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Re: [paho-dev] R: Paho Java Client retransmission implementation
|
>> Do you think it would be worthwhile to add this
feature to Paho?
>We quite deliberately removed message retry from the client side, but
>I don't recall the full reasoning (this was >2 years ago now). That
>said, it does presume you are running against a 'reliable' server,
>which, in the context of WebSphere MQ, you are and that you have a
>reliable TCP connection (reliable in the sense packets don't
>mysteriously go missing even if the connection is fragile).
>So, perhaps there is a place in for this behaviour in the client -
>Dave/Ian, can you recall why we did this?
One of the original MQTT Java clients
had retry capability built in. At the time it was added to handle badly
behaving TCPIP implementations on some "wireless" networks (in
the late 90s) . For instance a QOS 1 publish would be sent but puback on
occasions would never be received. This was not the fault of the server
but the fault of the network which never delivered the publish to the server.
As a result of improvements in TCPIP implementations the retry logic
was taken out of the Java client.
On the assertion thatf TCPIP behaves
correctly, iIf a problem occurs then either the "network" informs
the MQTT client or the MQTT client detects the problem via the keepalive.
When a problem occurs the MQTT client will tidy up and alert the app the
connection has been lost. When the app causes the MQTT client to reconnect
(after an abnormal disconnect or normal disconnect) it will ensure messages
that are still in-flight are delivered to the requested QOS.
Given the background are there still
reasons why retry might be useful?
All the best
Dave Locke
Senior Inventor, Pervasive and
Advanced Messaging Technologies
locke@xxxxxxxxxx
Dave
Locke/UK/IBM@ibmgb
7-246165
(int) +44 1962816165 (ext)
37274133
(mobex) +44 7764132584 (ext)
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From:
"Nicholas O'Leary"
<nick.oleary@xxxxxxxxx>
To:
General development
discussions for paho project <paho-dev@xxxxxxxxxxx>,
Date:
07/08/2012 15:08
Subject:
Re: [paho-dev]
R: Paho Java Client retransmission implementation
Sent by:
paho-dev-bounces@xxxxxxxxxxx
> Would I wait forever? Unacknowledged messages
will stay forever in internal in-memory queues and persistence?
With that code, yes you would wait forever. There is also
token.waitForCompletion(timeout) if you want to block for a certain
period before moving on, however that still leave the message in an
indeterminate state and unable to be resent.
> I might want to track the in-flight messages in the application and
retransmit them if they are not acknowledged by a suitable timeout.
> The spec allows for this by republishing the message using the original
message ID and setting the DUP flag in the MQTT PUBLISH header.
> However this is not possible in Paho with the public API used in Sample.java.
Correct, the api does not expose that sort of republishing capability.
> Do you think it would be worthwhile to add this feature to Paho?
We quite deliberately removed message retry from the client side, but
I don't recall the full reasoning (this was >2 years ago now). That
said, it does presume you are running against a 'reliable' server,
which, in the context of WebSphere MQ, you are and that you have a
reliable TCP connection (reliable in the sense packets don't
mysteriously go missing even if the connection is fragile).
So, perhaps there is a place in for this behaviour in the client -
Dave/Ian, can you recall why we did this?
> What would be the required effort?
Obviously it depends on the approach taken - here are a couple options
off the top of my head:
1. it should be fairly straight forward to add some retry logic into
ClientState, along with a config option for a retry-timeout in
ConnectOptions. The client would then do the retrying under the
covers; the application would not need to take any further action.
2. Alternatively, the DeliveryToken object could have a method
.resend() added to it, that would resubmit the message for delivery
(with the duplicate flag set). So, if
deliveryToken.waitForCompletion(timeout) timed out, the application
could choose whether to resend the message.
Option 1 means retries will just work, Option 2 gives the application
more control, but also more responsibility.
We would also have to consider the impact on the other clients in
Paho; we want to keep them feature compatible.
Regards,
Nick
On 7 August 2012 09:36, De Alti, Cristiano
<Cristiano.DeAlti@xxxxxxxxxxxx> wrote:
> Hi Nick,
> Thanks for the quick confirm.
> I know that the spec does not require it.
> More questions:
> 1) What happens if we publish a message with QoS>0 and wait for
a confirm that never comes in (taken from Sample.java):
>
> public void publish(String topicName, int qos, byte[]
payload) throws MqttException {
>
> // Connect to the server
> client.connect();
> log("Connected to "+brokerUrl);
>
> // Get an instance of the topic
> MqttTopic topic = client.getTopic(topicName);
>
> MqttMessage
message = new MqttMessage(payload);
> message.setQos(qos);
>
> // Publish the message
> log("Publishing at: "+System.currentTimeMillis()+
" to topic \""+topicName+"\" qos "+qos);
> MqttDeliveryToken token = topic.publish(message);
>
> // Wait until the message has been delivered
to the server
> token.waitForCompletion();
>
> // Disconnect the client
> client.disconnect();
> log("Disconnected");
> }
>
> Would I wait forever? Unacknowledged messages will stay forever in
internal in-memory queues and persistence?
>
> 2) Alternatively I might decide to not wait for completion and track
the confirms asynchronously in the callback:
> public void deliveryComplete(MqttDeliveryToken
token) {
> // Called
when a message has completed delivery to the
> // server.
The token passed in here is the same one
> // that was
returned in the original call to publish.
> // This allows
applications to perform asychronous
> // delivery
without blocking until delivery completes.
>
> // This sample
demonstrates synchronous delivery, by
> // using the
token.waitForCompletion() call in the main thread.
> }
>
> I might want to track the in-flight messages in the application and
retransmit them if they are not acknowledged by a suitable timeout.
> The spec allows for this by republishing the message using the original
message ID and setting the DUP flag in the MQTT PUBLISH header.
> However this is not possible in Paho with the public API used in Sample.java.
>
> We currently use our own MQTT client implementation but we would like
to switch to Paho because it will be actively maintained.
> However our client autonomously performs retries.
> Do you think it would be worthwhile to add this feature to Paho? What
would be the required effort?
>
> Thanks,
> Ciao,
> Cristiano
> ________________________________________
> Da: paho-dev-bounces@xxxxxxxxxxx [paho-dev-bounces@xxxxxxxxxxx] per
conto di Nicholas O'Leary [nick.oleary@xxxxxxxxx]
> Inviato: martedì 7 agosto 2012 10.03
> A: General development discussions for paho project
> Oggetto: Re: [paho-dev] Paho Java Client retransmission implementation
>
> Hi Cristiano,
>
> no, the paho client does not retry unacknowledged messages (except
on
> reconnect).
>
> Clients are not required to retry delivery of messages.
>
> Regards,
> Nick
>
> On 7 August 2012 08:12, De Alti, Cristiano
> <Cristiano.DeAlti@xxxxxxxxxxxx> wrote:
>> Hi,
>> Does Paho Java Client implement the retransmission of unacknowledged
>> messages sent with QoS level 1 or 2?
>> I'm browsing the code but I cannot find any references to a retry
mechanism.
>>
>> Ciao,
>> Cristiano
>>
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>>
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