We already allow it for the purge: org.hibernate.search.mapper.pojo.work.impl.PojoIndexedTypeIndexingPlan.IndexedEntityIndexingPlan#providedRoutingKey. Let's allow it for any kind of work: it will benefit to Infinispan, in particular.

The idea would simply be to add a parameter String providedRoutingKey to every add/addOrUpdate/delete method in org.hibernate.search.mapper.pojo.work.spi.PojoIndexingPlan.