Where are retained topics stored?

[Sy]

I am trying to trace a problem with our set-up, We have an old topic
that has been set-up with the wrong topic name, i.e.

CC/S1001/leakStatus

Where CC is a 2 letter abreviation representing the company code.
S1001 reperesents the site Identifier.

The problem is that this site has been set-up with the wrong company
code. I have created a replacement topic and this is now present in
the broker, the problem is so is the original.

Is there any way I can remove this topic so that isn't returned?

[Nicholas O'Leary]

Hi Sy,

it depends on which broker implementation you are using.

With RSMB (and I think mosquitto, but Roger would have to confirm),
you publish a message with an empty payload and the retained flag set.
This clears any previously retained message on that topic.

Regards,
Nick

> --
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[Sy]

Thank you, I believe we are using the Java Microbroker.

On Feb 6, 4:36 pm, "Nicholas O'Leary" <nick.ole...@gmail.com> wrote:
> Hi Sy,
>
> it depends on which broker implementation you are using.
>
> With RSMB (and I think mosquitto, but Roger would have to confirm),
> you publish a message with an empty payload and the retained flag set.
> This clears any previously retained message on that topic.
>
> Regards,
> Nick
>

> On 6 February 2012 16:16, Sy <simonaplat...@gmail.com> wrote:
>
>
>
>
>
>
>
> > I am trying to trace a problem with our set-up, We have an old topic
> > that has been set-up with the wrong topic name, i.e.
>
> > CC/S1001/leakStatus
>
> > Where CC is a 2 letter abreviation representing the company code.
> > S1001 reperesents the site Identifier.
>
> > The problem is that this site has been set-up with the wrong company
> > code.  I have created a replacement topic and this is now present in
> > the broker, the problem is so is the original.
>
> > Is there any way I can remove this topic so that isn't returned?
>
> > --

> > To learn more about MQTT please visithttp://mqtt.org

[Roger Light]

Hi,

> With RSMB (and I think mosquitto, but Roger would have to confirm),
> you publish a message with an empty payload and the retained flag set.
> This clears any previously retained message on that topic.

This is the case in mosquitto.

Cheers,

Roger

[Sy]

Thank you, I can confirm this worked a charm! Is this feature in the
specification?

Thanks again,
Simon

[Nicholas O'Leary]

Hi Sy,

glad to hear it works - means it is unlikely you are using MicroBroker
as that doesn't support this method of clearing retained messages. You
have to use its administration client to clear retained messages
out-of-band of your messaging.

When it became clear we needed a standard in-protocol way of clearing
them, we added the statement:
"A server may delete a retained message if it receives a message with
a zero-length payload and the Retain flag set on the same topic."

Note the "may" - it isn't currently a required feature of brokers.

Cheers,
Nick

[Sy]

I'm pretty sure it is the Microbroker...hang on, will sign onto box
and check...

Certainly looks like it, the installation is in a folder called
"microbroker". The jar files are:

BrokerMonitor.jar and wmqtt.jar

Start-up script contains:
/usr/java/jdk1.6.0_07/bin/java -cp jars/micro.jar:jars/micro-
mgr.jar:jars/micro-log.jar:jars/micro-registry.jar:jars/micro-
trace.jar:jars/wmqtt.jar com.ibm.micro.management.Manager -start -
name=Microbroker

I used the wmqttSample.jar to publish empty topics with the retain
flag set.

Hope that helps,

On Feb 6, 6:41 pm, "Nicholas O'Leary" <nick.ole...@gmail.com> wrote:
> Hi Sy,
>
> glad to hear it works - means it is unlikely you are using MicroBroker
> as that doesn't support this method of clearing retained messages. You
> have to use its administration client to clear retained messages
> out-of-band of your messaging.
>
> When it became clear we needed a standard in-protocol way of clearing
> them, we added the statement:
> "A server may delete a retained message if it receives a message with
> a zero-length payload and the Retain flag set on the same topic."
>
> Note the "may" - it isn't currently a required feature of brokers.
>
> Cheers,
> Nick
>

