mqttx 链接mysql数据库sql怎么写

        上两篇文件讲述了MQTT协议基础分解过程，这一章节主要讲述具体消息的实现。由于消息处理规范都差不多，接下来就介绍Connect和Publish两个比较复杂的消息分解代码，这两个消息在MQTT协议中算是比较复杂的两个，只能理解这两个的协议分析那对实现整个MQTT协议就不存在问题了。
参考文档 
        中文：vitsumoc.github.io/mqtt-v5-0-chinese.html
        英文:  docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-v5.0.html
        编程语言: C#
        完整代码: //github.com/beetlex-io/mqtt

Connect消息
        在实现之前还是需要先了解一下Connect的数据结构，这个消息也是MQTT中结构最复杂的一个了。消息包括一个可变头部的数据集和Payload的属性集。

	Description	7	6	5	4	3	2	1	0
Protocol Name
byte 1	Length MSB (0)	0	0	0	0	0	0	0	0
byte 2	Length LSB (4)	0	0	0	0	0	1	0	0
byte 3	‘M’	0	1	0	0	1	1	0	1
byte 4	‘Q’	0	1	0	1	0	0	0	1
byte 5	‘T’	0	1	0	1	0	1	0	0
byte 6	‘T’	0	1	0	1	0	1	0	0
Protocol Version
	Description	7	6	5	4	3	2	1	0
byte 7	Version (5)	0	0	0	0	0	1	0	1
Connect Flags
byte 8	User Name Flag (1) Password Flag (1) Will Retain (0) Will QoS (01) Will Flag (1) Clean Start(1) Reserved (0)	1	1	0	0	1	1	1	0
Keep Alive
byte 9	Keep Alive MSB (0)	0	0	0	0	0	0	0	0
byte 10	Keep Alive LSB (10)	0	0	0	0	1	0	1	0
Properties
byte 11	Length (5)	0	0	0	0	0	1	0	1
byte 12	Session Expiry Interval identifier (17)	0	0	0	1	0	0	0	1
byte 13	Session Expiry Interval (10)	0	0	0	0	0	0	0	0
byte 14		0	0	0	0	0	0	0	0
byte 15		0	0	0	0	0	0	0	0
byte 16		0	0	0	0	1	0	1	0

而对应Payload的可以变属性集就包括客户端ID和遗嘱属性集，遗嘱属性集又包含着属性集，这些详细都有在协议文档描述。有了这些规则就可以通过代码来分解协议了，在读写过程中相关内容顺序必须按照文档描述的顺序来读写（针对前一章描述的属性表内容则无须根据文档描述顺序，那个是似对应的属性编码来对应）。

• 读取协议代码

ProtocolName = parse.ReadString(stream);
Version = (byte)stream.ReadByte();
byte flats = (byte)stream.ReadByte();
Reserved = (MQTTParse.BIT_1 & flats) > 0;
ClearStart = (MQTTParse.BIT_2 & flats) > 0;
WillFalg = (MQTTParse.BIT_3 & flats) > 0;
WillRetian = (MQTTParse.BIT_6 & flats) > 0;
PasswordFlag = (MQTTParse.BIT_7 & flats) > 0;
UserFlag = (MQTTParse.BIT_8 & flats) > 0;
this.WillQos = (QoSType)((0b0001_1000 & flats) >> 3);
KeepAlive = parse.ReadUInt16(stream);
var ps = GetPropertiesStream();
ps.Read(parse, stream);
SessionExpiryInterval = ps;
ReceiveMaximum = ps;
MaximumPacketSize = ps;
TopicAliasMaximum = ps;
RequestResponseInformation = ps;
RequestProblemInformation = ps;
AuthenticationMethod = ps;
AuthenticationData = ps;
UserProperties = ps;
ClientID = parse.ReadString(stream);
if (WillFalg)
{
    this.WillProperty.Read(this, parse, stream);
    WillTopic = parse.ReadString(stream);
    WillPayload = parse.ReadBinary(stream);
}
if (UserFlag)
    Name = parse.ReadString(stream);
if (PasswordFlag)
    Password = parse.ReadString(stream);

