what constitutes an "acceptable" behavior of such a process. We capture this notion in our definition of the validity of an on-line change. We define an on-line change to be valid if some time after the change, the process reaches a reachable state of the new program version. Thus, validity ensures that following a change, the process starts behaving like the new version of the program after a "transition period". We first consider validity of on-line changes to programs written in sequential procedure based languages. For this purpose, a very simple model in which procedures and functions are not allowed is first considered. State is modelled as a mapping from variable names to values. For this model, we show that it is undecidable to find whether or not a given on-line change is valid. This result has important consequences. It means that computable necessary and sufficient conditions for validity of change can not be obtained. Undecidability in this simple model also

this paper, we describe a system for on-line software version change for software written in the C language. When the change is initiated by the user, the system instantiates a new process with the new version of the software, transfers state from the old process to the new one at an appropriate time, and transfers the control to the new process. The user sees a minimal delay in this switchover

Abstract not found

The fast evolution pace for cloud computing software is on a collision course with our growing reliance on cloud computing. On one hand, cloud software must have the agility to evolve rapidly, in order to remain competitive; on the other hand, more and more critical services become dependent on the cloud and demand high availability through firm Service Level Agreements (SLAs) for cloud infrastructures. This race between the needs to increase both the cloud upgrade frequency and the service availability is unsustainable. In this paper we highlight challenges and opportunities for upgrades in the cloud. We survey the release histories of several cloud applications to analyze their evolution pace, and we discuss the shortcomings with current cloud upgrade mechanisms. We outline several solutions for sustaining this evolution while improving availability, by focusing on the novel characteristics of cloud computing. By discussing several promising directions for realizing this vision, we propose a research agenda for the future of software upgrades in the cloud. 1.