Specifies the key to be used for caching configurations within a Dispatcher. If a Dispatcher has a CacheKeyExtractor, it will extract the cache key (an EntryFacets) and use it to constrain the configuration and store it keyed off that EntryFacets. The next time another entry is logged with the same extracted cache key, it will use the preconstrained configuration instead of the full configuration. You can use this to improve the performance of your dispatcher if you know the approximate distribution of entries it will be asked to dispatch.

If you are not concerned with performance, it is safe to ignore the CacheKeyExtractor entirely.