What are the disadvantages of using persistent connection in PDO

Question

In PDO, a connection can be made persistent using the PDO::ATTR_PERSISTENT attribute. According to the php manual -

Persistent connectio

Answer 1

Persistent connections are a good idea only when it takes a (relatively) long time to connect to your database. Nowadays that's almost never the case. The biggest drawback to persistent connections is that it limits the number of users you can have browsing your site: if MySQL is configured to only allow 10 concurrent connections at once then when an 11th person tries to browse your site it won't work for them.

PDO does not manage the persistence. The MySQL driver does. It reuses connections when a) they are available and the host/user/password/database match. If any change then it will not reuse a connection. The best case net effect is that these connections you have will be started and stopped so often because you have different users on the site and making them persistent doesn't do any good.

The key thing to understand about persistent connections is that you should NOT use them in most web applications. They sound enticing but they are dangerous and pretty much useless.

I'm sure there are other threads on this but a persistent connection is dangerous because it persists between requests. If, for example, you lock a table during a request and then fail to unlock then that table is going to stay locked indefinitely. Persistent connections are also pretty much useless for 99% of your apps because you have no way of knowing if the same connection will be used between different requests. Each web thread will have it's own set of persistent connections and you have no way of controlling which thread will handle which requests.

The procedural mysql library of PHP, has a feature whereby subsequent calls to mysql_connect will return the same link, rather than open a different connection (As one might expect). This has nothing to do with persistent connections and is specific to the mysql library. PDO does not exhibit such behaviour

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In General you could use this as a rough "ruleset"::

YES, use persistent connections, if:

• There are only few applications/users accessing the database, i.e. you will not result in 200 open (but probably idle) connections, because there are 200 different users shared on the same host.

• The database is running on another server that you are accessing over the network

• An (one) application accesses the database very often

NO, don't use persistent connections, if:

• Your application only needs to access the database 100 times an hour.

• You have many, many webservers accessing one database server

Using persistent connections is considerable faster, especially if you are accessing the database over a network. It doesn't make so much difference if the database is running on the same machine, but it is still a little bit faster. However - as the name says - the connection is persistent, i.e. it stays open, even if it is not used.

The problem with that is, that in "default configuration", MySQL only allows 1000 parallel "open channels". After that, new connections are refused (You can tweak this setting). So if you have - say - 20 Webservers with each 100 Clients on them, and every one of them has just one page access per hour, simple math will show you that you'll need 2000 parallel connections to the database. That won't work.

Ergo: Only use it for applications with lots of requests.