Theory Relational Database Elasticsearch Database Index Table Type Row Document Column Field Schema Mapping Index Everything is indexed SQL Query DSL SELECT * FROM table… GET http://… UPDATE table SET PUT http://…

User service construction User business model backend service: For user registration, due to the uniqueness of the mobile phone number, the most reliable method is to use the unique index guarantee of the database, and at the same time, catch the duplicatedkey exception and throw a meaningful mobile phone number registered exception.

Using SpringBoot to build a basic project The steps to initialize SprigBoot using idea Using spring initializer initialize springboot project Choosing jdk 1.8 Choosing springboot 2.1.5 Setting root path and package name: com.

Theory Traversal pre-order: root -> left sub-tree -> right sub-tree in-order: left sub-tree -> root -> right sub-tree post-order: left sub-tree -> right sub-tree -> root base case usually refers to the null child node below the leaf node.

Theory Integer –> null (Zero False Null principle) Queue First In First Out: Breadth-First Search related problems. Java api: enqueue: offer() - offer at the tail. dequeue: poll() - poll at the head.