幂等操作:多次执行没有副作用的操作。 示例:从数据资源检索值并打印值的操作 非幂等操作:多次执行会造成伤害的操作。(当它们改变某些值或状态时) 示例:从银行账户提款的操作

理解幂等运算的一个好例子可能是用远程钥匙锁汽车。

log(Car.state) // unlocked

Remote.lock();
log(Car.state) // locked

Remote.lock();
Remote.lock();
Remote.lock();
log(Car.state) // locked

锁是一个幂等运算。即使每次运行上锁都有一些副作用，比如眨眼，但不管你运行多少次上锁操作，汽车仍然处于相同的上锁状态。

简而言之，幂等运算意味着无论你做多少次幂等运算都不会得到不同的结果。

例如，根据HTTP规范的定义，GET、HEAD、PUT、DELETE是幂等操作;但是POST和PATCH不是。这就是为什么有时POST被PUT取代的原因。

my 5c: In integration and networking the idempotency is very important. Several examples from real-life: Imagine, we deliver data to the target system. Data delivered by a sequence of messages. 1. What would happen if the sequence is mixed in channel? (As network packages always do :) ). If the target system is idempotent, the result will not be different. If the target system depends of the right order in the sequence, we have to implement resequencer on the target site, which would restore the right order. 2. What would happen if there are the message duplicates? If the channel of target system does not acknowledge timely, the source system (or channel itself) usually sends another copy of the message. As a result we can have duplicate message on the target system side. If the target system is idempotent, it takes care of it and result will not be different. If the target system is not idempotent, we have to implement deduplicator on the target system side of the channel.

只是想提出一个真实的用例来证明幂等性。在JavaScript中，假设你定义了一堆模型类(就像MVC模型一样)。它的实现方式通常是这样的(基本示例):

function model(name) {
  function Model() {
    this.name = name;
  }

  return Model;
}

然后你可以像这样定义新的类:

var User = model('user');
var Article = model('article');

但如果你试图通过模型(' User ')从代码的其他地方获取User类，它会失败:

var User = model('user');
// ... then somewhere else in the code (in a different scope)
var User = model('user');

这两个User构造函数是不同的。也就是说,

model('user') !== model('user');

为了让它是幂等的，你只需要添加一些缓存机制，像这样:

var collection = {};

function model(name) {
  if (collection[name])
    return collection[name];

  function Model() {
    this.name = name;
  }

  collection[name] = Model;
  return Model;
}

通过添加缓存，每次你建模('user')它都是同一个对象，所以它是幂等的。所以:

model('user') === model('user');

幂等操作是一种可以应用多次而不改变结果(即系统状态)的操作、动作或请求，超出初始应用。

示例(web应用上下文):

IDEMPOTENT: Making multiple identical requests has the same effect as making a single request. A message in an email messaging system is opened and marked as "opened" in the database. One can open the message many times but this repeated action will only ever result in that message being in the "opened" state. This is an idempotent operation. The first time one PUTs an update to a resource using information that does not match the resource (the state of the system), the state of the system will change as the resource is updated. If one PUTs the same update to a resource repeatedly then the information in the update will match the information already in the system upon every PUT, and no change to the state of the system will occur. Repeated PUTs with the same information are idempotent: the first PUT may change the state of the system, subsequent PUTs should not.

非幂等性: 如果一个操作总是导致状态的变化，比如反复向用户发送相同的消息，导致每次都发送新消息并存储在数据库中，我们称该操作为NON-IDEMPOTENT。

NULLIPOTENT: 如果一个操作没有副作用，就像仅仅在网页上显示信息而没有对数据库进行任何更改(换句话说，你只是在读取数据库)，我们说这个操作是NULLIPOTENT。所有get都应该是无效的。

当谈论系统的状态时，我们显然忽略了无害的和不可避免的影响，如日志和诊断。