There are many different replica control techniques, used in different research communities. To understand when one replica management algorithm can be replaced by another, we need to describe more abstractly the consistency model, which captures the set of properties that an algorithm provides, and on which the clients rely (whether the clients are people or other programs). In this chapter we describe a few of the different consistency models that have been proposed, and we sketch a framework for thinking about consistency models. In particular, we show that there are several styles in which consistency models can be expressed, and we also propose some axes of variation among the consistency models.
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