We're having some constants declared to define Elasticsearch configuration properties, which currently expect a single global value, for example (but not only):

public static final String SERVER_URI = "hibernate.search.elasticsearch.host";
public static final String INDEX_MANAGEMENT_STRATEGY = "hibernate.search.elasticsearch.index_management_strategy";

To be consistent with the other configuration properties, these should rather be parsed independently on a per-backend base.
So the constants would only define the post-fix, and we'd document as valid configuration options properties like:

hibernate.search.default.elasticsearch.host = localhost
hibernate.search.animals.elasticsearch.host = otherhost
hibernate.search.animals.a.elasticsearch.host = AnimalsShardA

This would imply that we want to support using multiple Elasticsearch nodes, which is currently not a given and should be discussed first.