Delegation: Efficiently Rewriting History

Transaction delegation, as introduced in ACTA, allows a transaction to transfer responsibility for the operations that it has performed on an object to another transaction. Delegation can be used to broaden the visibility of the delegatee, and to tailor the recovery properties of a transaction model. Delegation has been shown to be useful in synthesizing Advanced Transaction Models. Withan efficient implementationof delegation it becomes practicable to realize various Advanced Transaction Models whose requirements are specified at a high level language instead of the current expensive practice of building them from scratch. In this paper we identify the issues in efficiently supporting delegation and hence advanced transaction models, and illustrate this with our solution in ARIES, an industrial-quality system that uses UNDO/REDO recovery. Since delegation is tantamount to rewriting history, a naı̈ve implementationcan entail frequent, costly log accesses, and can result in complicated recovery protocols. Our algorithm achieves the effect of rewriting history without rewriting the log, resulting in an implementation that realizes the semantics of delegation at minimal additionaloverhead and incurs no overhead when delegation is not used. Our work indicates that it is feasible to build efficient and robust, general-purpose machinery for Advanced Transaction Models. It also leads toward making recovery a firstclass concept within Advanced Transaction Models.