Node editing concurrency in Drupal

Default locking behavior in Drupal

When two users in Drupal are editing the same node (page, story, news, ...), only the results of the first one who saves his changes will be saved. The other one will get a nice message saying "This content has been modified by another user, changes cannot be saved.". This is what is called an optimistic locking strategy.

Let's get a closer look at this to fully understand what is going on here:
- User A hits 'Edit' on node 1234.
- User B hits 'Edit' on node 1234.
- User A is making some changes on the node form of 1234.
- User B is making some changes on the node form of 1234.
- User A hits the 'Save' button.
- Changes get saved.
- User B hits the 'Save' button.
- User B gets the message and his changes are not saved.

Why? Well, because otherwise User B would have overwritten the changes of User A.

If you want to know where this is done in the Drupal code, take a look at node_validate in the node module.

  if (isset($node->nid) && (node_last_changed($node->nid) > $node->changed)) {
    form_set_error('changed', t('This content has been modified by another user, changes cannot be saved.'));
  }

Alternative: Pessimistic locking

If you want to alter this default behaviour, you can do so. You can download a module called Checkout (content locking) that brings pessimistic locking in Drupal. This means the node will be "locked" whenever a user is editing the node, so no other user can edit the node while the first one is doing his thing.