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PHP : Features : Persistent Database Connections

Chapter 8. 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.


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.

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.


There are a couple of additional caveats to keep in mind when using persistent connections. One is that when using table locking on a persistent connection, if the script for whatever reason cannot release the lock, then subsequent scripts using the same connection will block indefinitely and may require that you either restart the httpd server or the database server. Another is that when using transactions, a transaction block will also carry over to the next script which uses that connection if script execution ends before the transaction block does. In either case, you can use register_shutdown_function() to register a simple cleanup function to unlock your tables or roll back your transactions. Better yet, avoid the problem entirely by not using persistent connections in scripts which use table locks or transactions (you can still use them elsewhere).

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(), ingres_pconnect(), msql_pconnect(), mssql_pconnect(), mysql_pconnect(), ociplogon(), odbc_pconnect(), ora_plogon(), pfsockopen(), pg_pconnect(), and sybase_pconnect().

Code Examples / Notes » features.persistent_connections


You can in fact provide a port for the connection, take a look at
Just use "hostname:port" for the server address.


Yes, with nonpersistent connections database connections last only while a database-related request is processed, thus reducing the load on the database server.
However, latency will be somewhat higher since a database connection must be opened before a request can be handeled.


To those using MySQL and finding a lot of leftover sleeping processes, take a look at MySQL's wait_timeout directive. By default it is set to 8 hours, but almost any decent production server will have been lowered to the 60 second range. Even on my testing server, I was having problems with too many connections from leftover persistent connections.


this one bit quite a bit of chunk out of my you-know-what. seems like if you're running multiple database servers on the same host (for eg. MySQL on a number of ports) you can't use pconnect since the port number isn't part of the key for database connections. especially if you have the same username and password to connect to all the database servers running on different ports. but then it might be php-MySQL specific. you might get a connection for an entirely different port than the one you asked for.


This may only pertain to Apache/MySQL:
After several hours of wrestling with MySQL "Access denied" messages, I determined that persistent database connections don't necessarily reflect subsequent privilege changes.
I loaded a PHP script attempting a LOAD DATA statement, and got an "Access denied" error. I granted FILE privileges to the MySQL user, and was able to run LOAD DATA statements from the terminal, but still got "Access denied" from my PHP pages.  When I switched from mysql_pconnect() to mysql_connect(), the problem went away; eventually I restarted apache to kill the persistent connection and switched back to mysql_pconnect(), and now everything works fine.


in response to web at nick, have you tried FLUSH PRIVILEGES. this should reload those privileges.


If you have multiple databases on the same server AND you are using persistent connections, you MUST prefix all the table names with the specific database name.
Changing the database using the xxx_select_db functions alters the database for the connection for all users who are sharing that connection (assuming PHP is running shared and not CGI/CLI).
If you have 2 databases (live and archive) and your script is talking to both, you cannot use 2 persistent connections and change the database for each one.
Internally, persistent connections are used even if you do not specify that you want to use persistent connections. This is why new_link was added to mysql_connect/mssql_connect (PHPV4.2.0+).


If anyone ever wonders why the number of idle db process (open connections) seems to grow even though you are using persistent connections, here's why:
"You are probably using a multi-process web server such as Apache. Since
database connections cannot be shared among different processes a new
one is created if the request happen to come to a different web server
child process."

christopher dot jones

For the oci8 extension it is not true that " [...] when using transactions, a transaction block will also carry over to the next script which uses that connection if script execution ends before the transaction block does.".  The oci8 extension does a rollback at the end scripts using persistent connections, thus ending the transaction.  The rollback also releases locks. However any ALTER SESSION command (e.g. changing the date format) on a persistent connection will be retained over to the next script.

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