Downside to using persistent connections?

Question

I have heard in the past that persistent connections are not good to use on a high traffic web server. Is this true, or does it only apply to apache's prefork mode? Would CGI mode have this problem?

This involves PHP, Apache, and Postgresql.

Answer 1

One problem with using persistent connections is that it doesn't really scale that well. If you have 5000 people connected, you need 5000 persistent connections. If you take away the need for persistence, you might be able to serve 10000 people with the same number of connections because they're able to share those connections when they're not using them.

Answer 2

Are PHP persistent connections evil ? -- in context of PHP and MySQL.

The reason behind using persistent connections is of course reducing number of connects which are rather expensive, even though they are much faster with MySQL than with most other databases.

The first problem with persistent connections...

If you’re establishing thousands of connections per second you normally do not keep it open for long time, but Operation System does. According to TCP/IP protocol Ports can’t be recycled instantly and have to spend some time in “FIN” stage waiting before they can be recycled.

The second problem... using too many MySQL server connections.

Some people simply do not realize you can increase *max_connections* variable and get over 100 concurrent connections with MySQL others were beaten by older Linux problems of not being able to have more than 1024 connections with MySQL.

Lets talk now about why Persistent connections were disabled in mysqli extension. Even though you could misuse persistent connections and get poor performance that was not the reason. The real reason is – you could get much more problems with it.

Persistent connections were added to PHP during times of MySQL 3.22/3.23 when MySQL was simple enough so you could recycle connections easily without any problems. In later versions number of problems however arose – If you recycle connection which has uncommitted transactions you run into trouble. If you happen to recycle connections with custom character set settings you’re in trouble back again, not to mention about possibly changed per session variables.