键值存储，也称为键值数据库，是一种非关系数据库。每个唯一标识符都存储为一个键及其关联值，这种数据配对称为“键值”对。

为了实现水平扩展，在服务器之间高效、均匀地分配请求/数据非常重要，一致哈希是实现这一目标的常用技术。

DESIGN A RATE LIMITER

System Design Interview CHAPTER 1：SCALE FROM ZERO TO MILLIONS OF USERS

Mediator pattern defines a separate (mediator) object that encapsulates the interaction between a set of objects and the objects delegate their interaction to a mediator object instead of interacting with each other directly.

Interpreter pattern is used to defines a grammatical representation for a language and provides an interpreter to deal with this grammar.

The command pattern encapsulates a request as an object, thereby letting us parameterize other objects with different requests, queue or log requests, and support undoable operations.

Captures and externalizes an object’s internal state so that it can be restored later, all without violating encapsulation.

Allows for one or more operation to be applied to a set of objects at runtime, decoupling the operations from the object structure.

迭代器模式（Iterator Design Pattern），也叫作游标模式（Cursor Design Pattern）。 迭代器模式将集合对象的遍历操作从集合类中拆分出来，放到迭代器类中，让两者的职责更加单一。

状态机有 3 个组成部分：状态（State）、事件（Event）、动作（Action）。

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

模板模式主要是用来解决复用和扩展两个问题，结合 Java Servlet、JUnit TestCase、Java InputStream、Java AbstractList 四个例子来具体讲解这两个作用。

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.