Abstract. In grid workflow systems, existing representative checkpoint selection strategies, which are used to select checkpoints for verifying fixed-time constraints at run-time execution stage, often select some unnecessary checkpoints and ignore some necessary ones. Consequently, overall temporal verification efficiency and effectiveness can be severely impacted. In this paper, we propose a new strategy that selects only necessary and sufficient checkpoints dynamically along grid workflow execution. Specifically, we introduce a new concept of minimum time redundancy as a key reference value for checkpoint selection. We also investigate its relationships with fixed-time constraint consistency. Based on these relationships, we present our strategy which can improve overall temporal verification efficiency and effectiveness significantly. 1

In grid workflow systems, a checkpoint selection strategy is responsible for selecting checkpoints for conducting temporal verification at run-time execution stage. Existing representative checkpoint selection strategies often select some unnecessary checkpoints and omit some necessary ones because they cannot adapt to the dynamics and uncertainty of run-time activity completion duration. In this paper, based on the dynamics and uncertainty of run-time activity completion duration, we develop a novel checkpoint selection strategy that can adaptively select not only necessary but also sufficient checkpoints. Specifically, we introduce a new concept of minimum time redundancy as a key reference parameter for checkpoint selection. An important feature of minimum time redundancy is that it can adapt to the dynamics and uncertainty of run-time activity completion duration. We develop a method on how to achieve minimum time redundancy dynamically along grid workflow execution, and investigate its relationships with temporal consistency. Based on the method and the relationships, we present our strategy and rigorously prove its necessity and sufficiency. The simulation evaluation further experimentally demonstrates such necessity and sufficiency and its significant improvement on checkpoint selection over other representative strategies.

Conventional upper bound constraint verification in grid workflow systems is based on the key assumption that an upper bound constraint only has two states: consistency and inconsistency. However, due to complexity of grid workflows and dynamic availability of participating grid services, this assumption is too restrictive as there may be some intermediate states. Therefore, in this paper, we introduce four states for an upper bound constraint. Namely, we treat conventional consistency as strong consistency and divide conventional inconsistency into weak consistency, weak inconsistency and strong inconsistency. Correspondingly, we develop their verification methods. For weak consistency, we present some algorithms on how to adjust it to strong consistency without triggering exception handling as in conventional work. For weak inconsistency, we analyse why it can rely on less costly exception handling than conventional work. The final evaluation demonstrates that our four-state approach can achieve better cost-effectiveness than the conventional two-state approach. 1.

In grid workflow systems, temporal correctness is critical to assure the timely completion of grid workflow execution. To monitor and control the temporal correctness, fixed-time constraints are often assigned to a grid workflow and then verified. A checkpoint selection strategy is used to select checkpoints along grid workflow execution for verifying fixed-time constraints. The problem of existing representative strategies is that they do not differentiate fixed-time constraints as once a checkpoint is selected, they verify all fixed-time constraints. However, these checkpoints do not need to be taken for those constraints whose consistency can be deduced from others. The corresponding verification of such constraints is consequently unnecessary and can severely impact the efficiency of overall temporal verification. To address the problem, in this paper, we develop a new temporal dependency based checkpoint selection strategy which can select checkpoints according to different fixed-time constraints. With our strategy, the corresponding unnecessary verification can be avoided. The comparison and experimental simulation further demonstrate that our new strategy can improve the efficiency of overall temporal verification significantly over the existing representative strategies.

Upper bound constraints are often set when complex scientific or business processes are modelled as grid workflow specifications. However, many existing processes such as climate modelling or international stock market analysis often have only one end-to-end upper bound constraint. This is not sufficient to control overall temporal correctness as we may not find temporal violations until the last activity. Then, it is too late to take any handling actions. Consequently, the execution results may not be useful and overall cost-effectiveness would be impacted. Therefore, in this paper, we systematically investigate how to set, verify and adjust sub-upper bound constraints within the timeframe of one end-to-end upper bound constraint so that we can control grid workflow execution locally. We develop corresponding setting, verification and adjustment methods and algorithms. The quantitative evaluation demonstrates that with sub-upper bound constraints, we can achieve better cost-effectiveness than only based on one end-to-end upper bound constraint. 1. Introduction and