最近遇到一个很严肃的问题，什么样的 events 算是合格的 events 结构。为什么需要定义source，定义在设计 events 事件体时是否有可以直接遵守的规范。针对这个问题，我收集了大量资料，从基础接口设计规范到CloudEvents 调研。索性也薄浅的了解了一些，就分享下这块的心得吧。有接口设计方面诉求的同学可以参考下。不过大部分也是研发的字段定义诉求了。
首先，我们先来了解下什么是 Events。
Events（以下统称事件）在系统设计中其实已经变的无处不在，它主要是接口级的事件描述。理论来讲定义接口和接口产生的事件是一个比较简单的事儿，但是现在缺乏一种对事件的统一描述，服务使用方和提供方往往要花费大量的时间沟通字段定义，凭直觉设计事件属性，并在将来的使用过程中疲于新增或修改事件的属性。所以在云上，定义事件其实有个 CNCF（云原生计算基金会） 牵头的 CloudEvents 标准协议来制定所有的服务事件规范。这其实也算是业内比较惯常使用的了（当然不包括 AWS，google 原因大家都懂， Azure 支持很到位）。
好了，这个应该很好理解，大体就是服务通过怎样的格式投递使用。
概念篇
那么从事件源到触发，整个流程是怎样的呢？下图很好的解释了里面的关系：
云服务代码中通常使用事件来连接不同的系统，其中一个系统中状态的改变会导致代码在另一个系统中执行。例如，当 Source 源接收到外部信号（例如HTTP或RPC）或检测到系统变化的值，它便会生成事件。Source 生成一条消息并将其中事件封装在协议中，事件到达目的地，触发事件数据并调用。
事件本身其实可以通过各种行业标准协议（例如HTTP，AMQP，MQTT，SMTP），开放源代码协议（例如Kafka，NATS）来传递，当然云厂商产生的事件其实一般是由厂商自定义的，这类事件也是本次探讨的核心。
action 的定义其实往往是指事件所附带的特定的事件触发。通常情况下，源是某种托管服务，而操作通常是 Serverless（无服务器化）的能力（例如AWS Lambda 或云函数 SCF）中的自定义代码。
定义篇
大体的概念大致梳理清楚了，那下面我们来看看最重要的定义，搞清楚定义，我们才能更好的研究事件的范围是如何的。
首先，我们来看下必选参数的定义：
id
类型： String
描述：事件标识，生产者必须确保source+id 对于每个不同的事件都是唯一的。如果重复事件被重新发送（例如网络错误），允许具有相同的id。消费者可以假设事件具有相同source且id重复。
source
类型： URI-reference描述：标识事件发生的上下文，通常将包括诸如事件源的类型，发布事件的组件或产生事件的过程之类的信息。一般由事件生产者定义数据背后的确切语法和语义。生产者必须确保source+id对于每个不同的事件都是唯一的。应用程序可以使用UUID，URN，DNS 授权或特定于整体技术架构的方案来创建唯一 source 标识符。
一个来源可以包括多个生产者。
type
类型： String
说明：该参数包含一个描述与源事件相关的事件类型。通常用于路由，策略执行等。格式应该由生产者定义，并且可能包含诸如的版本之类的信息。
其次，是非必选参数定义：
datacontenttype
类型：String描述：data 值的内容类型，该参数允许 data 赋予任何类型的内容。例如，通常使用 JSON 格式事件可能也会通过XML返回data信息，此时可以通过将此属性设置为“ application/xml”来通知使用者。
time
类型： Timestamp
描述：发生该事件的时间戳，如果不能确定发生时间，则生产者可以将此属性设置为其他时间（例如当前时间），但是所有生产者source必须保持一致。换句话说就是要么全部使用实际事件发生时间，要么全部使用相同的时间获取方式来确定所使用的时间戳。
核心参数的定义大抵如此了，其次是一些 Event Data，Attributes，和相关扩展字段，这里就不一一阐述了。这里给放个例子，也是 cloudevents 官方用例，可以供大家参考：
{
"specversion" : "1.0", // cloudevent版本信息 选择性填写即可
"type" : "com.github.pull.create",
"source" : "https://github.com/cloudevents/spec/pull",
"subject" : "123",
"id" : "A234-1234-1234",
"time" : "2020-04-05T17:31:00Z",
"comexampleextension1" : "value",
"comexampleothervalue" : 5,
"datacontenttype" : "text/xml",
"data" : "<much wow=\"xml\"/>"
}
总结篇
CloudEvents 的核心其实是定义了一组关于不同组件间传输事件的元数据，以及这些元数据应该如何出现在消息体中。
其主旨大抵如下：
- 事件规范化
- 降低平台集成难度
- 提高FaaS的可移植性
- 源事件可追踪
- 提升事件关联性
准确的事件体，事件信息才可以做出更稳定的系统架构，永远保持对事件的敬畏。
附 一些术语及定义：
Occurrence：发生，指事件逻辑上的发生，基于某种情况，事件出现了。
Event：事件，表示事件以及上下文的数据记录。可以根据事件中的信息决定路由，但事件本身并不包含路由信息。
Producer：生产者，真正创造事件的实例或组件
Source：源，事件发生的上下文，可以由多个producer组成。
Consumer：消费者，接收事件并对事件进行消费
Intermediary：中介，接收包含事件的消息（message），并转发给下一个接收方，类似路由器
Context：上下文，上下文元数据被封装到context attributes中。用来判断事件与其它系统的关系
Data：数据，也可以叫做 payload
EventFormat：事件格式，例如 json
Message：消息，封装事件并将其从source传递到destination。
Protocol：协议，可以是行业标准如 http，开源协议如 kafka 或者供应商协议如 AWS Kinesis
Protocol Binding：协议绑定，描述如何通过给定的协议收发事件，如何将事件放到消息里
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