Dynamic Rebinding for Concurrent Object Groups: Theory and practice

Abstract

In this paper we propose a new mechanism for Dynamic Rebinding, a particular kind of Dynamic Software Updating that focuses on modifying the workflow of a program. This mechanism is built upon the concurrency model of Concurrent Object Groups that is adopted in programming languages like Coboxes, Creol or ABS. Using this model, which extends and solves some of the limitations of Active Objects, it becomes possible for an update to wait for the program to reach a local quiescent state and then perform the update without creating any inconsistency in the program's state. We validate our mechanism by formally proving that i) no updates are performed when the program has not reached a local quiescent state, ii) an update is guaranteed to be applied if we first wait for the program to reach a local quiescent state, and iii) it is possible to perform different updates atomically. We also provide a type system that checks the good usage of the rebinding operator. We implemented a prototype supporting our mechanism in the toolchain of the ABS language, and we tested the implementation by supporting dynamic changes in the server policy of an industrial case study from the eCommerce Fredhopper platform, offered by SDL. Without our rebinding mechanism, supporting such dynamic changes in a provably correct manner would not have been so simple. Finally, we developed an implementation of type checking for the ABS language extended with our rebinding primitives, and successfully tested it for the case study discussed in the paper.