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Service level and consistency

Note: Version requirement

Service levels have been introduced in PECL mysqlnd_ms version 1.2.0-alpha. mysqlnd_ms_set_qos() is available with PHP 5.4.0 or newer.

Different types of MySQL cluster solutions offer different service and data consistency levels to their users. An asynchronous MySQL replication cluster offers eventual consistency by default. A read executed on an asynchronous slave may return current, stale or no data at all, depending on whether the slave has replayed all changesets from the master or not.

Applications using an MySQL replication cluster need to be designed to work correctly with eventual consistent data. In some cases, however, stale data is not acceptable. In those cases only certain slaves or even only master accesses are allowed to achieve the required quality of service from the cluster.

As of PECL mysqlnd_ms 1.2.0 the plugin is capable of selecting MySQL replication nodes automatically that deliver session consistency or strong consistency. Session consistency means that one client can read its writes. Other clients may or may not see the clients' write. Strong consistency means that all clients will see all writes from the client.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        }
    }
}

Service levels can be set in the plugins configuration file and at runtime using mysqlnd_ms_set_qos(). In the example the function is used to enforce session consistency (read your writes) for all future statements until further notice. The SELECT statement on the orders table is run on the master to ensure the previous write can be seen by the client. Read-write splitting logic has been adapted to fulfill the service level.

After the application has read its changes from the orders table it returns to the default service level, which is eventual consistency. Eventual consistency puts no restrictions on choosing a node for statement execution. Thus, the SELECT statement on the specials table is executed on a slave.

The new functionality supersedes the use of SQL hints and the master_on_write configuration option. In many cases mysqlnd_ms_set_qos() is easier to use, more powerful improves portability.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        },
        "failover" : "master"
    }
}

The eventual consistency service level can be used with an optional parameter to set a maximum slave lag for choosing slaves. If set, the plugin checks SHOW SLAVE STATUS for all configured slaves. In case of the example, only slaves for which Slave_IO_Running=Yes, Slave_SQL_Running=Yes and Seconds_Behind_Master <= 4 is true are considered for executing the statement SELECT item, price FROM daytrade.

Checking SHOW SLAVE STATUS is done transparently from an applications perspective. Errors, if any, are reported as warnings. No error will be set on the connection handle. Even if all SHOW SLAVE STATUS SQL statements executed by the plugin fail, the execution of the users statement is not stopped, given that master fail over is enabled. Thus, no application changes are required.

Note: Expensive and slow operation

Checking SHOW SLAVE STATUS for all slaves adds overhead to the application. It is an expensive and slow background operation. Try to minimize the use of it. Unfortunately, a MySQL replication cluster does not give clients the possibility to request a list of candidates from a central instance. Thus, a more efficient way of checking the slaves lag is not available.

Please, note the limitations and properties of SHOW SLAVE STATUS as explained in the MySQL reference manual.

To prevent mysqlnd_ms from emitting a warning if no slaves can be found that lag no more than the defined number of seconds behind the master, it is necessary to enable master fail over in the plugins configuration file. If no slaves can be found and fail over is turned on, the plugin picks a master for executing the statement.

If no slave can be found and fail over is turned off, the plugin emits a warning, it does not execute the statement and it sets an error on the connection.

{
    "myapp": {
        "master": {
            "master_0": {
                "host": "localhost",
                "socket": "\/tmp\/mysql.sock"
            }
        },
        "slave": {
            "slave_0": {
                "host": "127.0.0.1",
                "port": "3306"
            }
        }
    }
}

PHP Warning:  mysqli::query(): (mysqlnd_ms) Couldn't find the appropriate slave connection. 0 slaves to choose from. Something is wrong in %s on line %d
PHP Warning:  mysqli::query(): (mysqlnd_ms) No connection selected by the last filter in %s on line %d
[2000] (mysqlnd_ms) No connection selected by the last filter