Question

I have no idea how PDO’s connection pooling works, and I’m getting nowhere with searches.

First, for clarity, is PDO’s connection pooling automatic?

If not, how can I utilize it on an AJAX page?

I’ve clocked the time cost on my local wampserver dev of making a database connection via mysqli at 10ms per, and I’m trying to drive wait times as close to 0 as possible. This is one of my last issues.

Answer 1

If you are talking about persistent connections look here: http://php.net/manual/en/pdo.connections.php

Do persistent connections remain open across different AJAX requests?

Yes and the risk that they are never reused. Thanks to @carefulnow. I’ll quote his comments here:

Persistent connections and connection pooling are not the same thing
Connection pooling creates a lot of connections to start with, and then the application just grabs one of those already established connections. Persistent connections do establish new connections, but they do not close unless explicitly told to (a normal connection will close at the end of the script). Persistent connections are dangerous if not used carefully, as most databases implement a maximum connections count, and will refuse further connections when hit. For most cases, a normal connection (just calling the PDO constructor with a DSN and login details) will be fine.

I’ll explain it by showing the PHP code, and the resulting connection count in the MariaDB console monitor. Look at the following screenshots, and pay attention the the "run count" in the bottom right. You’ll see when using persistent connections, connections do not close, but instead go into sleep mode. Ignore connection #231, that’s the console monitor’s. https://i.stack.imgur.com/UkCjN.png, https://i.stack.imgur.com/vXaGQ.png, https://i.stack.imgur.com/gUlGA.png. As you can see, the time column indicates the length of inactivity, not the length of time connected, hence the console monitor’s time is always zero, because I’ve just executed SHOW PROCESSLIST;. I hope this clears everything up a bit, database connection management is quite tricky sometimes.

So looking at the screenshots the questions is open, how to pickup these persistent connections again. Some answers on StackOverflow say, that they are cached on thread level, so if you have more than one thread (which is highly expected) than you get many connections. This opens the risk to run into the connection limit of mysql. Before using persistant connections make sure you understand how they work (which I don’t, but it seems @carefulnow :-))

Is it possible to set a persistent connection to expire?

PHP doc says they are cached, therefore I assume they are cleanup after a time.

More on the php doc