Seems like a mostly straightforward arch. I have only one question (and it's a genuine question as I'm not sure under what circumstances it's being used and if

Seems like a mostly straightforward arch. I have only one question (and it's a genuine question as I'm not sure under what circumstances it's being used and if there is an issue that was being run up against). My question is why is Solr replication being run using a 'custom' solution rather than cloud/Zookeeper or similar, or even just the standard master/slave arch if the data was small enough?