I have not had a need for this, but another (home built PEAR) CMS I support for a client uses locking to prevent two users from being able to edit a piece of content at the same time. It can be irritating but could prevent loss of work.
I suppose if you have multiple site editors it could be invaluable.
Is it necessary? Is the module content lock a viable option?
I believe this is already implemented to some degree. If two users are editing the same piece of content, and one submits changes, the other user will receive this message when trying to do the same:
The content on this page has either been modified by another user, or you have already submitted modifications using this form. As a result, your changes cannot be saved.
It happens in the node_validate() function in case you're interested in checking out the code.
Below is a possible solution to address this question, which implements an edit-in-progress (EIP) feature using the Flag module, combined with the Rules module to get it to work. Read on for more details ...
Create a global (not user specific) flag, and say you name (label) it edit-in-progress flag (='EIP' for short). The purpose of this flag is like so:
Create a rule to disallow edit of a node if the EIP flag for a node is already set. To recognize a request to start an edit session for a node, use an URL like node/xyz/edit (with xyz = nid of the node).
Refer to my answer to the question about "How to restrict access to a node via node/12 and allow access via a path like content/sometitle?", which contains a rule that is pretty close to what's needed for this rule here:
Use the same Rules Event, i.e. "Drupal is initializing" (you want to catch this edit event via Rules BEFORE the edit session actually allowed to be started!).
Slightly adapt the regular expression, by adding /edit to it.
Add an extra condition to verify if the EIP-flag for this node is ALREADY turned on (which indicates some other edit session for this node was started already).
Adapt the Rules Action to (a) issue an appropriate message (that some other edit session is already started) and (b) redirect the user to whatever other path that fits your needs (e.g: just node/xzy).
Create a rule to SET the EIP flag for a node whenever an authorized user requests to start an edit session for a node. To recognize such request, use an URL like node/xyz/edit (with xyz = nid of the node).
Refer to my answer mentioned in Step 2, which contains a rule that is pretty close to what's needed for this rule here:
Use the same Rules Event, i.e. "Drupal is initializing" (you want to catch this edit event via Rules BEFORE the edit session actually allowed to be started!).
Slightly adapt the regular expression, by adding /edit to it.
Add an extra condition to verify if the EIP-flag for this node is NOT turned on (which indicates no other edit session for this node exists).
Adapt the Rules Action to SET the EIP flag.
Create a rule to UNSET the EIP flag for a node whenever an authorized user requests to end an edit session for a node.
Here is a summary of this rule:
Should you want to do/consider so, you should be able to merge the 2 Rules from Step 2 and Step 3 in a single Rule, using the Conditional Rules module. Refer to my answer to the question How to prevent a node being saved when using the Rules module? for a sample of how to use this module (for which not a lot of documentation seems to exist).
When two people edit the same page and one submits changes there is a hight risk that at the same time the second person has made some changes to the same page that will be lost. There, I find, exists the great use of this module.
To "unblock" the node for editing it you can clear cache from within the admin page UI. In Drupal 7 it is in main menu, first icon (Home) -> Flush all caches.
