## Connections

The MySQL server supports the use of different transport layers for connections. Connections use TCP/IP, Unix domain sockets or Windows named pipes.

The hostname `localhost` has a special meaning. It is bound to the use of Unix domain sockets. To open a TCP/IP connection to the localhost, `127.0.0.1` must be used instead of the hostname `localhost`.

Example #1 Special meaning of localhost

`<?php
$mysqli = new mysqli("localhost", "user", "password", "database");
echo $mysqli->host_info . "\n";
$mysqli = new mysqli("127.0.0.1", "user", "password", "database", 3306);
echo $mysqli->host_info . "\n";`

The above example will output:

    Localhost via UNIX socket
    127.0.0.1 via TCP/IP

Connection parameter defaults

Depending on the connection function used, assorted parameters can be omitted. If a parameter is not provided, then the extension attempts to use the default values that are set in the PHP configuration file.

Example #2 Setting defaults

    mysqli.default_host=192.168.2.27
    mysqli.default_user=root
    mysqli.default_pw=""
    mysqli.default_port=3306
    mysqli.default_socket=/tmp/mysql.sock

The resulting parameter values are then passed to the client library that is used by the extension. If the client library detects empty or unset parameters, then it may default to the library built-in values.

Built-in connection library defaults

If the host value is unset or empty, then the client library will default to a Unix socket connection on `localhost`. If socket is unset or empty, and a Unix socket connection is requested, then a connection to the default socket on `/tmp/mysql.sock` is attempted.

On Windows systems, the host name `.` is interpreted by the client library as an attempt to open a Windows named pipe based connection. In this case the socket parameter is interpreted as the pipe name. If not given or empty, then the socket (pipe name) defaults to `\\.\pipe\MySQL`.

If neither a Unix domain socket based not a Windows named pipe based connection is to be established and the port parameter value is unset, the library will default to port `3306`.

The mysqlnd library and the MySQL Client Library (libmysqlclient) implement the same logic for determining defaults.

Connection options

Connection options are available to, for example, set init commands which are executed upon connect, or for requesting use of a certain charset. Connection options must be set before a network connection is established.

For setting a connection option, the connect operation has to be performed in three steps: creating a connection handle with mysqli_init() or mysqli::__construct(), setting the requested options using mysqli::options(), and establishing the network connection with mysqli::real_connect().

Connection pooling

The mysqli extension supports persistent database connections, which are a special kind of pooled connections. By default, every database connection opened by a script is either explicitly closed by the user during runtime or released automatically at the end of the script. A persistent connection is not. Instead it is put into a pool for later reuse, if a connection to the same server using the same username, password, socket, port and default database is opened. Reuse saves connection overhead.

Every PHP process is using its own mysqli connection pool. Depending on the web server deployment model, a PHP process may serve one or multiple requests. Therefore, a pooled connection may be used by one or more scripts subsequently.

Persistent connection

If an unused persistent connection for a given combination of host, username, password, socket, port and default database cannot be found in the connection pool, then mysqli opens a new connection. The use of persistent connections can be enabled and disabled using the PHP directive mysqli.allow_persistent. The total number of connections opened by a script can be limited with mysqli.max_links. The maximum number of persistent connections per PHP process can be restricted with mysqli.max_persistent. Please note that the web server may spawn many PHP processes.

A common complain about persistent connections is that their state is not reset before reuse. For example, open and unfinished transactions are not automatically rolled back. But also, authorization changes which happened in the time between putting the connection into the pool and reusing it are not reflected. This may be seen as an unwanted side-effect. On the contrary, the name `persistent` may be understood as a promise that the state is persisted.

The mysqli extension supports both interpretations of a persistent connection: state persisted, and state reset before reuse. The default is reset. Before a persistent connection is reused, the mysqli extension implicitly calls mysqli::change_user() to reset the state. The persistent connection appears to the user as if it was just opened. No artifacts from previous usages are visible.

The mysqli::change_user() call is an expensive operation. For best performance, users may want to recompile the extension with the compile flag `MYSQLI_NO_CHANGE_USER_ON_PCONNECT` being set.

It is left to the user to choose between safe behavior and best performance. Both are valid optimization goals. For ease of use, the safe behavior has been made the default at the expense of maximum performance.

See also

- mysqli::__construct()
- mysqli_init()
- mysqli::options()
- mysqli::real_connect()
- mysqli::change_user()
- $mysqli::host_info
- MySQLi Configuration Options
- Persistent Database Connections

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