• Concepts
  • Architecture

  • Architecture
  • Architecture


    The mysqlnd replication and load balancing plugin
    is implemented as a PHP extension. It is written in C and operates
    under the hood of PHP. During the startup of the PHP interpreter,
    in the module init phase of the PHP engine, it gets registered as a
    mysqlnd plugin to
    replace selected mysqlnd C methods.

    At PHP runtime, it inspects queries sent from
    mysqlnd (PHP) to the MySQL server. If a query is recognized as
    read-only, it will be sent to one of the configured slave servers.
    Statements are considered read-only if they either start with
    SELECT, the SQL hint /*ms=slave*/ or a slave had
    been chosen for running the previous query, and the query started
    with the SQL hint /*ms=last_used*/. In all other cases,
    the query will be sent to the MySQL replication master server.

    For better portability, applications should use the
    MYSQLND_MS_LAST_USED_SWITCH predefined mysqlnd_ms
    , instead of their literal values, such as

    The plugin handles the opening and closing of
    database connections to both master and slave servers. From an
    application point of view, there continues to be only one
    connection handle. However, internally, this one public connection
    handle represents a pool of network connections that are managed by
    the plugin. The plugin proxies queries to the master server, and to
    the slaves using multiple connections.

    Database connections have a state consisting of,
    for example, transaction status, transaction settings, character
    set settings, and temporary tables. The plugin will try to maintain
    the same state among all internal connections, whenever this can be
    done in an automatic and transparent way. In cases where it is not
    easily possible to maintain state among all connections, such as
    when using BEGIN TRANSACTION, the plugin leaves it to the
    user to handle.