Quality of service

The MQTT protocol provides three qualities of service for delivering messages between clients and servers: "at most once", "at least once" and "exactly once".

Quality of service (QoS) is an attribute of an individual message being published. An application sets the QoS for a specific message by setting the MQTTAsync_message.qos field to the required value.

A subscribing client can set the maximum quality of service a server uses to send messages that match the client subscriptions. The MQTTAsync_subscribe() and MQTTAsync_subscribeMany() functions set this maximum. The QoS of a message forwarded to a subscriber thus might be different to the QoS given to the message by the original publisher. The lower of the two values is used to forward a message.

The three levels are:

QoS0, At most once: The message is delivered at most once, or it may not be delivered at all. Its delivery across the network is not acknowledged. The message is not stored. The message could be lost if the client is disconnected, or if the server fails. QoS0 is the fastest mode of transfer. It is sometimes called "fire and forget".

The MQTT protocol does not require servers to forward publications at QoS0 to a client. If the client is disconnected at the time the server receives the publication, the publication might be discarded, depending on the server implementation.

QoS1, At least once: The message is always delivered at least once. It might be delivered multiple times if there is a failure before an acknowledgment is received by the sender. The message must be stored locally at the sender, until the sender receives confirmation that the message has been published by the receiver. The message is stored in case the message must be sent again.

QoS2, Exactly once: The message is always delivered exactly once. The message must be stored locally at the sender, until the sender receives confirmation that the message has been published by the receiver. The message is stored in case the message must be sent again. QoS2 is the safest, but slowest mode of transfer. A more sophisticated handshaking and acknowledgement sequence is used than for QoS1 to ensure no duplication of messages occurs.

@page publish Publication example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "MQTTAsync.h"
 
#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "MQTT Examples"
#define PAYLOAD     "Hello World!"
#define QOS         1
#define TIMEOUT     10000L
 
volatile MQTTAsync_token deliveredtoken;
 
int finished = 0;
 
void connlost(void *context, char *cause)
{
        MQTTAsync client = (MQTTAsync)context;
        MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
        int rc;
 
        printf("\nConnection lost\n");
        printf("     cause: %s\n", cause);
 
        printf("Reconnecting\n");
        conn_opts.keepAliveInterval = 20;
        conn_opts.cleansession = 1;
        if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start connect, return code %d\n", rc);
                finished = 1;
        }
}
 
 
void onDisconnect(void* context, MQTTAsync_successData* response)
{
        printf("Successful disconnection\n");
        finished = 1;
}
 
 
void onSend(void* context, MQTTAsync_successData* response)
{
        MQTTAsync client = (MQTTAsync)context;
        MQTTAsync_disconnectOptions opts = MQTTAsync_disconnectOptions_initializer;
        int rc;
 
        printf("Message with token value %d delivery confirmed\n", response->token);
 
        opts.onSuccess = onDisconnect;
        opts.context = client;
 
        if ((rc = MQTTAsync_disconnect(client, &opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start sendMessage, return code %d\n", rc);
                exit(EXIT_FAILURE);
        }
}
 
 
void onConnectFailure(void* context, MQTTAsync_failureData* response)
{
        printf("Connect failed, rc %d\n", response ? response->code : 0);
        finished = 1;
}
 
 
void onConnect(void* context, MQTTAsync_successData* response)
{
        MQTTAsync client = (MQTTAsync)context;
        MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
        MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
        int rc;
 
        printf("Successful connection\n");
 
        opts.onSuccess = onSend;
        opts.context = client;
 
        pubmsg.payload = PAYLOAD;
        pubmsg.payloadlen = strlen(PAYLOAD);
        pubmsg.qos = QOS;
        pubmsg.retained = 0;
        deliveredtoken = 0;
 
        if ((rc = MQTTAsync_sendMessage(client, TOPIC, &pubmsg, &opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start sendMessage, return code %d\n", rc);
                exit(EXIT_FAILURE);
        }
}
 
 
int main(int argc, char* argv[])
{
        MQTTAsync client;
        MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
        MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
        MQTTAsync_token token;
        int rc;
 
        MQTTAsync_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
 
        MQTTAsync_setCallbacks(client, NULL, connlost, NULL, NULL);
 
        conn_opts.keepAliveInterval = 20;
        conn_opts.cleansession = 1;
        conn_opts.onSuccess = onConnect;
        conn_opts.onFailure = onConnectFailure;
        conn_opts.context = client;
        if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start connect, return code %d\n", rc);
                exit(EXIT_FAILURE);
        }
 
        printf("Waiting for publication of %s\n"
         "on topic %s for client with ClientID: %s\n",
         PAYLOAD, TOPIC, CLIENTID);
        while (!finished)
                #if defined(WIN32) || defined(WIN64)
                        Sleep(100);
                #else
                        usleep(10000L);
                #endif
 
        MQTTAsync_destroy(&client);
        return rc;
}