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Migrating from NServiceBus.Router to NServiceBus.Transport.Bridge

Component: Transport Bridge

This is a Preview project

This is a guide for migrating from the NService.Router community package to Particular Software's NServiceBus.Transport.Bridge package.

Router versus bridge

One important difference between NServiceBus.Transport.Bridge and NServiceBus.Router is that the NServiceBus.Transport.Bridge is supported by Particular Software. NServiceBus.Router is a community package and is not supported by Particular Software.

The bridge does not require what the NServiceBus.Router calls connectors.

NServiceBus.Router has the concept of connectors which it uses to configure endpoints so that messages are delivered to the correct destination. Connectors must be deployed alongside the endpoints they are routing. The bridge does not require connectors or any other similar component. Endpoints are not aware that a bridge is translating messages between transports so after migrating to the bridge, these connectors no longer need to be deployed with each endpoint. One of the benefits of this is that migration to a different transport becomes easier than with the NServiceBus.Router.

Steps to migrate

To migrate the following messages must be taken into consideration:

  • In-flight messages
  • Commands
  • Events
  • Additional messages

In-flight messages

In-flight messages are messages that were sent to an endpoint but have not been processed yet. That is, a message may have been sent using NServiceBus.Router but after migration, it may be processed by the transport bridge. Since the NServiceBus.Router changes the routing for messages so that they are sent directly to the Router component, this means that any in-flight messages will require the Router to be available to them, even when endpoints have already been configured to no longer use the Router anymore.

Because of this, the NServiceBus.Router must remain running until all in-flight messages have been delivered. The time this takes is specific to each system. If it is unclear what the right time is to stop the NServiceBus.Router, contact Particular Software for guidance.

Commands

Commands are the easiest to migrate as they have a specific route to a specific logical endpoint. After the transport bridge has been set up and running, every endpoint can remove any NServiceBus.Router-related configuration for routing commands using the connector. This implies that the original routing towards the specific endpoint can be restored and the bridge will make sure it is transferred to the correct transport.

Events

Events require more effort to migrate to the transport bridge from NServiceBus.Router. The issue is with the subscriptions that are registered within a message broker. The scenario could exist as seen in the graph below, where Endpoint B publishes an event and the event is sent to both the NServiceBus.Router and the NServiceBus.Transport.Bridge. As a result, Endpoint C could receive the same event twice.

Note that both events would have the same message identifier, which allows the outbox to de-duplicate the messages. Without the outbox, either the message handler must be idempotent or the event will be processed twice.

flowchart LR Br(Bridge) A[Endpoint A] <---> Br B[Endpoint B] <---> Br Br <---> C[Endpoint C] Br <---> D[Endpoint D] Ro[Router] <---> C B <---> Ro subgraph msmq A B end subgraph sql C D end

For this scenario, take the following steps to move from NServiceBus.Router to NServiceBus.Transport.Bridge:

  1. Stop both EndpointB and the Bridge.
  2. Configure the Bridge to register a publisher with Endpoint C for the event that Endpoint B publishes.
  3. Remove the subscription from the message broker for the Router.
  4. Add the subscription to the message broker for the Bridge.
  5. Start the Bridge.
  6. Start Endpoint B.

When the Bridge starts up, it will also create the subscription; it is not required to manually create the subscription in the message broker. However, be aware of when Endpoint B cannot be taken offline due to high availability, scaling out or service level agreements.

If Endpoint B cannot be taken offline, the time between step 3 and step 4 determines how many events will be lost.

Additional messages

There may be additional messages, like request/response messages, which route a message back to the endpoint it originated from. These must be routed back using the NServiceBus.Router component. The result is that as long as there are messages like these to be processed, the NServiceBus.Router component must be active within the system alongside the bridge.

Conclusion

Migrating from NServiceBus.Router to NServiceBus.Transport.Bridge is not difficult, but it requires careful consideration and analysis before the NServiceBus.Router component can be completely removed from a system.


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