Chapter 21. Persistent Database Connections
Persistent connections are links that do not close when
the execution of your script ends. When a persistent connection
is requested, PHP checks if there's already an identical
persistent connection (that remained open from earlier)
- and if it exists, it uses it. If it does not exist, it
creates the link. An 'identical' connection is a connection
that was opened to the same host, with the same username
and the same password (where applicable).
People who aren't thoroughly familiar with the way web
servers work and distribute the load may mistake persistent
connects for what they're not. In particular, they do not
give you an ability to open 'user sessions' on the same
link, they do not give you an ability to build up a transaction
efficiently, and they don't do a whole lot of other things.
In fact, to be extremely clear about the subject, persistent
connections don't give you any functionality that wasn't
possible with their non-persistent brothers.
Why?
This has to do with the way web servers work. There are
three ways in which your web server can utilize PHP to generate
web pages.
The first method is to use PHP as a CGI "wrapper".
When run this way, an instance of the PHP interpreter is
created and destroyed for every page request (for a PHP
page) to your web server. Because it is destroyed after
every request, any resources that it acquires (such as a
link to an SQL database server) are closed when it is destroyed.
In this case, you do not gain anything from trying to use
persistent connections -- they simply don't persist.
The second, and most popular, method is to run PHP as a
module in a multiprocess web server, which currently only
includes Apache. A multiprocess server typically has one
process (the parent) which coordinates a set of processes
(its children) who actually do the work of serving up web
pages. When a request comes in from a client, it is handed
off to one of the children that is not already serving another
client. This means that when the same client makes a second
request to the server, it may be served by a different child
process than the first time. When opening a persistent connection,
every following page requesting SQL services can reuse the
same established connection to the SQL server.
The last method is to use PHP as a plug-in for a multithreaded
web server. Currently PHP 4 has support for ISAPI, WSAPI,
and NSAPI (on Windows), which all allow PHP to be used as
a plug-in on multithreaded servers like Netscape FastTrack
(iPlanet), Microsoft's Internet Information Server (IIS),
and O'Reilly's WebSite Pro. The behavior is essentially
the same as for the multiprocess model described before.
Note that SAPI support is not available in PHP 3.
