Persistent connections are SQL 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).
注: There are other extensions that provide persistent connections, such as the IMAP extension.
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 SQL 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 each 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 serviced by a
different child process than the first time. What a
persistent connection does for you in this case it make it so
each child process only needs to connect to your SQL server
the first time that it serves a page that makes use of such a
connection. When another page then requires a connection to
the SQL server, it can reuse the connection that child
established earlier.
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.
If persistent connections don't have any added
functionality, what are they good for?
The answer here is extremely simple -- efficiency.
Persistent connections are good if the overhead to create a
link to your SQL server is high. Whether or not this overhead
is really high depends on many factors. Like, what kind of
database it is, whether or not it sits on the same computer
on which your web server sits, how loaded the machine the SQL
server sits on is and so forth. The bottom line is that if
that connection overhead is high, persistent connections help
you considerably. They cause the child process to simply
connect only once for its entire lifespan, instead of every
time it processes a page that requires connecting to the SQL
server. This means that for every child that opened a
persistent connection will have its own open persistent
connection to the server. For example, if you had 20
different child processes that ran a script that made a
persistent connection to your SQL server, you'd have 20
different connections to the SQL server, one from each
child.
Note, however, that this can have some drawbacks if you
are using a database with connection limits that are exceeded
by persistent child connections. If your database has a limit
of 16 simultaneous connections, and in the course of a busy
server session, 17 child threads attempt to connect, one will
not be able to. If there are bugs in your scripts which do
not allow the connections to shut down (such as infinite
loops), the database with only 16 connections may be rapidly
swamped. Check your database documentation for information on
handling abandoned or idle connections.
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An important summary. Persistent connections were
designed to have one-to-one mapping to regular connections.
That means that you should always be able to replace persistent
connections with non-persistent connections, and it won't
change the way your script behaves. It may (and probably
will) change the efficiency of the script, but not its
behavior!
See also fbsql_pconnect(),
ibase_pconnect(),
ifx_pconnect(), imap_popen(),
ingres_pconnect(),
msql_pconnect(),
mssql_pconnect(),
mysql_pconnect(),
OCIPLogon(),
odbc_pconnect(),
Ora_pLogon(),
pfsockopen(), pg_pconnect(), and
sybase_pconnect().