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Subject: Re: [mqtt] Recharter: Shared subscriptions and $share


Okay, let's say it is a property of the subscription and can be used with any topic. It would be helpful to me if we could flesh out the expected/desired behaviour in the following scenarios.

Both Andrews have implied the existence of a share name that goes alongside the topic. I presume that is so you can have more than one group of clients sharing the messages amongst themselves.

Eg client A, B and C subscribe to foo with a share name of alpha. Clients D, E and F subscribe to foo with a share name of beta. So there are effectively two groups of clients for which the messages are shared between the members of each group.

1. Is the share name scoped to the topic? If client G then subscribes to topic NotFoo with a share name of alpha, is that considered a different share?

2. If the share name is scoped to the topic, I presume more accurately it is scoped to the topic filter used in the subscription request? That is to say a sub to foo/# with share name gamma would not be related to a sub to foo/+ with share name gamma even when messages are published to foo/bar (which matches both filters)

3. I realise after I typed this one out its what Ken addresses below - what to do when a client has overlapping subs that are a mix of shared and non shared. I'm inclined to agree the cleanest answer would be to prohibit overlapping subs of mixed type.

I don't agree with the argument that using topic prefix is akin to adding point to point to mqtt and that using a share name on the subscription  isn't. The whole shared sub concept is, as far as the participating clients are concerned, queuing. But that's just semantics and not worth arguing over. (Because I expect one of you to say that in the share name case, you can still have normal subscribers on the topic receiving everything... To which I say, great, we now have a hybrid of queuing and pub/sub....)

Andrew S - I take your point that using a topic prefix on its own loses flexibility - that you cannot have a mix of subscribers. But you could combine it with the share name concept to allow different groups of subscribers. The key difference being if a client didn't provide a share name it would be put in a default share group - allowing 3.1.1 clients to partake without needing to know about share names. Just a thought.

I think my preference for a topic prefix approach came from the sense it had little, if any, impact on client implementation and on-the-wire formats. We could get shared sub behaviour quite easily. As has been identified, that approach has its limitations - the question is whether those limitations make it not fit for purpose.

The share name approach is more flexible at the cost of implementation complexity and client churn.

I think I find myself back on the fence at this point.






Nick O'Leary
IBM Emerging Technology Services

Twitter: @knolleary

IBM United Kingdom Ltd registered in England and Wales with number 741598 Registered office: PO Box 41, North Harbour, Portsmouth, Hants, PO6 3AU


Ken Borgendale --- Re: [mqtt] Recharter: Shared subscriptions and $share ---

From:"Ken Borgendale" <kwb@us.ibm.com>
To:mqtt@lists.oasis-open.org
Date:Fri, 8 Apr 2016 22:45
Subject:Re: [mqtt] Recharter: Shared subscriptions and $share


As a charter issue, whether or not $share is an attribute of a subscription or of a topic, we would be defining semantics of the topic space which is what the existing and proposed charter do not allow.

I agree with Andrew that making shared an attribute of the topic makes it a queue rather than a pub/sub topic. While queues do have some useful semantics, this would be a radical departure for MQTT and we would certainly need to make this an in-scope item if that is what we want to do.

It is perfectly reasonable for there to be multiple subscriptions to a topic, some shared and some not. There is not a significant semantic issue with this. The significant semantic issue comes only with overlapping shared and non-shared subscriptions by the same client. This is messy today (see MQTT-217) and shared subs is only likely to make it messier. We could solve this for shared subs by saying that a new shared subscription MUST be rejected if it overlaps an existing non-shared subscription and vice versa. We could make all overlapping subscriptions invalid, but that again would strain the compatibility clauses of the charter :
Changes to the input document should be compatible with implementations of previous versions of the standard such that it is possible for a client or server to implement multiple versions of the standard, allowing a client coded to an older version of the protocol to connect to and use a server that implements both the previous and current versions of the standard.

Specifying that either single or all subscriptions in a client must be honored in the case of overlapping subscriptions would most certainly break this.


Ken Borgendale -- kwb@us.ibm.com 1-207-805-6708 1-207-371-8082
Senior Programmer -- IBM MessageSight and Watson Internet of Things Connect - Architect


Inactive hide details for Andrew Schofield ---04/08/2016 04:30:19 PM---Hi Nick, If I've understood your comment correctly, you Andrew Schofield ---04/08/2016 04:30:19 PM---Hi Nick, If I've understood your comment correctly, you are suggesting that a topic

From: Andrew Schofield <andrew_schofield@uk.ibm.com>
To: mqtt@lists.oasis-open.org
Date: 04/08/2016 04:30 PM
Subject: Re: [mqtt] Recharter: Shared subscriptions and $share
Sent by: <mqtt@lists.oasis-open.org>





Hi Nick,
If I've understood your comment correctly, you are suggesting that a topic starting with the prefix "$shared" would actually treat all subscriptions as shared subscriptions. Unfortunately, I think that's a really bad idea. It's gone from N publishers->M subscribers to N publishers->1 subscription->M consumers. That's not pub/sub - it's just a queue with multiple consumers.

In a publish/subscribe system, ideally I want the publishers to be totally unaware of the number and availability of the subscribers. By having this rule for "$shared", if I want to use workload balancing among a set of consumers, I have to publish on a topic starting "$shared" and each message published can be consumed by at most one consumer. I can no longer dream up another purpose for the messages and add another subscription - it would just end up getting a small share of the existing shared subscription.

