Core metadata document comprises three: index, type, id
1、_index
Illustrates a document which is stored in the index, stored under the same index is similar document (field most of the same document) index name must be lowercase, can not begin with an underscore, it can not include a comma.
2、_type
Indicates that the document belongs to which type of index, the next index can have only one type, type name can be uppercase, lowercase may be, can not begin with an underscore, it can not include a comma.
3、_id:
Uniquely identifies the document, and index, together uniquely identify the type of a document can be manually specified value, this value may be generated by the es.