22.9.6.7. Predefined Constants

The constants below are defined by this extension, andwill only be available when the extension has eitherbeen compiled into PHP or dynamically loaded at runtime.

SQL hint related

Example 22.299. Example demonstrating the usage of mysqlnd_ms constants

The mysqlnd replication and load balancing plugin (mysqlnd_ms) performs read/write splitting. This directs write queries to a MySQL master server, and read-only queries to the MySQL slave servers. The plugin has a built-in read/write split logic. All queries which start with SELECT are considered read-only queries, which are then sent to a MySQL slave server that is listed in the plugin configuration file. All other queries are directed to the MySQL master server that is also specified in the plugin configuration file.

User supplied SQL hints can be used to overrule automatic read/write splitting, to gain full control on the process. SQL hints are standards compliant SQL comments. The plugin will scan the beginning of a query string for an SQL comment for certain commands, which then control query redirection. Other systems involved in the query processing are unaffected by the SQL hints because other systems will ignore the SQL comments.

The plugin supports three SQL hints to direct queries to either the MySQL slave servers, the MySQL master server, or the last used MySQL server. SQL hints must be placed at the beginning of a query to be recognized by the plugin.

For better portability, it is recommended to use the string constants MYSQLND_MS_MASTER_SWITCH , MYSQLND_MS_SLAVE_SWITCH and MYSQLND_MS_LAST_USED_SWITCH instead of their literal values.

<?php/* Use constants for maximum portability */$master_query = "/*" . MYSQLND_MS_MASTER_SWITCH . "*/SELECT id FROM test";/* Valid but less portable: using literal instead of constant */$slave_query = "/*ms=slave*/SHOW TABLES";printf("master_query = '%s'\n", $master_query);printf("slave_query = '%s'\n", $slave_query);?>

The above examples will output:

master_query = /*ms=master*/SELECT id FROM testslave_query = /*ms=slave*/SHOW TABLES

MYSQLND_MS_MASTER_SWITCH (string): SQL hint used to send a query to the MySQL replication master server.
MYSQLND_MS_SLAVE_SWITCH (string): SQL hint used to send a query to one of the MySQL replication slave servers.
MYSQLND_MS_LAST_USED_SWITCH (string): SQL hint used to send a query to the last used MySQL server. The last used MySQL server can either be a master or a slave server in a MySQL replication setup.

mysqlnd_ms_query_is_select related

MYSQLND_MS_QUERY_USE_MASTER (integer): If mysqlnd_ms_is_select returns MYSQLND_MS_QUERY_USE_MASTER for a given query, the built-in read/write split mechanism recommends sending the query to a MySQL replication master server.
MYSQLND_MS_QUERY_USE_SLAVE (integer): If mysqlnd_ms_is_select returns MYSQLND_MS_QUERY_USE_SLAVE for a given query, the built-in read/write split mechanism recommends sending the query to a MySQL replication slave server.
MYSQLND_MS_QUERY_USE_LAST_USED (integer): If mysqlnd_ms_is_select returns MYSQLND_MS_QUERY_USE_LAST_USED for a given query, the built-in read/write split mechanism recommends sending the query to the last used server.

mysqlnd_ms_set_qos, quality of service filter and service level related

MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL (integer): Use to request the service level eventual consistency from the mysqlnd_ms_set_qos. Eventual consistency is the default quality of service when reading from an asynchronous MySQL replication slave. Data returned in this service level may or may not be stale, depending on whether the selected slaves happen to have replicated the latest changes from the MySQL replication master or not.
MYSQLND_MS_QOS_CONSISTENCY_SESSION (integer): Use to request the service level session consistency from the mysqlnd_ms_set_qos. Session consistency is defined as read your writes. The client is guaranteed to see his latest changes.
MYSQLND_MS_QOS_CONSISTENCY_STRONG (integer): Use to request the service level strong consistency from the mysqlnd_ms_set_qos. Strong consistency is used to ensure all clients see each others changes.
MYSQLND_MS_QOS_OPTION_GTID (integer): Used as a service level option with mysqlnd_ms_set_qos to parameterize session consistency.
MYSQLND_MS_QOS_OPTION_AGE (integer): Used as a service level option with mysqlnd_ms_set_qos to parameterize eventual consistency.

Other

The plugins version number can be obtained using MYSQLND_MS_VERSION or MYSQLND_MS_VERSION_ID . MYSQLND_MS_VERSION is the string representation of the numerical version number MYSQLND_MS_VERSION_ID , which is an integer such as 10000. Developers can calculate the version number as follows.

Version (part)	Example
Major*10000	1*10000 = 10000
Minor*100	0*100 = 0
Patch	0 = 0
MYSQLND_MS_VERSION_ID	10000

MYSQLND_MS_VERSION (string): Plugin version string, for example, "1.0.0-prototype".
MYSQLND_MS_VERSION_ID (integer): Plugin version number, for example, 10000.