(In previous lessons 13 and 14, I said that HTTP is "stateless", which is both an advantage and a disadvantage. The advantage is that the server has no state difference and can be easily clustered. The disadvantage is that it can not support transaction operations requiring state records.Fortunately, the HTTP protocol is extensible, and the cookie technology invented later adds "memory capability" to http.What are cookies?I wonder if you have seen the classic film "memory fragments" directed by Christopher Nolan, in which the protagonist suffers from short-term amnesia and can't remember what happened recently.)