Read-Only Replicas

Learn how to configure replicas as read-only instances. This ensures data integrity by preventing accidental writes on the replica while allowing it to process replication events.

A common replication setup is to have the replicas read-only to ensure that no one accidentally updates them. If the replica has binary logging enabled and gtid_strict_mode is used, then any update that causes changes to the binary log will stop replication.

When the variable read_only is set to 1, no updates are permitted except from users with the SUPER privilege (<= MariaDB 10.5.1) or READ ONLY ADMIN privilege (>= MariaDB 10.5.2) or replica servers updating from a primary. Inserting rows to log tables, updates to temporary tables and OPTIMIZE TABLE or ANALYZE TABLE statements on temporary tables are excluded from this limitation.

If read_only is set to 1, then the SET PASSWORD statement is limited only to users with the SUPER privilege (<= MariaDB 10.5.1) or READ ONLY ADMIN privilege (>= MariaDB 10.5.2).

Attempting to set the read_only variable to 1 will fail if the current session has table locks or transactions pending.

The statement will wait for other sessions that hold table locks. While the attempt to set read_only is waiting, other requests for table locks or transactions will also wait until read_only has been set.

Older MariaDB Versions

If you are using an older MariaDB version with read-only replicas and binary logging enabled on the replica, and you need to do some changes but don't want to have them logged to the binary log, the easiest way to avoid the logging is to disable binary logging while running as root during maintenance:

SET sql_log_bin=0;
ALTER TABLE test ENGINE=rocksdb;

The above changes the test table on the replica to rocksdb without registering the change in the binary log.

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