mysqlnd-ms-php-failover-2

  • Concepts
  • Failover

  • Failover
  • Failover

    Failover

    By default, connection failover handling is left to
    the user. The application is responsible for checking return values
    of the database functions it calls and reacting to possible errors.
    If, for example, the plugin recognizes a query as a read-only query
    to be sent to the slave servers, and the slave server selected by
    the plugin is not available, the plugin will raise an error after
    not executing the statement.

    Default: manual
    failover

    It is up to the application to handle the error
    and, if required, re-issue the query to trigger the selection of
    another slave server for statement execution. The plugin will make
    no attempts to failover automatically, because the plugin cannot
    ensure that an automatic failover will not change the state of the
    connection. For example, the application may have issued a query
    which depends on SQL user variables which are bound to a specific
    connection. Such a query might return incorrect results if the
    plugin would switch the connection implicitly as part of automatic
    failover. To ensure correct results, the application must take care
    of the failover, and rebuild the required connection state.
    Therefore, by default, no automatic failover is performed by the
    plugin.

    A user that does not change the connection state
    after opening a connection may activate automatic failover. Please
    note, that automatic failover logic is limited to connection
    attempts. Automatic failover is not used for already established
    connections. There is no way to instruct the plugin to attempt
    failover on a connection that has been connected to MySQL already
    in the past.

    Automatic failover

    The failover policy is configured in the plugins
    configuration file, by using the failover configuration directive.

    Automatic and silent failover can be enabled
    through the failover configuration directive. Automatic
    failover can either be configured to try exactly one master after a
    slave failure or, alternatively, loop over slaves and masters
    before returning an error to the user. The number of connection
    attempts can be limited and failed hosts can be excluded from
    future load balancing attempts. Limiting the number of retries and
    remembering failed hosts are considered experimental features,
    albeit being reasonable stable. Syntax and semantics may change in
    future versions.

    Please note, since version 1.5.0 automatic failover
    is disabled for the duration of a transaction if transaction
    stickiness is enabled and transaction boundaries have been
    detected. The plugin will not switch connections for the duration
    of a transaction. It will also not perform automatic and silent
    failover. Instead an error will be thrown. It is then left to the
    user to handle the failure of the transaction. Please check, the
    trx_stickiness documentation how to do
    this.

    A basic manual failover example is provided within
    the error
    handling
    section.

    Standby servers

    Using weighted load balancing, introduced in
    PECL/mysqlnd 1.4.0, it is possible to configure standby servers
    that are sparsely used during normal operations. A standby server
    that is primarily used as a worst-case standby failover target can
    be assigned a very low weight/priority in relation to all other
    servers. As long as all servers are up and running the majority of
    the workload is assigned to the servers which have hight weight
    values. Few requests will be directed to the standby system which
    has a very low weight value.

    Upon failure of the servers with a high priority,
    you can still failover to the standby, which has been given a low
    load balancing priority by assigning a low weight to it. Failover
    can be some manually or automatically. If done automatically, you
    may want to combine it with the remember_failed option.

    At this point, it is not possible to instruct the
    load balancer to direct no requests at all to a standby. This may
    not be much of a limitation given that the highest weight you can
    assign to a server is 65535. Given two slaves, of which one shall
    act as a standby and has been assigned a weight of 1, the standby
    will have to handle far less than one percent of the overall
    workload.

    Failover and primary
    copy

    Please note, if using a primary copy cluster, such
    as MySQL Replication, it is difficult to do connection failover in
    case of a master failure. At any time there is only one master in
    the cluster for a given dataset. The master is a single point of
    failure. If the master fails, clients have no target to fail over
    write requests. In case of a master outage the database
    administrator must take care of the situation and update the client
    configurations, if need be.