Consider the following definition of a global state: A global state is a set of local states which occur simultaneously. This definition is based on physical time. We use the phrase “time domain” to refer to such a definition. A different definition of a global state is possible based on the “causal domain” (a world based on the causally precedes relation). In the causal domain, a global state is a set of local states which are all concurrent with each other. By concurrent, we mean that no two states have a cause and effect relationship with each other. A global state in the time domain is also a global state in the causal domain; if two states occur simultaneously, then they cannot have any cause-effect relationship. However, the converse is not true; two concurrent states may not occur simultaneously in a given execution.
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