Notes on Concurrency Control in Groupware

this paper. Our focus is on target object inconsistency. First, it is possible for users to damage the integrity of the collection of target objects by making concurrent updates. Second, although we may consider that different users view one collection of target objects, in reality they sometimes view different versions of the same conceptual object. There are several factors that mitigate against users being able to access a consistent set of target objects: . Conflicting updates. Two or more users may each attempt to make a series of updates concurrently which conflict with one another: if the updates were allowed to proceed without restraint they would leave the shared objects in a state that breaks the constraints of the application. This is a well-known problem in the database world, and the transaction mechanism exists to solve the problem in that world. . Operation latency. Users experience varying latencies when accessing data over a network. In general users cannot access exactly the same data at the same real time. . Disconnection. A network partition may disconnect a user or subgroup of users from the rest. Or a user may deliberately disconnect her mobile computer from the network. If these users continue working, their copies of target objects may diverge. . Replication. Often shared data are replicated for efficient access and reliability. Each user's workstation or each of several server machines (or both) holds a copy, so that users are `close' to at least one copy of the data. Because of variable communication latencies and variable message arrival order, the copies of the data may diverge. The degree of agreement implied by consistency depends upon the application. For example, in the USENET newsgroup system, users observing the `latest' postings to a g..