• Concepts
  • Read-write splitting

  • Read-write splitting
  • Read-write splitting

    Read-write splitting

    The plugin executes read-only statements on the
    configured MySQL slaves, and all other queries on the MySQL master.
    Statements are considered read-only if they either start with
    SELECT, the SQL hint /*ms=slave*/, or if a slave
    had been chosen for running the previous query and the query starts
    with the SQL hint /*ms=last_used*/. In all other cases,
    the query will be sent to the MySQL replication master server. It
    is recommended to use the constants
    of /*ms=slave*/. See also the list of mysqlnd_ms

    SQL hints are a special kind of standard compliant
    SQL comments. The plugin does check every statement for certain SQL
    hints. The SQL hints are described within the mysqlnd_ms constants
    documentation, constants that are exported by the extension. Other
    systems involved with the statement processing, such as the MySQL
    server, SQL firewalls, and SQL proxies, are unaffected by the SQL
    hints, because those systems are designed to ignore SQL

    The built-in read-write splitter can be replaced by
    a user-defined filter, see also the user filter documentation.

    A user-defined read-write splitter can request the
    built-in logic to send a statement to a specific location, by
    invoking mysqlnd_ms_is_select().


    The built-in read-write splitter is not aware of
    multi-statements. Multi-statements are seen as one statement. The
    splitter will check the beginning of the statement to decide where
    to run the statement. If, for example, a multi-statement begins
    with SELECT 1 FROM DUAL; INSERT INTO test(id) VALUES (1);
    the plugin will run it on a slave although the statement
    is not read-only.