[rabbitmq-discuss] How to handle full exchange history delivery

Wed Jun 1 16:15:26 BST 2011

Thanks Jon, I misunderstood you probably.

WRT this:

When I was writing it, I thought it would be really easy to, when a new queue is bound to this exchange, go fetch all the messages in that bucket and send them to the queue.

I’m not sure it is, that was my main question actually. How do you figure out how to join the messages stored in the bucket and those arriving live so that the last message you send to the queue from those picked up from the bucket is followed exactly by its logical successor from those coming live?

Say I have messages 1..100 being published to the exchange, each time one arrives it’s stored. The exchange has received the first 50 messages and a client performs a binding of a queue to the exchange. So you start pulling the messages from the storage and sending them to the queue before forwarding any live messages. While you are pulling them out of the storage messages 51..60 are published to the exchange, how do you deal with them? Do you always pull from the storage? When do you stop pulling from the storage and relay to the queue directly what is being published to the exchange if you don’t know whether any new messages have arrived while you were pulling from the storage?
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Simone Busoli
Direzione risorse umane e segreteria generale – Sistemi informativi gestione sportiva
Collaboratore
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From: Jon Brisbin [mailto:jon at jbrisbin.com]
Sent: Wednesday, June 01, 2011 4:59 PM
To: Busoli, Simone
Cc: rabbitmq-discuss at lists.rabbitmq.com
Subject: Re: [rabbitmq-discuss] How to handle full exchange history delivery

Maybe I wasn't being clear:

I've already written a custom exchange that stores every message it receives in Riak (http://wiki.basho.com/). I'm going to be talking about it next week at Erlang Factory in London, as a matter of fact. :)

https://github.com/jbrisbin/riak-exchange

When I was writing it, I thought it would be really easy to, when a new queue is bound to this exchange, go fetch all the messages in that bucket and send them to the queue.

Since I'm using Riak, the messages themselves are not all stored in memory.

Thanks!

Jon Brisbin
http//jbrisbin.com

________________________________
From: "Simone Busoli" <Simone.Busoli at ferrari.com>
To: rabbitmq-discuss at lists.rabbitmq.com
Sent: Wednesday, June 1, 2011 8:43:58 AM
Subject: Re: [rabbitmq-discuss] How to handle full exchange history delivery

That’s what I thought as well, although I’m thinking about a couple of issues about it:

•         Lack of knowledge about Erlang, which is the language I’d have to use to build a custom exchange, I guess

•         Amount of memory it would require to store the entire history in memory. It needn’t be in memory actually, but I’m not sure what would be the recommended way if the storage has to be carried out by exchange itself. Mnesia perhaps, would it make sense?
_______________________________________________________________
Simone Busoli
Direzione risorse umane e segreteria generale – Sistemi informativi gestione sportiva
Collaboratore
Ferrari S.p.A.     Via Ascari 55/57, 41053, Maranello (MO); Italy
T (+39) 0536-949.850
E simone.busoli at ferrari.com<mailto:simone.busoli at ferrari.com>  www.ferrari.com<http://www.ferrari.com/>

From: Jon Brisbin [mailto:jon at jbrisbin.com]
Sent: Wednesday, June 01, 2011 3:38 PM
To: Busoli, Simone
Cc: rabbitmq-discuss at lists.rabbitmq.com
Subject: Re: [rabbitmq-discuss] How to handle full exchange history delivery

I've been seriously considering adding this functionality to the Riak-backed custom exchange. It was something I thought I would add first time through but didn't.

You'd have to use a custom exchange. Not sure if that's an issue in your scenario or not.

Thanks!

Jon Brisbin
http//jbrisbin.com
________________________________
From: "Simone Busoli" <Simone.Busoli at ferrari.com>
To: rabbitmq-discuss at lists.rabbitmq.com
Sent: Wednesday, June 1, 2011 5:52:24 AM
Subject: [rabbitmq-discuss] How to handle full exchange history delivery

Hi,

I am trying to achieve a scenario in which messages are published from some data source into a RabbitMQ exchange and clients should be able to receive the full history of messages flown through that exchange, regardless of the exact moment in time when they connect.

I’ve been thinking to solve it by setting up a proxy which subscribes to the exchange where messages are published using its own queue, keeps an internal storage of the messages and accepts requests from clients.
When a request from a client arrives it just replays the full message history to the client by publishing the messages to the client queue (supplied in the ReplyTo field), then binds the client queue to the messages exchange, so that the client from this moment onwards starts receiving the “live” messages, and is pretty much unaware of what’s happened; it simply just receives the whole history.

Now I am under the impression that there’s a fundamental flaw in this reasoning, since the proxy cannot know when the message history has completed and he can switch the client to the live stream, because while it is replaying historical messages to the client other live messages might be arriving on the proxy’s queue, thus getting into an endless loop.

I’m probably missing something obvious, any advice about how others would approach this scenario is appreciated.

Simone

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