def get_cache(self, cache_name, miss_fn):
        """
        Get an L{AsyncLRUCache} object with the given name.  If such an object
        does not exist, it will be created.  Since the cache is permanent, this
        method can be called only once, e.g., in C{startService}, and it value
        stored indefinitely.

        @param cache_name: name of the cache (usually the name of the type of
        object it stores)
        @param miss_fn: miss function for the cache; see L{AsyncLRUCache}
        constructor.
        @returns: L{AsyncLRUCache} instance
        """
        try:
            return self._caches[cache_name]
        except KeyError:
            max_size = self.config.get(cache_name, self.DEFAULT_CACHE_SIZE)
            assert max_size >= 1
            c = self._caches[cache_name] = lru.AsyncLRUCache(miss_fn, max_size)
            return c