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Managing Update Conflicts in Bayou, a Weakly Connected Replicated Storage System

Douglas B. Terry, Marvin M. Theimer, Karin Petersen, Alan J. Demers, Mike J. Spreitzer, and Carl H. Hauser

Summary

In this paper, the author present Bayou storage system, which provides an infrastructure for collaborative applications that manages the conflicts introduced by concurrent activity while relying only on the weak connectivity available for mobile computing. Unlike previous works, Bayou does not provide transparent replicated data support. The authors assume that applications must be aware that they may read weakly consistent data and also that their write operations may conflict with those of other users and applications. Bayou supports application-specific mechanisms to detect and resolve the update conflicts that naturally arise in such a system. Besides, it ensures that replicas move towards eventual consistency. Bayou also defines a protocol by which the resolution of update conflicts stabilized.

Contributions

  • The authors introduce per-update dependency checks and merge procedures as a general mechanism for application-specific conflict detection and resolution.
  • The authors define two states of an update, committed and tentative, which relate to whether or not the conflicts potentially introduced by the update have been ultimately resolved.
  • The authors present mechanisms for managing these two states of an update both from the perspective of the clients and the storage management requirements of the replicas.
  • The authors describe how replicas move towards eventual consistency.
  • The authors discuss how security is provided in a system like Bayou.

Strengths

  • The client can read or write to any replica without explicit coordination with other replicas.
  • Bayou provides high availability. It does not mark conflicting data as unavailable until a person resolves the conflict.
  • Bayou provides support for clients that may choose to access only stable data.

Weaknesses

  • Bayou does not provide transparent replicated data support for existing file systems and database applications.
  • Applications of Bayou should deal with the weakly consistency and write conflicts by themselves. They should be involved in the detection and resolution of conflicts.
  • Bayou introduces expensive merge procedure.