Suggested by Guillaume Smet

Instead of having this:

hibernate.search.default_backend es
hibernate.search.backends.es.type elasticsearch
hibernate.search.backends.es.index_defaults.foo bar
hibernate.search.indexes.Book.foo bar2

We would have this:

hibernate.search.default_backend es
hibernate.search.backends.es.type elasticsearch
hibernate.search.backends.es.index_defaults.foo bar
hibernate.search.backends.es.indexes.Book.foo bar2

Which makes more sense, since the options available to configure the index "Book" actually depend on the backend in which this index is defined.

As a bonus it also feels more "tidy" since the index properties are nested under the right backend. It's particularly visible in a YAML configuration, like in Spring Boot:

spring.jpa.properties:
    hibernate.search:
        default_backend: es
        backends.es:
            type: elasticsearch
            index_defaults:
                foo: bar
            indexes.Book:
                foo: bar2

However, in order for this to be possible, we will need to change how we configure an index to point to a certain backend. Currently this is done in the configuration:

hibernate.search.backends.es.type elasticsearch
hibernate.search.backends.lucene.type lucene

hibernate.search.default_backend es

# Override the default
hibernate.search.indexes.Book.backend lucene

Which obviously makes no sense if the index properties are nested under the backend name.

One solution would be to configure the targeted backend in the mapper:

@Indexed(backend = "lucene") // New attribute
public class Book {
... 
}

The default (when the "backend" attribute is not defined) being to rely on hibernate.search.default_backend.