Implemented support for persistent connections in PDO and mysqli drivers

[morozov]

Q	A
Type	improvement
BC Break	no
Fixed issues	#2315

As of #3080, DBAL doesn't use a custom PDO statement class which enables the use of persistent connections with PDO drivers. Additionally, the support for persistent connections has been implemented for the mysqli driver.

[morozov]

At the moment, I don't see a reliable way to test the persistence of a connection across supported platforms/drivers.

In pseudo-code, a test may look like following:

$conn = connect($persistent = false);
$id1 = $conn->id;

$conn = connect($persistent = true);
$id2 = $conn->id;

assertNotEquals($id1, $id2);

$conn = connect($persistent = true);
$id3 = $conn->id;

assertEquals($id2, $id3);

The results are:

1. The approach above works for MySQL (SELECT CONNECTION_ID()) and PostgreSQL (SELECT pg_backend_pid()).
2. SQL Server and Oracle return the same ID even for non-persistent connections (SELECT @@SPID and SELECT SYS_CONTEXT('USERENV', 'SID') FROM DUAL respectively).
3. I didn't find a way to obtain the connection ID for IBM DB2 and SQL Anywhere.

[Majkl578]

Can we at least test that persistent connection flag is active when persistent parameter is enabled?
For PDO, it can be done using PDO::getAttribute(). For mysqli, it can be done using mysqli::get_connection_stats().

[morozov]

Can we at least test that persistent connection flag is active when persistent parameter is enabled?
For PDO, it can be done using PDO::getAttribute().

Good idea. Done.

For mysqli, it can be done using mysqli::get_connection_stats().

If we use the active_persistent_connections key from there, the test may pass if there is a persistent connection in the server, not necessarily the current one. Let's keep the test skipped for non-PDO drivers.

[Majkl578]

I'd prefer array_key_exists() over more dangerous ! empty().

[morozov]

In this case it's irrelevant. array_key_exists() over a mistyped key will have the same effect.

[morozov]

I'm thinking of introducing a ConnectionParameters object which will be just a typed structure which will also contain username and password if they are specified. It could avoid optional $username and $pasword in driver/connection methods. Any suggestions/ideas?

[jwage]

Nice! quick and easy win 👍

[dasmfm]

Hi, @morozov, can you please backport this fix to upcoming 2.9.3 or 2.10.0?
It looks like small, but very useful feature.

[morozov]

No, it depends on #3080 which is a breaking change.