Currently if we do this:

Then the value for property "foo" will be applied to all indexes regardless of their backend.

I doubt this is wise: if we end up with multiple properties with the same name in different backends, this may lead to unexpected results, or simply to failing bootstrap because value "bar" sense for one backend type but not for another.

If we were to us per-backend index defaults, on the other hand:

... then no confusion would be possible. And this syntax would not be much worse for users who only have one backend.

Also, this syntax would remove the problems users may face when they have multiple indexes and name one of them "default" (this index's configuration will be automatically used as a default for all other indexes...).

Note that in order to introduce this syntax, we would have make the setting for the "default backend" a separate property, e.g.: