Oracle 5.0 Reference Manual Download

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MySQL Improved Extension (

Mysqli

)

2314

For setting a connection option, the connect operation has to be performed in three steps: creating a
connection handle with

mysqli_init

, 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 a unused persistent connection for a given combination of host, username, password, socket,
port and default database can not 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

function 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.

Oracle 5.0, Reference Manual

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