The crawl phase indicates the action the crawl is currently performing. Each crawl will will transition to the following phase if a set of conditions is met. Not all crawl types, transition through all phases. These are the phases for each crawl type:
Each crawl phase has a set of possible states. The allowable states for a given phase will be a subset of all the possible states, with some states not being allowed in a given phase (as they do not make sense – “pausing” in the idle phase for instance). The diagram shows the crawl phases and the allowable states.