A node may use its own platform (OS, hardware) and database engine independently of other nodes. Also child nodes can be installed without Áú»¢¶Ä²© frontend.
It may be practical to use less powerful hardware with Áú»¢¶Ä²© server running SQLite or MySQL MyISAM while nodes of higher levels may use combination of a better hardware with MySQL InnoDB, Oracle or PostgreSQL backend.