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外匯交易FX是什麼?

飞飞 ​

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FX CRM—An Essential Tool for Forex Brokers

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MQTT协议,终于有人讲清楚了

飞飞 ​

  • mqtt协议
  • 1 MQTT协议特点
    • 发布和订阅
    • QoS(Quality of Service levels)
    • 2.1 MQTT固定头
    • 2.2 MQTT可变头 / Variable header
    • 2.3 Payload消息体
    • 3.1 MQTT服务器搭建
    • 3.2 MQTT Client

    mqtt协议

    MQTT(Message Queuing Telemetry Transport,消息队列遥测传输协议),是一种基于 发布/订阅 ( publish/subscribe )模式的“轻量级”通讯协议,该协议构建于TCP/IP协议上,由IBM在1999年发布。

    MQTT最大优点在于,用极少的代码和有限的带宽,为连接远程设备提供实时可靠的消息服务

    1 MQTT协议特点

    MQTT是一个基于客户端-服务器的消息发布/订阅传输协议。

    MQTT协议当前版本为,2014年发布的MQTT v3.1.1。除标准版外,还有一个简化版 MQTT-SN ,该协议主要针对嵌入式设备,这些设备一般工作于TCP/IP网络,如:ZigBee。

    MQTT 与 HTTP 一样,MQTT 运行在传输控制协议/互联网协议 (TCP/IP) 堆栈之上。

    发布和订阅

    这是一种消息传递模式,消息不是直接从发送器发送到接收器(即点对点),而是由 MQTT server (或称为 MQTT 外匯交易FX是什麼? Broker)分发的。

    MQTT 服务器是发布-订阅架构的核心

    它可以非常简单地在Raspberry Pi或NAS等单板计算机上实现,当然也可以在大型机或 Internet 服务器上实现。

    客户端(也称为节点)是一种智能设备,如微控制器或具有 TCP/IP 堆栈和实现 MQTT 协议的软件的计算机。

    消息在允许过滤的主题下发布。主题是分层划分的 UTF-8 字符串。不同的主题级别用斜杠 / 作为分隔符号。

    • 光伏发电站是发布者( Publisher )。
    • 主要主题( 外匯交易FX是什麼? Topic )级别是 "PV" ,这个工厂发布两个子级别 "sunshine" 和 "data" ;
    • "PV/sunshine" 是一个布尔值(true/fault,也可以是 1/0),充电站需要它来知道是否应该装载电动汽车(仅在阳光普照时 :))。
    • 充电站(EVSE)是订阅者,订阅 "PV/sunshine" 从服务器获取信息。
    • "PV/data" 另一方面,以 kW 为单位传输工厂产生的瞬时功率,并且该主题可以例如通过计算机或平板电脑订阅,以生成一天内传输功率的图表。

    QoS(Quality of Service levels)

    服务质量是 MQTT 的一个重要特性。当我们使用 TCP/IP 时,连接已经在一定程度上受到保护。但是在无线网络中,中断和干扰很频繁,MQTT 在这里帮助避免信息丢失及其服务质量水平。这些级别在发布时使用。如果客户端发布到 MQTT 服务器,则客户端将是发送者,MQTT 服务器将是接收者。当MQTT服务器向客户端发布消息时,服务器是发送者,客户端是接收者。

    QoS 1 承诺消息将至少传送一次给订阅者。

    使用 QoS 2,我们保证消息仅传送到目的地一次。为此,带有唯一消息 ID 的消息会存储两次,首先来自发送者,然后是接收者。QoS 级别 2 在网络中具有最高的开销,因为在发送方和接收方之间需要两个流。

    2 MQTT 数据包结构

    • 固定头(Fixed header) ,存在于所有 MQTT 数据包中,表示数据包类型及数据包的分组类标识;
    • 可变头(Variable header) ,存在于部分 MQTT 数据包中,数据包类型决定了可变头是否存在及其具体内容;
    • 消息体(Payload) ,存在于部分 MQTT 数据包中,表示客户端收到的具体内容;

    2.1 MQTT 固定头

    固定头 存在于所有 MQTT 数据包中,其结构如下:

    MQTT 消息类型 / message type

    **位置:**byte 1, bits 7-4。

    名称流方向描述

    标识位 / DUP

    **位置:**byte 1, bits 3-0。

      外匯交易FX是什麼?
    • DUP :发布消息的副本。用来在保证消息的可靠传输,如果设置为 1,则在下面的变长中增加MessageId,并且需要回复确认,以保证消息传输完成,但不能用于检测消息重复发送。
    • QoS 发布消息的服务质量(前面已经做过介绍),即:保证消息传递的次数
      • 00 :最多一次,即: 外匯交易FX是什麼?
      • 01 :至少一次,即:>=1
      • 10 :一次,即:=1
      • 11 :预留

      剩余长度(Remaining Length)

      2.2 MQTT 可变头 / Variable header

      Bit7 — 0

      PUBLISH (QoS > 0)、PUBACK、PUBREC、PUBREL、PUBCOMP、

      2.3 Payload 消息体

      Payload 消息体是 MQTT 数据包的第三部分,CONNECT、SUBSCRIBE、SUBACK、UNSUBSCRIBE四种类型的消息 有消息体:

      • CONNECT 外匯交易FX是什麼? ,消息体内容主要是:客户端的ClientID、订阅的Topic、Message以及用户名和密码
      • SUBSCRIBE ,消息体内容是一系列的要订阅的主题以及 QoS 。
      • SUBACK ,消息体内容是服务器对于 SUBSCRIBE 所申请的主题及 QoS 进行确认和回复。
      • UNSUBSCRIBE ,消息体内容是要订阅的主题。

      3 环境搭建

      3.1 MQTT服务器搭建

      • Mosquitto: https:// mosquitto.org/
      • VerneMQ: https:// vernemq.com/
      • EMQTT: http:// emqtt.io/

      本文将使用 Mosquitoo 进行测试,进入到安装页面,下载自己电脑的系统所适配的程序;

      按住 Shift ,右键鼠标点击空白处,然后打开 Powershell ,正常打开一个终端软件即可;

      • 输入 ./mosquitto.exe -h 可以查看相应的帮助;
      • 输入 ./mosquitto.exe -p 10086 ,就开启了MQTT服务,监听的地址是 127.0.0.1 ,端口是 10086 ;

      3.2 MQTT Client

      这里我使用的是自己编译了一个 QT mqtt client 程序,是基于Qt的官方库进行编译的,下面打开这个软件,下一期简单介绍一下如何完成这个客户端,并设置好相应参数: