Question

When using session and namespaces, when do IO operations really occur (since sessions are stored as files somewhere on the server)?

As soon as I declare a new instance of *Zend_Session_Namespace*?

$myNamespace = new Zend_Session_Namespace('myNamespace');

As soon as I read or write into a variable of the namespace?

$myNamespace = new Zend_Session_Namespace('myNamespace');
$myNamespace->someVar = 3;
$myVar = $myNamespace->someVar;

I would like to know which operation is really expensive (IO read/write).

Answer 1

Zend_Session_Namespace uses native PHP sessions.
According the official PHP documentation:

When PHP shuts down (or when session_write_close() is called), PHP will internally encode the $_SESSION superglobal and pass this along with the session ID to the the write callback. After the write callback has finished, PHP will internally invoke the close callback handler.

As you can read, the session is written when the script shuts down.