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| Pub/Sub and retained messages, only one can be retained? |
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| ChrisH |
 Posted: Sat Feb 28, 2009 7:17 am Post subject: Pub/Sub and retained messages, only one can be retained? |
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Newbie
Joined: 26 Feb 2009
Posts: 6
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Hi first off let me warn you I'm a newbie to MQ, so please bear with me.
We're working on some POC stuff for the company and were planning on putting up (publishing) a bunch of files then having a subscriber read them spit out some data from them and go to the next one.
In reading the documentation about Topics and Retaining them, I saw this:
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| Only one publication can be retained at each node of the topic tree. That means if there already is a retained publication for this topic, published by any other application, it is replaced with this publication. It is recommended that you do not have more than one publisher retaining messages on the same topic. |
Does this mean that a publication can really only be "transitory"? By that I mean that only one topic can be retained on the topic at a time.
If so then I guess the best way is to have a queue and let it hold them and then let the topic grab from that queue and publish them.
Any better ideas or clarification you guys could give me would help.
Christopher |
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| fjb_saper |
| Posted: Sat Feb 28, 2009 12:07 pm Post subject: |
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Grand High Poobah
Joined: 18 Nov 2003
Posts: 20756
Location: LI,NY
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Think of a publication of the NY exchange.
When you register your subscription you would like to know the latest values of all stocks...
But you are not interested in stock history....
However the publication happens on event. How can a new subscriber get the latest values without it being published all the time? Retention!
Ok so the topic tree would be something like topic://stocks/nyse/ticker
This way you could subscribe to topic://stocks/nyse# and get the latest values for all tickers. Once a new value is published it replaces the retained value of the same ticker.
If your topic tree looks more like topic://stocks/ticker/nyse you could subscribe to a specific ticker and see what the values throughout the different exchanges are.... (Euronext, nyse, hangsei ...)
The retained value is then always the last published.
As a subscriber you can retrieve the retained value at subscription time and thus get the latest even though you missed it at publication time.
What the documentation is saying is that this might get messy very quickly if you have multiple publishers publishing on a retained topic. Whose publication do you expect as being the retained one? This is why you should have a unique publishing authority per retained topic. Multiple topics can share that unique publishing authority. (i.e. stock exchange publishing the official rates)...
Have fun 
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| ChrisH |
| Posted: Sun Mar 01, 2009 7:35 am Post subject: Pub/Sub and retained messages, only one can be retained? |
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Newbie
Joined: 26 Feb 2009
Posts: 6
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Right and that makes sense. So if for example I wanted to be able to have all the replies that have gone on as far as this topic published to any group of subscribers, I'd have to have a queue (MQQueue) or some other repository set up to hold/stage them, then put them each on the Topic to be shot out to any current subscribers right?
Just making sure I'm doing things (within reason) the BP way and the best way.
Thanks saper,
Christopher |
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| zpat |
| Posted: Sun Mar 01, 2009 8:26 am Post subject: |
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Jedi Council
Joined: 19 May 2001
Posts: 5866
Location: UK
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Use durable subscriptions to get all published messages sent to a queue.
Otherwise only the last one is retained by the broker.
Depends whether you are dealing with event messages or status update messages. |
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| mqjeff |
| Posted: Sun Mar 01, 2009 9:14 am Post subject: |
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Grand Master
Joined: 25 Jun 2008
Posts: 17447
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It's really not clear what you want to happen, nor why you want to use Pub/Sub for this.
A retained publication allows a new subscriber to receive the last publication on a topic, rather than having to wait for the next publication.
A durable subscription allows a subscriber to receive all publications made *after* subscription, regardless of whether the subscriber is active at the time of publication or not.
There's nothing that allows a new subscriber to get *all* publications on a topic, since the beginning of time. It doesn't make any sense in a pub/sub model.
If, somehow, you really need to be able to have a new subscriber get the entire history of the world when it subscribes, you need to implement something to hold that history and provide it on request. But you would never ever want to replay that history through the "normal" topic - because then everyone would be getting the entire history of the world.
Go back to the whiteboard. Think about the situation that you need to build for from a producer side and from a consumer side. Maybe you just need to load the files into a database, and have a request/reply model to retrieve records from the database. |
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| fjb_saper |
| Posted: Sun Mar 01, 2009 9:29 am Post subject: Re: Pub/Sub and retained messages, only one can be retained? |
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Grand High Poobah
Joined: 18 Nov 2003
Posts: 20756
Location: LI,NY
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| ChrisH wrote: |
Right and that makes sense. So if for example I wanted to be able to have all the replies that have gone on as far as this topic published to any group of subscribers, I'd have to have a queue (MQQueue) or some other repository set up to hold/stage them, then put them each on the Topic to be shot out to any current subscribers right?
Just making sure I'm doing things (within reason) the BP way and the best way.
Thanks saper,
Christopher |
Looks like you are getting confused with a different model here.
pub/sub is really good at what it does but it does not cater for the full "Magazine" model.
So pub/sub would only give you the current issue of a Magazine... allowing for retained publication of such.
However if a subscriber comes in late (durable or not) and wants back issues there is no way to cater to that.
What you can do is create a library (DB) that subscribes to the magazine and stores any publication it receives. Now if you want back copies you as a subscriber have to call the library service and request them. This is not part of a pub/sub because all subscribers are obviously not interested in what you (the particular subscriber) want. This should be a request / response service and it could even be asynchronous.
To go back to the wall street analogy, if you want the rates for a particular stock over the last 5 years, the general population subscribed to the instant rates could care less about it. It would even be detrimental to publish it as it would affect the retained value and could introduce an incorrect (obsolete) rate.
Have fun.
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| ChrisH |
| Posted: Sun Mar 01, 2009 10:08 am Post subject: Pub/Sub and retained messages, only one can be retained? |
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Newbie
Joined: 26 Feb 2009
Posts: 6
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| Yeah it's seeming that I'm not looking at the right model I agree and the Pub/Sub makes sense in the other contexts. Thanks for your help and I'll try and see what I can come up with as a better model (or set of models) to handle the situation. |
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