NServiceBus: Sharing Message DLLs

I have been recently looking into NServiceBus, as I thought messaging would be a good way to reduce dependencies between systems. However, one of the first things that struck me is that the message publisher and all subscribers have to share the message definition DLL. What would happen in this scenario?:

Say there is one central system that handles client data. Whenever a client record is changed, it publishes a message, containing name and address. This has ten subscribers, which update their local copy of the data on receipt of the message.

One day, requirements change and one of the subscribers need the clients phone number as well. The message, the publishe开发者_开发百科r, and the affected subscriber are all updated to handle the phone number, and they are all recompiled and released.

Will all nine other subscribers continue unaffected? Will they carry on as normal with the old Message DLL, or will they all need to be updated with the new DLL, recompiled and released as well?

The NServiceBus architecture is designed to be resilient to message structure changes (especially where the changes involve adding information like in your scenario). See the Versioning Sample page on the NServiceBus site.

It is not the case that you can handle versioning in NSB like they outline in the Versioning Sample.

You can do this if you are implementing NSB in a send/receive scenario. In this instance even though the contract is a messages DLL, the same DLL version does not need to be shared between senders and receivers. This is because providing the XML on the wire will de-serialize cleanly on the receiver end all will be well.

However, this completely breaks down in a pub-sub scenario. In this case, there is a dependency on the exact same version of the messaging assembly being shared between publisher and subscribers. This means the version, public key token etc all need to be identical. The reason for this is the subscription mechanism.

When your subscriber starts up it will send a subscription message to the publisher, who will then enter the subscription in the subscription data store. This subscription is for messages originating in a specific assembly version.

If the publisher then updates it's version of the messages DLL and receives a message which it needs to publish, it will do a lookup against the subscriptions it holds and evaluates each one in turn. Because the subscription exists for a previous version of the messages assembly the evaluation process will ignore that subscription entry, and therefore no message will be sent to the subscriber.

You need to be aware of this hard-dependency in the pub-sub scenario.

Hope this helps.

Edit

As of version 3.x of NServiceBus as long as your messages assembly major version is shared between publisher and subscriber then pub-sub will work as normal.