MQTT：掉线重连 (Reconnect)
cleansession对重连的影响
在使用MQTT同步库：#include "MQTTClient.h" 时，若client 与 broker 意外断开了连接，重连逻辑如何实现呢？其中 cleansession 对重连逻辑实现有什么影响呢？
分别对cleansession的两种情况进行测试和验证。
📌 注：MQTT Version = MQTT3.1.1
cleansession=1
当cleansession=1时，client断开连接后，broker中会清除会话的信息。当该client重连后，broker中重新创建新的会话ID，client需要重新订阅主题，从主题消息的最新的位置开始消费。
MQTTClient_subscribe_cleansession1.c
#include "windows.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include "MQTTClient.h"
#define ADDRESS "tcp://xx.xx.xx.xx:1883"
#define CLIENTID "ClientSub_Host62"
#define TOPIC "datetime1"
#define QOS 1
#define TIMEOUT 10000L
static int run = 1;
volatile MQTTClient_deliveryToken deliveredtoken;
static void sighandler(int signum)
{
if (signum == SIGINT)
{
printf("catch signal:SIGINT\n");
run = 0;
}
}
void delivered(void *context, MQTTClient_deliveryToken dt)
{
printf("Message with token value %d delivery confirmed\n", dt);
deliveredtoken = dt;
}
int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message)
{
printf("Message arrived\n");
printf(" topic: %s\n", topicName);
printf(" message: %.*s\n", message->payloadlen, (char*)message->payload);
MQTTClient_freeMessage(&message);
MQTTClient_free(topicName);
return 1;
}
void connlost(void *context, char *cause)
{
printf("\nConnection lost\n");
printf(" cause: %s\n", cause);
MQTTClient client = (MQTTClient )context;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
conn_opts.keepAliveInterval = 10;
conn_opts.cleansession = 1;
while (1)
{
if (MQTTClient_connect(client, &conn_opts) == MQTTCLIENT_SUCCESS)
{
printf("Reconnection Successful\n");
break;
}
else
{
printf("Reconnection Failure\n");
}
Sleep(1000);
}
while (1)
{
if (MQTTClient_subscribe(client, TOPIC, QOS) == MQTTCLIENT_SUCCESS)
{
printf("Resubscribe Successful\n");
break;
}
else
{
printf("Resubscribe Failure\n");
}
Sleep(1000);
}
}
int main(int argc, char* argv[])
{
MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
int rc;
if ((rc = MQTTClient_create(&client, ADDRESS, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to create client, return code %d\n", rc);
rc = EXIT_FAILURE;
goto exit;
}
if ((rc = MQTTClient_setCallbacks(client, client, connlost, msgarrvd, delivered)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to set callbacks, return code %d\n", rc);
rc = EXIT_FAILURE;
goto destroy_exit;
}
conn_opts.keepAliveInterval = 10;
conn_opts.cleansession = 1;
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to connect, return code %d\n", rc);
rc = EXIT_FAILURE;
goto destroy_exit;
}
printf("Subscribing to topic %s for client %s using QoS%d\n", TOPIC, CLIENTID, QOS);
if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to subscribe, return code %d\n", rc);
rc = EXIT_FAILURE;
goto destroy_exit;
}
signal(SIGINT, sighandler);
while (run)
{
Sleep(100);
}
if ((rc = MQTTClient_disconnect(client, TIMEOUT)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to disconnect, return code %d\n", rc);
rc = EXIT_FAILURE;
}
destroy_exit:
MQTTClient_destroy(&client);
exit:
getchar();
return rc;
}
cleansession=0
当cleansession=0时，client断开连接后，broker中会记录刚掉线会话的信息，例如：订阅的主题、未消费的主题消息。当该会话重连时，不需要对掉线前的主题再次订阅。重连成功后，接着消费掉线前的位置消费（即历史消息），历史消息的最大数量取决于broker中队列的大小。EMQX中的队列最多可以保留1000条消息
MQTTClient_subscribe_cleansession0.c
#include "windows.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include "MQTTClient.h"
#define ADDRESS "tcp://xx.xx.xx.xx:1883"
#define CLIENTID "ClientSub_Host62"
#define TOPIC "datetime1"
#define QOS 1
#define TIMEOUT 10000L
static int run = 1;
volatile MQTTClient_deliveryToken deliveredtoken;