What I would seek to enable is exactly what you didn't like: I want subscribers that can take all of the messages to be able to have them all, and subscribers who cannot keep up with the message rate to be able to share the stream of messages with like-minded consumers.

So, I think sharing is a property of the subscription, not the topic. I prefer having a separate share name which is not part of the topic name.

Thanks,
Andrew

Andrew Schofield
Chief Architect, Hybrid Cloud Messaging
Senior Technical Staff Member
IBM Cloud Application Services

IBM United Kingdom Limited
Mail Point 211
Hursley Park
Winchester
Hampshire
SO21 2JN

Phone ext. 37248357 (External:+44-1962-818357), DE2J22
Internet mail: andrew_schofield@uk.ibm.com



From:
Nicholas O'Leary/UK/IBM@IBMGB
To:
mqtt@lists.oasis-open.org
Date:
08/04/2016 19:46
Subject:
[mqtt] Recharter: Shared subscriptions and $share
Sent by:
<mqtt@lists.oasis-open.org>





There are two fundamental approaches here.

1. The client chooses whether a subscription is shared or not
2. The ‘shared’ nature is a property of the topic itself


If the client gets to choose, then you need to define what happens if clients A and B subscribe to topic foo with the shared flag set, but client C subscribes without the shared flag set. I think trying to support that type of combination would overly complicate implementations. We could say that in that scenario client C’s sub is rejected, but it doesn’t feel like the right approach to me.

I think it is much cleaner for the shared nature to be a pre-existing property of the topic - in that way no changes are needed in client implementations at all, as demonstrated by the fact some broker implementations already support shared subs within the confines of 3.1.1.

The question is then whether it should be done by virtue of a well-known topic prefix - $shared, or have it as a property on the topic that can be administratively set on the broker.

The advantage of the latter is that any topic could be set as a shared topic - and would be within the constraints of our charter to not define topic spaces. The downside is it requires administrative action to use the feature - something that isn’t required anywhere else in the protocol. To deploy a new application that requires a shared sub would now require co-ordinated administrative action on the broker - that doesn’t feel lightweight to me and would fail the Just Works test that so much of MQTT benefits from.


That brings me to the conclusion that the cleanest solution is to define any topic that is prefixed with $shared as being a shared topic. We shouldn’t let the current text of the charter, which we’re currently redrafting anyway, stop us making the right technical choices for the protocol.


Nick O'Leary
IBM Emerging Technology Services

Twitter: @knolleary

IBM United Kingdom Ltd registered in England and Wales with number 741598 Registered office: PO Box 41, North Harbour, Portsmouth, Hants, PO6 3AU



Brian Raymor --- [mqtt] Recharter: Shared subscriptions and $share ---
From: "Brian Raymor" <Brian.Raymor@microsoft.com>
To: mqtt@lists.oasis-open.org
Date: Mon, 4 Apr 2016 16:25
Subject: [mqtt] Recharter: Shared subscriptions and $share




Forwarding to the mailing list for broader awareness and discussion.


The conversation is related to this out-of-scope item in the draft charter:


·
The TC will not identify MQTT topics nor prescribe any mechanism or convention for classification of MQTT topics or topic spaces.

…Brian


From:
Andrew Banks [mailto:andrew_banks@uk.ibm.com]
Sent:
Friday, April 1, 2016 12:35 PM
To:
Ken Borgendale <kwb@us.ibm.com>
Cc:
Brian Raymor <Brian.Raymor@microsoft.com>
Subject:
Re: MQTT shared subs and charter

Ken, using the $share topic prefix seems the more natural way to go to me.

To avoid using the $ prefix we would still have to encode the sharename into the subscribe packet, probably using the same metadata encoding method as for the Publish Metadata. The existence of the sharename should signal the shared subscription and so remove the need to use one of the Qos bits. However, the Subscribe packet can carry multiple subscriptions and SubAck can carry multiple outcomes so that is where it starts to look unnatural.

The $ prefix was reserved in the V3.1.1 specification for uses such as this, and it is being used in practice, so I think we should alter the charter to make its use allowed.



Andrew Banks
Telephone (44) 1962 816123




From:
Ken Borgendale/Austin/IBM
To:
Andrew Banks/UK/IBM@IBMGB
Date:
31/03/2016 22:18
Subject:
MQTT shared subs and charter




The issue I have with charter and shared subs in the prohibition in the charter of defining topics. I would prefer to keep this charter limitation but it means that using $shared or any other $ topic to indicate shared subs would not be allowed. I think it makes much more sense to use one of the bits in the QoS byte of SUBSCRIBE for this purpose. This is especially true if we also implement nolocal in this way, as there needs to be an explanation of the intersection of shared subs and nolocal. The problem is that there is no corresponding bit on UNSUBSCRIBE but perhaps we should have an options byte per UNSUBSCRIBE and on UNSUBACK. This makes them symetical with SUBSCRIBE and allows for a return status on individual UNSUBSCRIBES.


On the issue of shared subs, how you specify shared subs is the simple issue. Some of the semantics of shared subs are very messy especially any text around overlapping subs and shared subs.






Ken Borgendale --
kwb@us.ibm.com 1-207-805-6708 1-207-371-8082
Senior Programmer -- IBM MessageSight - Architect


Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU


Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU



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