==============
Record locking
==============

In Jam.py, application you can implement a record locking while users 
concurrently edit a record.

Jam.py uses optimistic locking model, also referred to as optimistic concurrency 
control. 

When an application executes the 
:doc:`edit_record </refs/client/item/m_edit_record>`
method, it receives the current version of the record from the server and 
saves it. When the user starts saving the record, the server application checks 
the current version of the record. If it differs from the stored value (another 
user changed it while the record were being edited), 
the application warns the user and prohibits saving.

This record locking mechanism is very easy to implement.

To do so you need to create an item that will store record version.

Open 
:doc:`project parameters <project/parameters>`
and click the button to the right of the
**Lock item** input:

.. image:: _images/history_item.png
	:align: center
	:alt: History item
	
In the dialog that will appear click on the **Create lock item** button: 

.. image:: _images/create_lock_item.png
	:align: center
	:alt: New history item	

After that you must set **Edit lock** attribute in the
:doc:`Item Editor Dialog <items/item_editor_dialog>`:

.. image:: _images/item_editor_dialog.png
	:align: center
	:alt: Keep history attribute