Session start-on-demand

.. versionadded:: 2.3

Control over session "start-on-demand" was added in Symfony 2.3.

In versions 2.1-2.2, Symfony Sessions automatically invoked ``$session->start()`` when

any attempt was made to access session data (effectively 'start on demand').

From Symfony 2.3 this behaviour can be controlled.

There are three modes defined by

:class:`Symfony\\Component\\HttpFoundation\\Session\\Storage\\SessionStorageInterface`

The settings are as follows:

- ``SessionStorageInterface::NO_START_ON_DEMAND_STRICT`` - The session will not be started on demand

and any attempt to read or write session data will result in a ``\RuntimeException``

- ``SessionStorageInterface::START_ON_DEMAND`` - The session will be started if it hasn't already been

when any attempt is made to read or write session data. This setting reflects the default behaviour

since Symfony 2.1

- ``SessionStorageInterface::NO_START_ON_DEMAND_LAX`` - The sessions will not be started on demand

when session data is read or written to. It will allow access to the unitialized ``BagInterface``.

If this session is subsequently started manually after data is written to a ``BagInterface`` will

be overwritten (by the session data read from persistence).

You can configure these by injecting a configured storage engine into the session::

* `name`_

* `mock_name`_

* `auto_start`_

* `on_demand`_

on_demand

**type**: ``string`` **default**: ``on``

Can be values

- ``on`` - start automatically if not started upon session read/write

- ``off`` - do not start session automatically on data read/write, if an attempt is

make to do so, throw a ``\RuntimeException``

- ``off_lax`` - do not start session automatically on data read/write, but if an attempt

is made to read or write to the session, allow access to the relevent bag.

If data is written to the bags and a session is subsequently started, it will be

overwritten.

auto_start

**type**: ``Boolean`` **default**: ``false``

This controls the ``SessionListener`` which will automatically start the session

during the Request cycle.

name

**type**: ``string``

Sets the session cookie name

mock_name

**type**: ``string``

Sets the mock session cookie name