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.

当一个系统中存在大量重复对象的时候，如果这些重复的对象是不可变对象，我们就可以利用享元模式将对象设计成享元，在内存中只保留一份实例，供多处代码引用。

Compose objects into tree structure to represent part-whole hierarchies.Composite lets client treat individual objects and compositions of objects uniformly.

门面模式为子系统提供一组统一的接口，定义一组高层接口让子系统更易用。