Messaging Bridge

The NServiceBus Messaging Bridge allows NServiceBus endpoints to connect to other endpoints that are not using the same transport using the Messaging Bridge Pattern.

The bridge is transparent to sending and receiving endpoints. That is, endpoints are not aware of the bridge or that it is transferring messages to a different transport. Endpoints send and receive messages to and from logical endpoints as they normally would if there were no bridge involved.

Why use the bridge

The messaging bridge enables several scenarios:

• Migrating from one transport to another. E.g. migration from MSMQ to a cloud-native transport. The bridge allows endpoints to be migrated one at a time rather than all at once.
• Using multiple transports because of pricing considerations. Instead of relying only on a cloud-native transport (which may be more expensive), less mission critical endpoints could operating on a cheaper transport.
• Using a transport that best fits non-functional considerations. E.g. A transport that allows bridging geographical locations, while some endpoints on a more secure transport only process private information.

More details on these scenarios are provided in the messaging bridge scenarios article.

Bridge configuration

NServiceBus.Transport.Bridge is packaged as a host-agnostic library. It can be hosted in a console application, a Windows service, a Docker container, or any service that supports the Microsoft Generic Host, similar to how endpoints are hosted.

The diagram below shows a simple MSMQ-to-AzureServiceBus configuration involving two endpoints. The two transports are used for demonstration purposes. The bridge supports all transports.

The following code enables the Sales endpoint hosted on the MSMQ transport to communicate with the Billing endpoint hosted on the AzureServiceBus transport.

var msmq = new BridgeTransport(new MsmqTransport());
var asb = new BridgeTransport(new AzureServiceBusTransport(connectionString));

msmq.HasEndpoint("Sales");
asb.HasEndpoint("Billing");

bridgeConfiguration.AddTransport(msmq);
bridgeConfiguration.AddTransport(asb);

The life cycle of the bridge is managed by the .NET Generic Host.

Errors

When the messaging bridge encounters an error while transferring a message between transports it will move the message to the bridge.error queue, which is specific to the messaging bridge. An additional header NServiceBus.Transport.Bridge.FailedQ is added to allow moving a message back to its originating queue, so the messaging bridge can pick them up again.

Consistency

If the bridge moves messages across different transport types or different brokers of the same type (e.g. Azure ServiceBus namespaces, or RabbitMQ virtual hosts), ReceiveOnly is the only supported transaction mode. In this mode, messages that are moved across the bridge may be duplicated if some infrastructure-related issue prevents the message from being moved to the target transport. To address this either ensure that handlers are idempotent or enable deduplication of messages using the outbox.

The bridge uses this mode if at least one of the configured transports is unable to use distributed transactions.

Distributed transactions

If all configured transports support the TransactionScope transaction mode, the bridge uses that transaction mode so that no messages are duplicated during message transfer between transports. This enables migration from the MSMQ transport to the SQL Server transport without having to make changes to the endpoints.

If ReceiveOnly transaction mode is preferred, the following configuration may be used:

var bridgeConfiguration = new BridgeConfiguration();

bridgeConfiguration.RunInReceiveOnlyTransactionMode();