Invalid operation exception, when trying to auto subscribe to events when using Brokered transports

[indualagarsamy]

When using brokered transports that offer support for centralized pub/sub like RabbitMQ and ActiveMQ, the EndpointMappings to specify event subscriptions are not needed. However, without it, an InvalidOperation exception was being thrown.

Steps to Repro

1. Have a pub/sub sample that uses RabbitMQ as transport
2. Add the appropriate assembly redirects for the correct version of NServiceBus.Core and NServiceBus.Host
3. Remove the EndpointMessageMappings in the unicast bus section for the subscriber.
4. Notice the error upon startup:

2013-12-09 10:46:09,043 [23] ERROR NServiceBus.Unicast.UnicastBus [(null)] <(null)> - System.InvalidOperationException: No destination could be found for message type Messages.Events.IGotRadarStatistics. Check the <MessageEndpointMappings>
section of the configuration of this endpoint for an entry either for this specific message type or for its assembly.
   at NServiceBus.Unicast.UnicastBus.Subscribe(Type messageType, Predicate`1 condition) in y:\BuildAgent\work\31f8c64a6e8a2d7c\src\NServiceBus.Core\Unicast\UnicastBus.cs:line 377
   at NServiceBus.Unicast.UnicastBus.Subscribe(Type messageType) in y:\BuildAgent\work\31f8c64a6e8a2d7c\src\NServiceBus.Core\Unicast\UnicastBus.cs:line 328
   at NServiceBus.AutomaticSubscriptions.AutoSubscriber.Start() in y:\BuildAgent\work\31f8c64a6e8a2d7c\src\NServiceBus.Core\AutomaticSubscriptions\AutoSubscriber.cs:line 24
   at NServiceBus.Unicast.UnicastBus.<>c__DisplayClass1f.<Start>b__1d() in y:\BuildAgent\work\31f8c64a6e8a2d7c\src\NServiceBus.Core\Unicast\UnicastBus.cs:line 786 could not be started.

Workaround
Add the message mappings in app.config.

NOTE: This used to work in version 4.2

[indualagarsamy]

The following code sets the DoNotRequireExplicitRouting to true:
https://github.com/Particular/NServiceBus/blob/develop/src/NServiceBus.Core/AutomaticSubscriptions/AutoSubscribe.cs#L21

However, the code here does not respect it:
https://github.com/Particular/NServiceBus/blob/4.3.0/src/NServiceBus.Core/Unicast/UnicastBus.cs#L372

[indualagarsamy]

@andreasohlund @SimonCropp -- thoughts??
Until we fix #1838 or do a proper refactoring moving the for loop in here:
https://github.com/Particular/NServiceBus/blob/develop/src/NServiceBus.Core/Unicast/UnicastBus.cs#L371-390 to MessageDrivenSubscriptionManager

Proposed Fix
I don't think we can rely on DoNotRequireExplicitRouting in https://github.com/Particular/NServiceBus/blob/develop/src/NServiceBus.Core/AutomaticSubscriptions/AutoSubscribe.cs#L21
What if AutoSubscribe feature is explicitly disabled?

How about?
Add the following check before https://github.com/Particular/NServiceBus/blob/develop/src/NServiceBus.Core/Unicast/UnicastBus.cs#L371

if (Feature.IsEnabled<MessageDrivenSubscriptions>() == false)
{
        // We are dealing with a brokered transport wired for auto pub/sub.
        SubscriptionManager.Subscribe(messageType, null);
        return;
}

and add the following check before
https://github.com/Particular/NServiceBus/blob/develop/src/NServiceBus.Core/Unicast/UnicastBus.cs#L421

if (Feature.IsEnabled<MessageDrivenSubscriptions>() == false)
{
        // We are dealing with a brokered transport wired for auto pub/sub.
        SubscriptionManager.Unsubscribe(messageType, null);
        return;
}

[SimonCropp]

looks good to me.

although can the code be

if (!Feature.IsEnabled<MessageDrivenSubscriptions>())
{
        // We are dealing with a brokered transport wired for auto pub/sub.
        SubscriptionManager.Unsubscribe(messageType, null);
        return;
}

[SimonCropp]

also. if !Feature.IsEnabled<MessageDrivenSubscriptions>() and addresses.Count > 0 should an exception be thrown?

[indualagarsamy]

@SimonCropp - I think not. Because for ActiveMQ and RabbitMQ, users don't need to specify any message mappings in the app.config. In which case, address.Count will be zero.

[SimonCropp]

@indualagarsamy agreed. but if someone has put some message mapping in the config, that will be ignored, shouldn't we tell them?

[indualagarsamy]

So, if addresses.Count > 0, how about we log a warning, stating:

MessageEndpointMappings for event {0} has been defined in the app.config in the UnicastBusConfig section.
MessageEndpointMappings are not necessary for defining event subscriptions, since you are using a brokered transport which supports pub/sub natively.
MessageEndpointMappings is only needed for specifying commands, i.e. Bus.Send destinations.

[SimonCropp]

sounds good