Towards Robust Optimistic Approaches

Optimism is a well-known technique to enhance the performance of distributed protocols. Optimistic approaches exploit properties exhibited by the system with certain likelihood, (i.e., that certain kinds of scenarios will prevail over others) to outperform the corresponding conservative protocol. These properties are usually referred as optimistic assumptions (e.g., an optimistic assumption is that reliably multicast messages in a LAN are spontaneously totally ordered). When the optimistic assumption holds, the optimistic approach is more efficient than the conservative one. However, this gain usually implies a tradeoff. That is, if the optimistic assumption does not hold, the optimistic approach is less efficient than the conservative one. This is due to the need to undo or repair the incorrect actions and the dismissal of work already done. This is precisely the Achilles’ heel of traditional optimistic approaches. Therefore, what is crucial for an optimistic approach to be successful is that the resulting gains of optimism outweigh the penalties imposed by optimism failures. Researchers have long recognized the potential benefits of using optimism and have proposed optimistic versions of conventional distributed protocols [BHG87,Ped01]. However, despite the many optimistic approaches suggested in distributed computing, they are not that common in industrial applications. The main reason for this reluctance is that whenever the optimistic assumption fails, the protocol behaves worse that the conventional protocol. This behavior might imply more messages, or undoing part of the work. It is our opinion that to increase the applicability of optimistic protocols they need to be enriched with safeguards that limit the consequences of those scenarios where the optimistic assumptions do not hold. These safeguards make optimistic approaches more robust and efficient, and therefore, more applicable. As a consequence, in those periods during which the optimistic assumptions do not hold frequently enough, the system will not degrade to unacceptable levels. In this paper, we try to point out the possible causes of the lack of success of some optimistic protocols, and show which directions can be taken to overcome these shortcomings in order to diminish the existing reluctance in industry for this kind of protocols. We think that optimistic protocols will play a crucial role in the upcoming widearea distributed systems. Despite that bandwidth will grow more and more, latency will always be a problem in WANs due to physical limitations.