(PECL mysqlnd_ms < 1.2.0)
mysqlnd_ms_set_qos — Sets the quality of service needed from the cluster
$connection
, int $service_level
[, int $service_level_option
[, mixed $option_value
]] )Sets the quality of service needed from the cluster. A database cluster delivers a certain quality of service to the user depending on its architecture. A major aspect of the quality of service is the consistency level the cluster can offer. An asynchronous MySQL replication cluster defaults to eventual consistency for slave reads: a slave may serve stale data, current data, or it may have not the requested data at all, because it is not synchronous to the master. In a MySQL replication cluster, only master accesses can give strong consistency, which promises that all clients see each others changes.
PECL/mysqlnd_ms hides the complexity of choosing appropriate nodes to achieve a certain level of service from the cluster. The "Quality of Service" filter implements the necessary logic. The filter can either be configured in the plugins configuration file, or at runtime using mysqlnd_ms_set_qos().
Similar results can be achieved with PECL mysqlnd_ms < 1.2.0, if using SQL hints to force the use of a certain type of node or using the master_on_write plugin configuration option. The first requires more code and causes more work on the application side. The latter is less refined than using the quality of service filter. Settings made through the function call can be reversed, as shown in the example below. The example temporarily switches to a higher service level (session consistency, read your writes) and returns back to the clusters default after it has performed all operations that require the better service. This way, read load on the master can be minimized compared to using master_on_write, which would continue using the master after the first write.
connectionA PECL/mysqlnd_ms connection handle to a MySQL server of the type PDO_MYSQL, mysqli or ext/mysql for which a service level is to be set. The connection handle is obtained when opening a connection with a host name that matches a mysqlnd_ms configuration file entry using any of the above three MySQL driver extensions.
service_level
The requested service level: MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL,
MYSQLND_MS_QOS_CONSISTENCY_SESSION or
MYSQLND_MS_QOS_CONSISTENCY_STRONG.
service_level_option
An option to parameterize the requested service level.
The option can either be MYSQLND_MS_QOS_OPTION_GTID
or MYSQLND_MS_QOS_OPTION_AGE.
The option MYSQLND_MS_QOS_OPTION_GTID can be used
to refine the service level MYSQLND_MS_QOS_CONSISTENCY_SESSION.
It must be combined with a fourth function parameter, the
option_value. The option_value
shall be a global transaction ID obtained from
mysqlnd_ms_get_last_gtid(). If set, the
plugin considers both master servers and asynchronous slaves for session
consistency (read your writes). Otherwise, only masters are
used to achieve session consistency. A slave is considered up-to-date and
checked if it has already replicated the global transaction ID from
option_value. Please note, searching appropriate slaves
is an expensive and slow operation. Use the feature sparsely, if the
master cannot handle the read load alone.
The MYSQLND_MS_QOS_OPTION_AGE option can be combined
with the MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL
service level, to filter out asynchronous slaves that lag more seconds behind
the master than option_value. If set, the plugin
will only consider slaves for reading if
SHOW SLAVE STATUS reports
Slave_IO_Running=Yes,
Slave_SQL_Running=Yes and
Seconds_Behind_Master <= option_value. Please note,
searching appropriate slaves is an expensive and slow operation.
Use the feature sparsely in version 1.2.0. Future versions may improve the
algorithm used to identify candidates. Please, see the MySQL reference
manual about the precision, accuracy and limitations of the MySQL administrative
command SHOW SLAVE STATUS.
option_value
Parameter value for the service level option. See also the
service_level_option parameter.
Returns TRUE if the connections service level
has been switched to the requested. Otherwise, returns FALSE
Note:
mysqlnd_ms_set_qos() requires PHP >= 5.4.0 and PECL mysqlnd_ms >= 1.2.0. Internally, it is using a mysqlnd library C functionality not available with PHP 5.3.
Example #1 mysqlnd_ms_set_qos() example
<?php
/* Open mysqlnd_ms connection using mysqli, PDO_MySQL or mysql extension */
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
/* Of course, your error handling is nicer... */
die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));
/* Session consistency: read your writes */
$ret = mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_SESSION);
if (!$ret)
die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
/* Will use master and return fresh data, client can see his last write */
if (!$res = $mysqli->query("SELECT item, price FROM orders WHERE order_id = 1"))
die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
/* Back to default: use of all slaves and masters permitted, stale data can happen */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL))
die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
?>