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Multi-Level Consistency

When it comes to keeping replicas mutually consistent, existing systems use either a pessimistic, a strict, or an optimistic strategy.3.3 In a system with mobile computers it might be a good idea to have multi-level consistency, due to the variation in quality of communication. Multi-level consistency is means that applications can specify whether they want to be pessimistic, strict, or optimistic when using a particular file, and maybe even have varying degrees of pessimism and optimism.

By introducing multi-level consistency applications can adapt their behaviour according to the state of their environment (i.e., the characteristics of communication) and/or user demands. They can relax their consistency requirements as the quality of communication decreases in order to achieve higher availability or reduce cost, and strengthen them again when suited.

In a system with multi-level consistency (using session semantics) pessimism will naturally have higher precedence than optimism. If an application uses a file optimistically (e.g., opens it for writing), and then later another application use the same file pessimistically, then the second application will get a lock on the file, and thereby hindering updates done by the first application. Thus pessimism should be used with caution, as it might decrease availability (for other applications/clients than the one being pessimistic).

In Coda, Ficus, and Bayou conflicts are always possible because they use optimistic replication only. In a system with multi-level consistency conflicts can be avoided if it is in the interest of the application (and thereby user). The application decides when to be optimistic (and to what extent) and when not to; optimism is not layed down a priori by the system.



Footnotes

... strategy.3.3
Albeit, under different definitions of the terms!

next up previous contents index
Next: Conflict Detection Up: Replica Control Strategies Previous: Optimistic   Contents   Index

michael@garfield.dk
2000-10-13