NServiceBus sending information when a subscriber subscribes

I'm just getting started with NServiceBus and I've got a very basic implementation up and running - much like the PubSub sample application.

What I want to do is to send some information when a subscriber subscribes.

For example, lets say I have a database with products that are categorized. My publisher will check the database every N seconds and will send messages when a new product开发者_开发知识库 is added to the database.

But each subscriber is only interested in a particular category and I want to send that category when they subscribe so that the publisher knows the check the database for new products in that category.

The categories are dynamic, so I can't create different messages for the different categories. So for that reason I assume that all the subscribers have to subscribe to the same published IMessage.

NOTE: I do also wish to use this information to then filter the messages NServiceBus sends to subscribers, but I thought I would post that as a separate question.

You can implement the IAuthorizeSubscriptions interface to get a callback at the publisher when a subscriber requests to subscribe. If you wanted to include some additional meta-data in the subscription request, you'll need to use Bus.OutgoingHeaders - then you can look at that info at the publisher and decided what to do with it.

I haven't worked with NServiceBus for quite some time but I don't think NServiceBus has any events to notify you when a subscription has taken place.

Be that as it may I think you may be approaching your solution on a level that is too fine.

You usually subscribe to events that the business is typically interested in. In your case when a new product is added. A simpler solution would be to simply subscribe to say a ProductAddedEvent and have your subscriber ignore any messages that it is not interested in.

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We make use of pipelines for various parts of the bus and you have given me an idea to include a SubscriptionRequested event on the StartupPipeline.