Major Description
HCatInputFormat has a cache feature that allows HCat to cache hive client connections to the metastore, so as to not keep reinstantiating a new hive server every single time. This uses a guava cache of hive clients, which only evicts entries from cache on the next write, or by manually managing the cache.
So, in a single threaded case, where we reuse the hive client, the cache works well, but in a massively multithreaded case, where each thread might perform one action, and then is never used, there are no more writes to the cache, and all the clients stay alive, thus keeping ports open.