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Re: [paho-dev] R: R: R: Paho Java Client retransmission implementation 
Hi Cristiano,
I must admit I was relying on what other people had told me about the changes in the spec regarding packet retries, and I had assumed that retrying during a session was forbidden. It seems it is not.
In my experience, clients retrying packets without some knowledge of when the broker is able to accept new messages again tends to exacerbate the problem. Floods of retried packets are no help to an overwhelmed broker. Delaying the acks is a better alternative.
On message ordering, I think it is up to the broker to allow ordering to be maintained "for an ordered topic" as the spec puts it. If the broker did not ack message 2, then it should do so before processing messsage 3. Unless the topic is unordered. 

Ian

On 03/30/2016 01:48 PM, De Alti, Cristiano wrote:

Hi Ian,


the reason that Paho clients do not retry QoS 1 and 2 messages within a
session is that the MQTT 3.1.1 specification dictates this behaviour.

Do you refer to "4.4 Message delivery retry" in the 10 December 2015 spec?
When a Client reconnects with CleanSession set to 0, both the Client and Server MUST re-send any unacknowledged PUBLISH Packets (where QoS > 0) and PUBREL Packets using their original Packet Identifiers [MQTT-4.4.0-1]. This is the only circumstance where a Client or Server is REQUIRED to redeliver messages.

It seems to me that the latest 3.1.1 spec does neither forbid nor discourages message delivery retries in the same connection.
Please correct me where I'm wrong:

* Clean Session = false. In our experience this is rarely used and difficult to implement correctly on the broker (because of the state it has to maintain).
    Message delivery retries are performed just after a client reconnects.
* Clean Session = true. This is the default configuration on our devices (and in Eclipse Kura).
    Message delivery retries are not performed on reconnect because the broker is not required to maintain the client state across reconnects.
    In this configuration, how the QoS semantics has to be understood?
    QoS == 1. If the client does not retry and the application does not receive the acknowledge, what should happen?
    "At least once" is actually "At most once" since, if the application retried, it would send another message with a different message ID and DUP flag not set.
    QoS == 2. Same as above.

On the other hand, if the client implements retries in the same connection, and given the absence of any window, how is the message order affected?
The client sends message 1,2,3 in this order. The broker does not acknowledge message 2. After a while the client retries delivering message 2.
In the meantime did the broker deliver messages 1 and 3 to subscribers? Does it have to detect the gap in the message ID sequence and holdoff delivery of message 3 until message 2 has been received?


One option for the broker is to delay
the acknowledgement of QoS 1 and 2 messages, rather than ignore them
entirely.

Interesting suggestion. We'll explore this possibility with our broker.


We are discussing this problem of flow control right now in the MQTT
specification work group

Thanks for the links. I'll take a look.


In the meantime, if there are changes to behaviour we can make in the
Paho clients which help but remain conformant to the MQTT 3.1.1
specification, we will be happy to look into them.  The inflight message
queue is only small in some client implementations - in the C
asynchronous client for example there is no limit, except for the number
of MQTT packet ids (64k), but I would not recommend going anywhere near
64k outstanding messages!

Increasing the size of the inflight message queue can avoid the deadlock but there is still the possibility that
unacknowledged messages will never be delivered (if the broker deliberately does not acknowledge them).
It seems there is no easy solution with the current implementation.


Some client implementations (not in Paho) have the ability to limit
throughput on the client side, but I think that is not dynamically
dependent on the broker workload.

Right, the broker cannot rely on the client being "nice".

Regards,
   Cristiano
________________________________________
Da: paho-dev-bounces@xxxxxxxxxxx <paho-dev-bounces@xxxxxxxxxxx> per conto di Ian Craggs <icraggs@xxxxxxxxxxxxxxxxxxxxxxx>
Inviato: martedì 29 marzo 2016 12.01
A: paho-dev@xxxxxxxxxxx
Oggetto: Re: [paho-dev] R: R: R: Paho Java Client retransmission implementation

Hi Cristiano,

the reason that Paho clients do not retry QoS 1 and 2 messages within a
session is that the MQTT 3.1.1 specification dictates this behaviour.
This was a change from MQTT 3.1.   One option for the broker is to delay
the acknowledgement of QoS 1 and 2 messages, rather than ignore them
entirely.

We are discussing this problem of flow control right now in the MQTT
specification work group: https://issues.oasis-open.org/browse/MQTT-257,
with the goal of having some resolution in the next version of MQTT.
Undoubtedly there will be some ability to negatively acknowledge publish
packets in the next version of MQTT, as everyone agrees this is needed
(https://issues.oasis-open.org/browse/MQTT-236).  This in itself would
provide another mechanism for the broker to manage message rates.

In the meantime, if there are changes to behaviour we can make in the
Paho clients which help but remain conformant to the MQTT 3.1.1
specification, we will be happy to look into them.  The inflight message
queue is only small in some client implementations - in the C
asynchronous client for example there is no limit, except for the number
of MQTT packet ids (64k), but I would not recommend going anywhere near
64k outstanding messages!

