Quasi - Synchronous Checkpointing : Models

Checkpointing algorithms are classiied as synchronous and asynchronous in the literature. In synchronous checkpointing, processes synchronize their checkpointing activities so that a globally consistent set of checkpoints is always maintained in the system. Synchronizing checkpointing activity involves message overhead and process execution may have to be suspended during the checkpointing coordination, resulting in performance degradation. In asynchronous checkpointing, processes take checkpoints without any coordination with others. Asynchronous checkpointing provides maximum autonomy for processes to take checkpoints; however, some of the checkpoints taken may not lie on any consistent global checkpoint, thus making the checkpointing efforts useless. Asynchronous checkpointing algorithms in the literature can reduce the number of useless checkpoints by making processes take communication induced checkpoints , besides asynchronous checkpoints. We call such algorithms, quasi-synchronous. In this paper, we present a theoretical framework for characterizing and classifying such algorithms. The theory not only helps to classify and characterize the quasi-synchronous checkpointing algorithms, but also helps analyzing the properties and limitations of the algorithms belonging to each class; it also provides guidelines for designing and evaluating such algorithms.