I'm executing a query with ".../_search?_source=cacheId,expirationTime" and also trying "/_search?_source=expirationTime,cacheId", and the response is always:
( ... ) "_source": {"expirationTime":1558724355237,"cacheId":"96|0hghj56776|13259|"} ( ... )
So, what's the order of the elements within the _source in the response? Because it seems it's not the order requested in the query. It's important for us because we are directly "parsing" the response as string.