&reftitle.constants; &extension.constants;

SQL hint related

## 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);
    ?>

&examples.outputs;

    master_query = /*ms=master*/SELECT id FROM test
    slave_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.