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## Connection pooling and switching

The replication and load balancing plugin changes the semantics of a PHP MySQL connection handle. The existing API of the PHP MySQL extensions (mysqli, mysql, and PDO_MYSQL) are not changed in a way that functions are added or removed. But their behaviour changes when using the plugin. Existing applications do not need to be adapted to a new API, but they may need to be modified because of the behaviour changes.

The plugin breaks the one-by-one relationship between a mysqli, mysql, and PDO_MYSQL connection handle and a MySQL network connection. And a mysqli, mysql, and PDO_MYSQL connection handle represents a local pool of connections to the configured MySQL replication master and MySQL replication slave servers. The plugin redirects queries to the master and slave servers. At some point in time one and the same PHP connection handle may point to the MySQL master server. Later on, it may point to one of the slave servers or still the master. Manipulating and replacing the network connection referenced by a PHP MySQL connection handle is not a transparent operation.

Every MySQL connection has a state. The state of the connections in the connection pool of the plugin can differ. Whenever the plugin switches from one wire connection to another, the current state of the user connection may change. The applications must be aware of this.

The following list shows what the connection state consists of. The list may not be complete.

- Transaction status
- Temporary tables
- Table locks
- Session system variables and session user variables
- The current database set using USE and other state chaining SQL commands
- Prepared statements
- HANDLER variables
- Locks acquired with GET_LOCK()

Connection switches happen right before queries are executed. The plugin does not switch the current connection until the next statement is executed.

> Note: Replication issues

> See also the MySQL reference manual chapter about » replication features and related issues. Some restrictions may not be related to the PHP plugin, but are properties of the MySQL replication system.

Broadcasted messages

The plugins philosophy is to align the state of connections in the pool only if the state is under full control of the plugin, or if it is necessary for security reasons. Just a few actions that change the state of the connection fall into this category.

The following is a list of connection client library calls that change state, and are broadcasted to all open connections in the connection pool.

If any of the listed calls below are to be executed, the plugin loops over all open master and slave connections. The loop continues until all servers have been contacted, and the loop does not break if a server indicates a failure. If possible, the failure will propagate to the called user API function, which may be detected depending on which underlying library function was triggered.

Library call | Notes | Version
--- | --- | ---
change_user() | Called by the mysqli_change_user() user API call. Also triggered upon reuse of a persistent mysqli connection. | Since 1.0.0.
select_db | Called by the following user API calls: mysql_select_db(), mysql_list_tables(), mysql_db_query(), mysql_list_fields(), mysqli_select_db(). Note, that SQL USE is not monitored. | Since 1.0.0.
set_charset() | Called by the following user API calls: mysql_set_charset(). mysqli_set_charset(). Note, that SQL SET NAMES is not monitored. | Since 1.0.0.
set_server_option() | Called by the following user API calls: mysqli_multi_query(), mysqli_real_query(), mysqli_query(), mysql_query(). | Since 1.0.0.
set_client_option() | Called by the following user API calls: mysqli_options(), mysqli_ssl_set(), mysqli_connect(), mysql_connect(), mysql_pconnect(). | Since 1.0.0.
set_autocommit() | Called by the following user API calls: mysqli_autocommit(), PDO::setAttribute(PDO::ATTR_AUTOCOMMIT). | Since 1.0.0. PHP >= 5.4.0.
ssl_set() | Called by the following user API calls: mysqli_ssl_set(). | Since 1.1.0.

Broadcasting and lazy connections

The plugin does not proxy or "remember" all settings to apply them on connections opened in the future. This is important to remember, if using lazy connections. Lazy connections are connections which are not opened before the client sends the first connection. Use of lazy connections is the default plugin action.

The following connection library calls each changed state, and their execution is recorded for later use when lazy connections are opened. This helps ensure that the connection state of all connections in the connection pool are comparable.

Library call | Notes | Version
--- | --- | ---
change_user() | User, password and database recorded for future use. | Since 1.1.0.
select_db | Database recorded for future use. | Since 1.1.0.
set_charset() | Calls set_client_option(MYSQL_SET_CHARSET_NAME, charset) on lazy connection to ensure charset will be used upon opening the lazy connection. | Since 1.1.0.
set_autocommit() | Adds SET AUTOCOMMIT=0|1 to the list of init commands of a lazy connection using set_client_option(MYSQL_INIT_COMMAND, "SET AUTOCOMMIT=...%quot;). | Since 1.1.0. PHP >= 5.4.0.

Caution

# Connection state

The connection state is not only changed by API calls. Thus, even if PECL mysqlnd_ms monitors all API calls, the application must still be aware. Ultimately, it is the applications responsibility to maintain the connection state, if needed.

Charsets and string escaping

Due to the use of lazy connections, which are a default, it can happen that an application tries to escape a string for use within SQL statements before a connection has been established. In this case string escaping is not possible. The string escape function does not know what charset to use before a connection has been established.

To overcome the problem a new configuration setting server_charset has been introduced in version 1.4.0.

Attention has to be paid on escaping strings with a certain charset but using the result on a connection that uses a different charset. Please note, that PECL/mysqlnd_ms manipulates connections and one application level connection represents a pool of multiple connections that all may have different default charsets. It is recommended to configure the servers involved to use the same default charsets. The configuration setting server_charset does help with this situation as well. If using server_charset, the plugin will set the given charset on all newly opened connections.

Architecture
Transaction handling
Concepts
PHP Manual