Some client implementations (not in Paho) have the ability to limit
throughput on the client side, but I think that is not dynamically
dependent on the broker workload.

Ian


On 03/24/2016 11:23 AM, De Alti, Cristiano wrote:

Hello,
     I'd like to resume this old thread about retrying the delivery of
messages published with QoS > 0 in the same MQTT connection (i.e between
two MQTT CONNECTCs).

The rationale is that the broker, to operate in a stable way, besides
limiting the rate of MQTT connections, may also have to throttle
incoming MQTT PUBLISH messages.

The scenario involves thousands of MQTT connections with clients storing
messages while they are offline.

If the network connecting these client to the Internet experiences a
temporary failure or the broker is restarted, there will be both
connection and message bursts that the broker will need to handle reliably.
Another reason for limiting the rate of connections and messages is to
prevent a DoS.

Under these circumstances, the broker might decide to drop incoming
PUBLISH messages.

For messages with QoS == 0, the broker just drops the PUBLISH message.

For messages with QoS > 0, the broker does not acknowledge the PUBLISH
message.

As far as I know, Paho does not retry the delivery of inflight (QoS > 0)
messages in the same MQTT connection and the inflight message queue
capacity is quite limited.

If the broker does not acknowledge these messages, the inflight message
queue will never be flushed and no more messages are accepted by Paho.

If the broker keeps responding to pings there is no way we can recover
from this deadlock.

One can object that rate limiting should happen at the TCP layer (for
example using iptables) but this proved to be ineffective.

Rate limiting at the tcp level is quite poor as it only works on new tcp
connections and does not allow to create granular rules. Rate limiting
at the MQTT level gives us the opportunity to create advanced logic to
limit the rate of new connections (as with tcp) but also the rate of all
other messages based on various MQTT information such as topic, clientid
or user.

I'd like to hear your thoughts on this.

Regards,
     Cristiano

On 09/08/2012 12:20, De Alti, Cristiano wrote:

Hi,
It was a bug in my code. The test now succeeds. Thanks for you support.

Ciao,
    Cristiano
________________________________________
Da: paho-dev-bounces@xxxxxxxxxxx [paho-dev-bounces@xxxxxxxxxxx] per conto di Nicholas O'Leary [nick.oleary@xxxxxxxxx]
Inviato: mercoledì 8 agosto 2012 20.07
A: General development discussions for paho project
Oggetto: Re: [paho-dev] R: R: Paho Java Client retransmission implementation

Hi Christiano,

the scenario you describe is a pretty basic one that is well covered
by routine testing the client gets hit with - although it is always
possible you're hitting a edge case somewhere.


Most of the time there are many messages in-flight (10-15 max).

The paho client allows a maximum of 10 in-flight messages at any one time.


At the end I sleep 10 seconds before disconnecting and I check if the
tracking list is empty.
It can happen it's not empty. Thus some messages are not acknowledged.

Does your test include a separate subscriber so you can confirm what
messages are actually delivered?

If you don't timeout after 10 seconds, but leave it running for longer
do the messages eventually get acknowledged? It is possible your 10
second timeout is not giving the client enough time to deliver all
1000 messages given the 10 in-flight limit.

Regards,
Nick

On 8 August 2012 17:29, De Alti, Cristiano
<Cristiano.DeAlti@xxxxxxxxxxxx> wrote:

Hi Dave,
I've slightly modified Sample.java in order to publish 1000 messages with
QoS = 2 on my Mosquitto broker at localhost (on Linux). There aren't any
other connections to the broker.
I'm publishing at the fastest possible rate without waiting for the
completion. I'm tracking the delivery tokens in a list and delete them when
the confirm token comes in asynchronously in the callback. Most of the time
there are many messages in-flight (10-15 max).
At the end I sleep 10 seconds before disconnecting and I check if the
tracking list is empty.
It can happen it's not empty. Thus some messages are not acknowledged (I
still don't know if Paho or Mosquitto drops messages or it's a bug with how
I track messages). I cannot imagine a more reliable connection since both
the broker and the client are running on localhost.

I'm sure that TCP/IP implementations are reliable enough but brokers may
be not.

I think I'll try to add the republish method to the Paho API in order to
allow for republishing/resubscribing at the application layer. If it works I
will submit the patches to you.

Ciao,
    Cristiano
________________________________
Da: paho-dev-bounces@xxxxxxxxxxx [paho-dev-bounces@xxxxxxxxxxx] per conto
di Dave Locke [locke@xxxxxxxxxx]
Inviato: mercoledì 8 agosto 2012 17.44

A: General development discussions for paho project
Oggetto: 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)
Fringe Bluepages with fuel injection
My Cattail: Share files in IBM and save your in-box






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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Paho Project Lead; Committer on Mosquitto

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Paho Project Lead; Committer on Mosquitto

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