Specification and Verification Using Alloy of Optimistic Access Control for Distributed Collaborative Editors

Distributed Collaborative Editors are interactive systems where several and dispersed users edit concurrently shared documents. Generally, these systems rely on data replication and use safe coordination protocol which ensures data consistency even though the users's updates are executed in any order on different copies. Controlling access in such systems is a challenging problem, as they need dynamic access changes and low latency access to shared documents. In [1], a flexible access control protocol is proposed; it is based on replicating the shared document and its authorization policy at the local memory of each user. To deal with latency and dynamic access changes, an optimistic access control technique is used where enforcement of authorizations is retroactive. However, verifying whether the combination of access control and coordination protocols preserves the data consistency is a hard task since it requires examining a large number of situations. In this paper, we specify this access control protocol in the first-order relational logic with Alloy, and we verify that it preserves the correctness of the system on which it is deployed in such a way that the access control policy is enforced identically at all participating user sites and, accordingly, the data consistency remains still maintained.