The below description applies to PECL/mysqlnd_ms < 1.1.0-beta. It is not valid for later versions.
The plugin is using its own configuration file. The configuration file holds information on the MySQL replication master server, the MySQL replication slave servers, the server pick (load balancing) policy, the failover strategy and the use of lazy connections.
The PHP configuration directive mysqlnd_ms.ini_file is used to set the plugins configuration file.
The configuration file mimics standard the php.ini format. It consists of one or more sections. Every section defines its own unit of settings. There is no global section for setting defaults.
Applications reference sections by their name. Applications use section names as the host (server) parameter to the various connect methods of the mysqli, mysql and PDO_MYSQL extensions. Upon connect the mysqlnd plugin compares the hostname with all section names from the plugin configuration file. If hostname and section name match, the plugin will load the sections settings.
Example #1 Using section names example
[myapp] master = localhost slave = 192.168.2.27 slave = 192.168.2.28:3306 [localhost] master = localhost:/tmp/mysql/mysql.sock slave = 192.168.3.24:3305 slave = 192.168.3.65:3309
/* All of the following connections will be load balanced */
$mysqli = new mysqli("myapp", "username", "password", "database");
$pdo = new PDO('mysql:host=myapp;dbname=database', 'username', 'password');
$mysql = mysql_connect("myapp", "username", "password");
$mysqli = new mysqli("localhost", "username", "password", "database");
Section names are strings. It is valid to use a section name such as 192.168.2.1, 127.0.0.1 or localhost. If, for example, an application connects to localhost and a plugin configuration section [localhost] exists, the semantics of the connect operation are changed. The application will no longer only use the MySQL server running on the host localhost but the plugin will start to load balance MySQL queries following the rules from the [localhost] configuration section. This way you can load balance queries from an application without changing the applications source code.
The master, slave and pick configuration directives use a list-like syntax. Configuration directives supporting list-like syntax may appear multiple times in a configuration section. The plugin maintains the order in which entries appear when interpreting them. For example, the below example shows two slave configuration directives in the configuration section [myapp]. If doing round-robin load balancing for read-only queries, the plugin will send the first read-only query to the MySQL server mysql_slave_1 because it is the first in the list. The second read-only query will be send to the MySQL server mysql_slave_2 because it is the second in the list. Configuration directives supporting list-like syntax result are ordered from top to bottom in accordance to their appearance within a configuration section.
Example #2 List-like syntax
[myapp] master = mysql_master_server slave = mysql_slave_1 slave = mysql_slave_2
Here is a short explanation of the configuration directives that can be used.
URI of a MySQL replication master server. The URI follows the syntax hostname[:port|unix_domain_socket].
The plugin supports using only one master server.
Setting a master server is mandatory. The plugin will report a warning upon connect if the user has failed to provide a master server for a configuration section. The warning may read (mysqlnd_ms) Cannot find master section in config. Furthermore the plugin may set an error code for the connection handle such as HY000/2000 (CR_UNKNOWN_ERROR). The corresponding error message depends on your language settings.
URI of one or more MySQL replication slave servers. The URI follows the syntax hostname[:port|unix_domain_socket].
The plugin supports using one or more slave servers.
Setting a slave server is mandatory. The plugin will report a warning upon connect if the user has failed to provide at least one slave server for a configuration section. The warning may read (mysqlnd_ms) Cannot find slaves section in config. Furthermore the plugin may set an error code for the connection handle such as HY000/2000 (CR_UNKNOWN_ERROR). The corresponding error message depends on your language settings.
Load balancing (server picking) policy. Supported policies: random, random_once (default), roundrobin, user.
If no load balancing policy is set, the plugin will default to random_once. The random_once policy picks a random slave server when running the first read-only statement. The slave server will be used for all read-only statements until the PHP script execution ends.
The random policy will pick a random server whenever a read-only statement is to be executed.
If using roundrobin the plugin iterates over the list of configured slave servers to pick a server for statement execution. If the plugin reaches the end of the list, it wraps around to the beginning of the list and picks the first configured slave server.
Setting more than one load balancing policy for a configuration section makes only sense in conjunction with user and mysqlnd_ms_set_user_pick_server(). If the user defined callback fails to pick a server, the plugin falls back to the second configured load balancing policy.
Failover policy. Supported policies: disabled (default), master.
If no failover policy is set, the plugin will not do any automatic failover (failover=disabled). Whenever the plugin fails to connect a server it will emit a warning and set the connections error code and message. Thereafter it is up to the application to handle the error and, for example, resent the last statement to trigger the selection of another server.
If using failover=master the plugin will implicitly failover to a slave, if available. Please check the concepts documentation to learn about potential pitfalls and risks of using failover=master.
Controls the use of lazy connections. Lazy connections are connections which are not opened before the client sends the first connection.
It is strongly recommended to use lazy connections. Lazy connections help to keep the number of open connections low. If you disable lazy connections and, for example, configure one MySQL replication master server and two MySQL replication slaves, the plugin will open three connections upon the first call to a connect function although the application might use the master connection only.
Lazy connections bare a risk if you make heavy use of actions which change the state of a connection. The plugin does not dispatch all state changing actions to all connections from the connection pool. The few dispatched actions are applied to already opened connections only. Lazy connections opened in the future are not affected. If, for example, the connection character set is changed using a PHP MySQL API call, the plugin will change the character set of all currently opened connection. It will not remember the character set change to apply it on lazy connections opened in the future. As a result the internal connection pool would hold connections using different character sets. This is not desired. Remember that character sets are taken into account for escaping.
If set, the plugin will use the master server only after the first statement has been executed on the master. Applications can still send statements to the slaves using SQL hints to overrule the automatic decision.
The setting may help with replication lag. If an application runs an INSERT the plugin will, by default, use the master to execute all following statements, including SELECT statements. This helps to avoid problems with reads from slaves which have not replicated the INSERT yet.
Transaction stickiness policy. Supported policies: disabled (default), master.
The setting requires 5.4.0 or newer. If used with PHP older than 5.4.0, the plugin will emit a warning like (mysqlnd_ms) trx_stickiness strategy is not supported before PHP 5.3.99.
If no transaction stickiness policy is set or, if setting trx_stickiness=disabled, the plugin is not transaction aware. Thus, the plugin may load balance connections and switch connections in the middle of a transaction. The plugin is not transaction safe. SQL hints must be used avoid connection switches during a transaction.
As of PHP 5.4.0 the mysqlnd library allows the plugin to monitor the autocommit mode set by calls to the libraries trx_autocommit() function. If setting trx_stickiness=master and autocommit gets disabled by a PHP MySQL extension invoking the mysqlnd library internal function call trx_autocommit(), the plugin is made aware of the begin of a transaction. Then, the plugin stops load balancing and directs all statements to the master server until autocommit is enabled. Thus, no SQL hints are required.
An example of a PHP MySQL API function calling the mysqlnd library internal function call trx_autocommit() is mysqli_autocommit().
Although setting trx_stickiness=master, the plugin cannot be made aware of autocommit mode changes caused by SQL statements such as SET AUTOCOMMIT=0.