[Koha] Is zebra server needed when using Elastic Search with 17.11-02 ?

Hi, Starting from version 17.11-02 we have made extensive changes to koha to get elastic search working in koha.   We don't run the zebra server but we see an error in the about box saying that it is not running.  Everything seems to work but we are wondering what functions need zebra even though we only use ES ? Best Regards, Dave