static void sighandler(int signum)
{
if (signum == SIGINT)
{
printf("catch signal:SIGINT\n");
run = 0;
}
}
void delivered(void *context, MQTTClient_deliveryToken dt)
{
printf("Message with token value %d delivery confirmed\n", dt);
deliveredtoken = dt;
}
int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message)
{
printf("Message arrived\n");
printf(" topic: %s\n", topicName);
printf(" message: %.*s\n", message->payloadlen, (char*)message->payload);
MQTTClient_freeMessage(&message);
MQTTClient_free(topicName);
return 1;
}
void connlost(void *context, char *cause)
{
MQTTClient pclient = (MQTTClient )context;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
printf("\nConnection lost\n");
printf(" cause: %s\n", cause);
conn_opts.keepAliveInterval = 10;
conn_opts.cleansession = 0;
while (1)
{
if (MQTTClient_connect(pclient, &conn_opts) == MQTTCLIENT_SUCCESS)
{
printf("Reconnection Successful\n");
break;
}
else
{
printf("Reconnection Failure\n");
}
Sleep(1000);
}
}
int main(int argc, char* argv[])
{
MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
int rc;
if ((rc = MQTTClient_create(&client, ADDRESS, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to create client, return code %d\n", rc);
rc = EXIT_FAILURE;
goto exit;
}
if ((rc = MQTTClient_setCallbacks(client, client, connlost, msgarrvd, delivered)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to set callbacks, return code %d\n", rc);
rc = EXIT_FAILURE;
goto destroy_exit;
}
conn_opts.keepAliveInterval = 10;
conn_opts.cleansession = 0;
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to connect, return code %d\n", rc);
rc = EXIT_FAILURE;
goto destroy_exit;
}
printf("Subscribing to topic %s for client %s using QoS%d\n", TOPIC, CLIENTID, QOS);
if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to subscribe, return code %d\n", rc);
rc = EXIT_FAILURE;
goto destroy_exit;
}
signal(SIGINT, sighandler);
while (run)
{
Sleep(100);
}
if ((rc = MQTTClient_disconnect(client, TIMEOUT)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to disconnect, return code %d\n", rc);
rc = EXIT_FAILURE;
}
destroy_exit:
MQTTClient_destroy(&client);
exit:
getchar();
return rc;
}
反思总结
疑点1
这里看以下重连回调函数原型：
/**
* This is a callback function. The client application
* must provide an implementation of this function to enable asynchronous
* notification of the loss of connection to the server. The function is
* registered with the client library by passing it as an argument to
* MQTTClient_setCallbacks(). It is called by the client library if the client
* loses its connection to the server. The client application must take
* appropriate action, such as trying to reconnect or reporting the problem.
* This function is executed on a separate thread to the one on which the
* client application is running.
* @param context A pointer to the <i>context</i> value originally passed to
* MQTTClient_setCallbacks(), which contains any application-specific context.
* @param cause The reason for the disconnection.
* Currently, <i>cause</i> is always set to NULL.
*/
typedef void MQTTClient_connectionLost(void* context, char* cause);
注释中说明了参数: void* context，这个指针指向上下文，通过函数 MQTTClient_setCallbacks() 的第二参数传递进去的。
函数 MQTTClient_setCallbacks() 中传入的所有的回调函数原型的第一个参数都是void* context，那么这里所说的指向上下文是什么意思呢？
其实就是 MQTTClient client mqtt客户端句柄就是联系上下文的，包括关联到：会话丢失回调函数、消息接收回调函数、消息发送成功回调函数
疑点2
在 会话丢失回调函数中，为什么要把连接操作放在一个while(1)语句中？
client连接到broker后，会话丢失回调函数被注册，等待会话断开才被激活。若重连没有成功就离开了会话丢失回调函数，那么再也不会进来了，所以一定要保证重连成功才能离开。
在会话丢失后，通过 会话丢失回调函数 重连成功，一段时间后，若会话又一次丢失，会话丢失回调函数还有效吗？
是的
