Concurrent

This module allows users to concurrently edit the same nodes without overwriting each other's changes. When one user modifies a field on the node the field will be marked changed and read only until the node has been saved. Any other user editing the same node will see those fields locked, but can still edit other fields.

When a field in the node edit form is changed an ajax call is made saving a cache entry that marks that field as changed by the current user.
The fields are checked on node edit page load and at 3 second intervals so users already editing a node will see fields get locked while they are working on the node.

Concurrency is defined per node type.