• 写入协议代码

parse.WriteString(stream, ProtocolName);
stream.WriteByte(Version);
byte flats = 0x0;
if (Reserved)
    flats |= MQTTParse.BIT_1;
if (ClearStart)
    flats |= MQTTParse.BIT_2;
if (WillFalg)
    flats |= MQTTParse.BIT_3;
flats |= (byte)((byte)this.WillQos << 3);
if (this.WillRetian)
    flats |= MQTTParse.BIT_6;
if (this.UserFlag)
    flats |= MQTTParse.BIT_7;
if (this.PasswordFlag)
    flats |= MQTTParse.BIT_8;
stream.WriteByte(flats);
parse.WriteUInt16(stream, KeepAlive);
var ps = GetPropertiesStream()
    + SessionExpiryInterval
    + ReceiveMaximum
    + MaximumPacketSize
    + TopicAliasMaximum
    + RequestResponseInformation
    + RequestProblemInformation
    + AuthenticationMethod
    + AuthenticationData
    + UserProperties;


ps.Write(parse, stream);
parse.WriteString(stream, ClientID);
if (WillFalg)
{
    this.WillProperty.Write(this, parse, stream);
    parse.WriteString(stream, WillTopic);
    if (WillPayload == null)
        WillPayload = new byte[0];
    parse.WriteBinary(stream, WillPayload);
}
if (UserFlag)
    parse.WriteString(stream, Name);
if (PasswordFlag)
    parse.WriteString(stream, Password);

完整代码
//github.com/beetlex-io/mqtt/blob/main/BeetleX.MQTT.Protocols/V5/Messages/Publish.cs

Publish消息
        该消息的结构并不复，带一个可变头部的数据块和推送内容的二进制结构；这个二进制则根据双方约定来进行编码处理，具体可以是JSON,UTF字符等。

Description	7	6	5	4	3	2	1	0
Topic Name
byte 1	Length MSB (0)	0	0	0	0	0	0	0	0
byte 2	Length LSB (3)	0	0	0	0	0	0	1	1
byte 3	‘a’ (0x61)	0	1	1	0	0	0	0	1
byte 4	‘/’ (0x2F)	0	0	1	0	1	1	1	1
byte 5	‘b’ (0x62)	0	1	1	0	0	0	1	0
Packet Identifier
byte 6	Packet Identifier MSB (0)	0	0	0	0	0	0	0	0
byte 7	Packet Identifier LSB (10)	0	0	0	0	1	0	1	0
Property Length
byte 8	No Properties	0	0	0	0	0	0	0	0

 以上是Publish消息的可变头结构，包括主题名称，消息ID和属性集；剩下的消息问题即是提交的消息体。

• 读取协议代码

Topic = parse.ReadString(stream);
if (QoS != QoSType.MostOnce)
    Identifier = parse.ReadUInt16(stream);
var ps = GetPropertiesStream();
ps.Read(parse, stream);
PayloadFormatIndicator = ps;
MessageExpiryInterval = ps;
TopicAlias = ps;
ResponseTopic = ps;
CorrelationData = ps;
SubscriptionIdentifier = ps;
ContentType = ps;
UserProperties = ps;
var len = (int)(stream.Length - stream.Position);
var payload = MQTTMessage.RentPayloadBuffer(len);
stream.Read(payload, 0, len);
Payload = new ArraySegment<byte>(payload, 0, len);

• 写入协议代码 

base.OnWrite(parse, stream, session);
parse.WriteString(stream, Topic);
if (QoS != QoSType.MostOnce)
    parse.WriteUInt16(stream, Identifier);
var ps = GetPropertiesStream() 
     + PayloadFormatIndicator 
     + MessageExpiryInterval 
     + TopicAlias 
     + ResponseTopic 
     + CorrelationData 
     + SubscriptionIdentifier 
     + UserProperties;
ps.Write(parse, stream);
stream.Write(Payload.Array, Payload.Offset, Payload.Count);

完整代码
//github.com/beetlex-io/mqtt/blob/main/BeetleX.MQTT.Protocols/V5/Messages/Publish.cs

        通以上两个消息协议代码的实现已经可以体现出MQTT协议的分析处理，其实MQTT协议的处理并不复杂，为了方便程序中的操作只能在针对协议定义对应的结构体繁琐工作。

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