster and perform a state transfer (IST/SST). It is a security feature to prevent unauthorized nodes from joining.

MariaDB [mysql]> DESCRIBE wsrep_allowlist;

+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| ip    | char(64) | NO   | PRI | NULL    |       |
+-------+----------+------+-----+---------+-------+

To add a new node to the allowlist, you can INSERT its IP address:

INSERT INTO mysql.wsrep_allowlist(ip) VALUES('18.193.102.155');

If a node attempts to join and its IP address is not in the allowlist, the join will fail. The DONOR nodes will log a warning similar to this:

[Warning] WSREP: Connection not allowed, IP 3.70.155.51 not found in allowlist.

The joining node will fail with a connection timeout error.

wsrep_cluster

This table contains a single row with a high-level view of the cluster's identity, state, and capabilities.

Attribute	Description
cluster_uuid	The unique identifier for the cluster.
view_id	Corresponds to the wsrep_cluster_conf_id status variable, representing the current membership view ID.
view_seqno	The global transaction sequence number associated with this cluster view.
protocol_version	The wsrep protocol version in use.
capabilities	A bitmask of capabilities provided by the Galera library.

You can query its contents like this:

SELECT * FROM mysql.wsrep_cluster\G

*************************** 1. row ***************************
   cluster_uuid: bd5fe1c3-7d80-11e9-8913-4f209d688a15
        view_id: 3
     view_seqno: 2956
protocol_version: 4
   capabilities: 184703

wsrep_cluster_members

This table provides a real-time list of all the nodes that are currently members of the cluster component.

Node	Description
node_uuid	The unique identifier for each individual node.
cluster_uuid	The UUID of the cluster this node belongs to.
node_name	The human-readable name of the node, set by the wsrep_node_name parameter.
node_incoming_address	The IP address and port where the node is listening for client connections.

Querying this table gives you a quick overview of the current cluster membership:

SELECT * FROM mysql.wsrep_cluster_members ORDER BY node_name\G

*************************** 1. row ***************************
           node_uuid: e39d1774-7e2b-11e9-b5b2-7696f81d30fb
        cluster_uuid: bd5fe1c3-7d80-11e9-8913-4f209d688a15
           node_name: galera1
node_incoming_address: AUTO
*************************** 2. row ***************************
           node_uuid: eb8fc512-7e2b-11e9-bb74-3281cf207f60
        cluster_uuid: bd5fe1c3-7d80-11e9-8913-4f209d688a15
           node_name: galera2
node_incoming_address: AUTO
*************************** 3. row ***************************
           node_uuid: 2347a8ac-7e2c-11e9-b6f0-da90a2d0a563
        cluster_uuid: bd5fe1c3-7d80-11e9-8913-4f209d688a15
           node_name: galera3
node_incoming_address: AUTO

wsrep_streaming_log

This table contains metadata for Streaming Replication transactions that are currently in progress. Each row represents a write-set fragment. The table is typically empty unless a large or long-running transaction with streaming enabled is active.

Fragment	Description
node_uuid	The UUID of the node where the streaming transaction originated.
trx_id	The transaction identifier.
seqno	The sequence number of the specific write-set fragment.
flags	Flags associated with the fragment.
frag	The binary log events contained in the fragment.

Example of querying the table during a streaming transaction:

-- Enable streaming for the session
SET SESSION wsrep_trx_fragment_unit='statements';
SET SESSION wsrep_trx_fragment_size=1;

-- Start a transaction
START TRANSACTION;
INSERT INTO my_table VALUES (100);

-- Query the log table in the same session
SELECT node_uuid, trx_id, seqno, flags
FROM mysql.wsrep_streaming_log;

+--------------------------------------+--------+-------+-------+
| node_uuid                            | trx_id | seqno | flags |
+--------------------------------------+--------+-------+-------+
| a006244a-7ed8-11e9-bf00-867215999c7c |     26 |     4 |     1 |
+--------------------------------------+--------+-------+-------+

_{This page is licensed: CC BY-SA / Gnu FDL}