> On 6 February 2012 18:29, Sy <simonaplat...@gmail.com> wrote:
>
>
>
>
>
>
>
> > Thank you, I can confirm this worked a charm!  Is this feature in the
> > specification?
>
> > Thanks again,
> > Simon
>
> > On Feb 6, 6:11 pm, Roger Light <ro...@atchoo.org> wrote:
> >> Hi,
>
> >> > With RSMB (and I think mosquitto, but Roger would have to confirm),
> >> > you publish a message with an empty payload and the retained flag set.
> >> > This clears any previously retained message on that topic.
>
> >> This is the case in mosquitto.
>
> >> Cheers,
>
> >> Roger
>
> > --

> > To learn more about MQTT please visithttp://mqtt.org

[Nicholas O'Leary]

Interesting.

> jars/micro.jar:jars/micro-
> mgr.jar:jars/micro-log.jar:jars/micro-registry.jar:jars/micro-
> trace.jar:jars/wmqtt.jar com.ibm.micro.management.Manager -start -
> name=Microbroker

I don't recognise any of those jar files as being what MicroBroker
supplies, nor is com.ibm.micro.management.Manager a class I know of..
but that could easily be a bit of bespoke code someone's written to
startup the broker and the jar files could have been repackaged.

The com.ibm.micro bit _is_ the main MB package, so it is likely you
have got a microbroker under the covers there somewhere.

Curious.

N

> To learn more about MQTT please visit http://mqtt.org

[Sy]

Its all before my time at the company. I think Andy Stanford set-up
the broker himself, several years ago.

On Feb 6, 6:52 pm, "Nicholas O'Leary" <nick.ole...@gmail.com> wrote:
> Interesting.
>
> > jars/micro.jar:jars/micro-
> > mgr.jar:jars/micro-log.jar:jars/micro-registry.jar:jars/micro-
> > trace.jar:jars/wmqtt.jar com.ibm.micro.management.Manager -start -
> > name=Microbroker
>
> I don't recognise any of those jar files as being what MicroBroker
> supplies, nor is com.ibm.micro.management.Manager a class I know of..
> but that could easily be a bit of bespoke code someone's written to
> startup the broker and the jar files could have been repackaged.
>
> The com.ibm.micro bit _is_ the main MB package, so it is likely you
> have got a microbroker under the covers there somewhere.
>
> Curious.
>
> N
>

[Nicholas O'Leary]

ahhh, that would explain a lot.. :)

> To learn more about MQTT please visit http://mqtt.org

[DanAnderson]

Hi,

Any client can send this message and the broker clears the retained
messages on the specific topic for all users?

Or am I misunderstanding?

Thanks
Dan

[Sy]

Certainly looks that way, these topics were originally created by a
process running on one of our servers. The topic was unpublished by
publishing the same topic with a blank payload.

[Roger Light]

Hi,

> Any client can send this message and the broker clears the retained
> messages on the specific topic for all users?

That's correct. The retained message doesn't belong to any user, so anybody can clear it - assuming they have permission to write to that topic of course.

Cheers,

Roger

[DanAnderson]

Hi,

Given that as true - is this a good idea?

If it doesn't belong to me - should I be able to delete it?

I'm speculating that a retained message might be important in some
scenarios.

Maybe the broker (or a specified client) should be the client that can
do this?

Thanks
Dan

[Paul Fremantle]

Surely this would be based on the topic security? In that case if I have the right to publish, I can replace the current retained message with any message I like, so deleting it is no worse.

I think this part of the spec ought to be a SHOULD not a MAY because it seems that there ought to be a standard way (rather than requiring out-of-band) to delete a topic.

Paul

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--
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[DanAnderson]

Hi,

Yeah, I'm re-reading that and the respective section of the spec and
that makes sense.

So, anything you don't want messed with; you put into a separate topic
and set the security for that as you wish.

Probably hair-splitting: Does this "delete the topic"? What should
happen if there are QoS > 0, Clean Session = 0 messages pending
delivery on this topic?

Thanks
Dan

[Nicholas O'Leary]

Hi Dan,

No, this does not delete the topic - it just removes the retained message.

Nick