Hi,
I'm very, very interested to this topic. Can I try to summarize my understanding of how Paho works?
1. Messages published with QoS > 0 are considered "in-flight messages" and
the acknowledgement phase/stage/state of every in-flight message is tracked in the persistence.
2. Paho does not retry the delivery of an in-flight message within the same "session"
(I mean before a clean disconnect or the connection is lost).
For example, if it does not receive a PUBACK to a message published with QoS == 1,
it does not resend the original PUBLISH.
Same for messages published with QoS == 2.
2.1 There is a maximum number of in-flight messages (10).
If messages cannot be delivered (for example slow or broken link, e.g. GPRS),
Paho will not accept more messages from the application.
Note that the TCP/MQTT connection can still be healthy
(accordingly to the value of the keep-alive intervals) even if the link is broken.
3. If Paho disconnects cleanly from the broker or the connection is lost,
Paho will retry the delivery of in-flight messages in the next session but ONLY
if its clean session flag is set to false. Otherwise the persistence will be cleared.
4 Not Paho specific, the semantics of QoS==2 can only be guaranteed across sessions established with clean session set to false.
So, for example it won't be guaranteed if a client connects first with clean session==true,
publishes some messages with QoS==2, loses the connection and
reconnects later with clean session==false.
If I'm not wrong Paho will not clear the persistence on the second session and might
resend PUBLISH messages (with the duplicate flag set).
On the broker side, since the first session was clean, the client state may have been deleted.
Will the broker accept the duplicates as new messages in the second session or not?
It's a corner case, I know...
Ciao,
Cristiano
On 20/10/2015 11:04, James Sutton1 wrote:
Hi Peiyuan,
In the example, if the MQTT Broker is not contactable then an MqttException will be thrown.
Console output when working:
paho/EclipsePahoMavenExample [ java -cp target/my-app-1.0-SNAPSHOT-jar-with-dependencies.jar org.eclipse.paho.App master * ] 9:53 AM
Connecting to broker: tcp://iot.eclipse.org:1883
Connected
Publishing message: Message from MqttPublishSample
Message published
Disconnected
When the broker cannot be contacted:
paho/EclipsePahoMavenExample [ java -cp target/my-app-1.0-SNAPSHOT-jar-with-dependencies.jar org.eclipse.paho.App master * ] 9:53 AM
Connecting to broker: tcp://iot.ceclipse.org:1883
reason 32103
msg Unable to connect to server
loc Unable to connect to server
cause java.net.ConnectException: Connection refused
excep Unable to connect to server (32103) - java.net.ConnectException: Connection refused
Unable to connect to server (32103) - java.net.ConnectException: Connection refused
at org.eclipse.paho.client.mqttv3.internal.TCPNetworkModule.start(TCPNetworkModule.java:79)
at org.eclipse.paho.client.mqttv3.internal.ClientComms$ConnectBG.run(ClientComms.java:619)
at java.lang.Thread.run(Thread.java:781)
Caused by: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:358)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:219)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:201)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:616)
at org.eclipse.paho.client.mqttv3.internal.TCPNetworkModule.start(TCPNetworkModule.java:70)
... 2 more
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Kind regards,
James Sutton
Software Engineer - IoT Foundation - MQTT Open Source Projects
Technical Trojan - Wimbledon Project
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----- Original message -----
From: fang peiyuan
<fang_peiyuan@xxxxxxxxx>
Sent by:
paho-dev-bounces@xxxxxxxxxxx
To: "paho-dev@xxxxxxxxxxx"
<paho-dev@xxxxxxxxxxx>
Cc:
Subject: [paho-dev] how to use the new mqttPresistence, Qos and the acknowledge from the MQTT broker
Date: Mon, Oct 19, 2015 7:34 AM
hi my name is peiyuan, currenly i am able to write a simple publisher and subscriber using java code and eclipse paho client thank to James. Now i would like to go into QoS service and new MqttPresistence
. So is there any tutorial or sample i can learn from it.
At the same time i had problem in my current mqttpublisher how to return the connack from the broker server that i had connect to it? Because i want to know let say i forget to turn on my mosquitto server
and hence i am not able to connect but currently even if i did not turn on my server, my client is still not a null and it till show is connected after i went through the debug mode
my current code
Mqttclient client = new Mqttclient (TCPaddress, ClientID, new Memorypresistence)
if (client != null)
{
client .connect;
if (!client.isconnected)
{
client.publish("topic", new MqttMessage("Message".getbytes));
}
thank you
peiyuan
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