The protocols described so far do not provide hard guarantees for the degree of replica divergence--consistency is achieved only eventually.
To alleviate this problem, Pangaea introduces an option, called the ``red button'', to provide users confirmation of update delivery. The red button, when pressed for a particular file, sends harbingers for any pending updates to neighboring replicas. These harbingers (and corresponding updates) circulate among replicas as described in Section 5.1.2. A replica, however, does not acknowledge a harbinger until all the graph neighbors to which it forwarded the harbinger acknowledge it or time out (to avoid deadlocking, a replica replies immediately when it receives the same harbinger twice). The user who pressed the red button waits until the operation is fully acknowledged or some replicas time out, in which case the user is presented with the list of unavailable replicas.
This option gives the user confirmation that her updates have been delivered to remote nodes and allows her to take actions contingent upon stable delivery, such as emailing her colleagues about the new contents. The red button, however, still does not guarantee a single-copy serializability, as it cannot prevent two users from changing the same file simultaneously.