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The cache is persistent for a running calibre instance. Restarting calibre wipes the cache. That doesn't matter because you rarely query the same book over more than one session. The purpose of the cache is to not do the expensive isbn->asin and asin->cover_url mappings more than once.
For example the cover plugin has a has_cover and a get_cover method, you dont want both of them to do the mapping.
Amazon typically only knows about the ISBNs for books that it sells. There is no guarantee that the isbn provided by the user or by the other metadata download plugins is one amazon knows about. Therefore, xisbn is used to find all the isbns for "related" editions and the amazon is queried for them if the original isbn is not found.
look at the google or isbndb plugins for basic metadata examples
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