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Akka Http Notes

在快3年多的Scala项目编程中, Akka是我见过的比较高质量的scala库, 其核心抽象是一种基于Actor的编程模型, 同时在这个核心抽象上, 提供一组工具库, 用户只需要按Actor形式写业务逻辑, 框架会帮你处理好底层的消息传递, 高并发和IO问题. Akka在工业场景下, 很接底气, 比如有很多微服务, 服务的性能各有差异, 这时候你需要整合这些微服务, 完成比如广告投放, 在线推荐, 事故检测等业务, Akka的业务抽象就会有很大的用处.

而最近系统看了Akka-HTTP, 我个人比较喜欢这个库在meta-programming方面的应用, akka-http把一个老生常谈的HTTP库实现的很优雅, 设计和抽象值得推敲, 时间有限, 就看了一周, 以下是一些最近对我帮助比较大的总结, 如果以后有空会继续完善

1.Akka HTTP 优势

定位: 用于处理复杂业务的Library, 不是一个MVC Framework(such as Play)

  • DSL with convenient pathMatchers
  • Streaming: 流式传输, 速率限制
  • Interacting with actor easy

2.核心数据结构和抽象

  • HTTP Module
    • HttpRequest
      • a method (GET, POST, etc.)
      • a URI (see URI model for more information)
      • a seq of headers
      • an entity (body data): HttpEntity
        • HttpEntity类型
          • Strict: 消息体小, 内存可放, String or ByteString
          • Default: Streaming Data Source, know data size
          • Chunked: unknown length
          • Multipart.BodyPart: streaming entity, unknown length
        • 大小控制: max-content-length
      • a protocol
    • HttpResponse
      • a status code
      • a Seq of headers
      • an entity (body data)
      • a protocol
    • Headers
      • content-type: 在HttpEntity中定义
      • Content-length
      • User-agent: 自动添加
      • Date: 自动添加
    • Other supporting types and tools:
      • Uri, HttpMethods, MediaTypes, StatusCodes, Attributes
      • Parsing/Rendering
  • URI Module
    • model, handling special characters
      • foo://example.com:8042/over/there?name=ferret#nose
      • scheme://authority/path/query?fragment
    • Extract directives
      • Uri.query()
      • PathDirectives
  • Marshalling/Unmarshalling
    • Marshalling(Serialization/Pickling): Convert high level object to low level wire format
      • Function: A => Future[List[Marshalling[B]]]
        • Future: 支持异步
        • List: 序列化后的对象, 提供多种格式
        • Marshalling[B]: 可以访问MediaType, HttpCharset这些属性; 延迟Marshalling的调用, 执行content negotiation
      • 常见的Mashallers
        • type ToEntityMarshaller[T] = Marshaller[T, MessageEntity]
        • type ToByteStringMarshaller[T] = Marshaller[T, ByteString]
        • type ToHeadersAndEntityMarshaller[T] = Marshaller[T, (immutable.Seq[HttpHeader], MessageEntity)]
        • type ToResponseMarshaller[T] = Marshaller[T, HttpResponse]
        • type ToRequestMarshaller[T] = Marshaller[T, HttpRequest]
      • Support Implicit Resolution
    • Unmarshalling(Deserialization)
      • From low level MessageEntity to high level type T
      • Unmarshaller[A, Future[B]]
      • Support implicit resolution
    • Sparay-Json support

3.Server API

  • High Level: Routing DSL, content negotiation, static content serving
    • https://doc.akka.io/docs/akka-http/current/routing-dsl/index.html
    • 设计思路:
      • 更可读, 可维护性更好, 可组合, 可扩展的API构建方式
      • 以Directives构建Route Tree
      • Route Tree可以转换为底层的Flow
    • Route的接口定义:
      • type Route = RequestContext => Future[RouteResult]
        • RequestContext: 提供比HttpRequest更丰富的内容
        • RouteResult: 是一个ADT(algebraic data type), 一般从RouteDirectives创建(complete, reject, redirect)
      • Composing Route
        • transformation: 代理给inner route处理
        • filtering: condition and reject
        • chaining: concat指令, 构建Route Tree, 不建议使用~符号
      • Sealing a route
        • 可以返回response的route
    • Directives是构建route的核心
      • 创建route
        • val route: Route = { ctx => ctx.complete(“yeah”) }
        • val route: Route = _.complete(“yeah”)
        • val route = complete(“yeah”)
      • 其他功能
        • Transform/Filter/Extract incoming RequestContext
        • Chain RouteResult
        • Complete a request

4.代码重要的模块

  • Akka-http: High level functionality, dsl, marshalling/unmarshalling
  • Akka-http-core: low level http protocols
  • Akka-http-tools
    • Akka-http-testkit
    • Akka-http2-support
    • Akka-http-spray-json
    • Akka-http-xml

5.执行层

  • 基于akka stream
  • Timeout
    • Client/Server: IdleTimeout: 链接如果一段时间没有request和response
    • Client:
      • akka.http.client.connecting-timeout
      • akka.http.host-connection-pool.max-connection-lifetime: This timeout configures a maximum amount of time, while the connection can be kept open
    • Server:
      • akka.http.server.request-timeout 20s
      • akka.http.server.bind-timeout
      • Linger timeout: server implementation will keep a connection open after all data has been delivered to the network layer
  • Caching
Akka, Scala

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