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MassTransit Channel

The Hermodr.Publisher.MassTransit package adds a publish channel that routes CloudEvent instances through a MassTransit bus, making Hermodr broker-agnostic — any MassTransit-supported transport (RabbitMQ, Azure Service Bus, Amazon SQS, Kafka, In-Memory, …) can be used transparently.

Installation​

dotnet add package Hermodr.Publisher.MassTransit

Prerequisite: MassTransit must already be configured in your application with at least one transport registered. See the MassTransit documentation for setup details.

Registration​

Inline configuration​

using Hermodr;

builder.Services
    .AddEventPublisher()
    .AddMassTransit(options =>
    {
        // Leave DestinationAddress null to publish (fan-out).
        // Set it to send to a specific endpoint.
        options.DestinationAddress  = new Uri("queue:order-events");
        options.MapAttributesToHeaders = true;
    });

Minimal registration (no options)​

builder.Services
    .AddEventPublisher()
    .AddMassTransit();

From appsettings.json​

builder.Services
    .AddEventPublisher()
    .AddMassTransit("Events:MassTransit");
// appsettings.json
{
  "Events": {
    "MassTransit": {
      "DestinationAddress": "queue:order-events",
      "MapAttributesToHeaders": true
    }
  }
}

Options reference​

MassTransitPublishOptions

PropertyTypeEffective defaultDescription
DestinationAddressUri?nullWhen null, the event is published via IPublishEndpoint (fan-out). When set, the event is sent to that specific queue/exchange via ISendEndpointProvider.
MapAttributesToHeadersbool?trueMaps CloudEvent attributes (id, type, source, time, datacontenttype, …) to MassTransit message headers. null → use channel default (effective default: true).

Nullable value-type properties use null as the sentinel meaning "inherit from the channel-level default". Set them in the channel registration to establish the baseline; set them in a per-call override only when you need to deviate from that baseline for a specific delivery.

How it works​

  1. The MassTransitPublishChannel wraps a CloudEvent in a CloudEventMessage — an ICloudEventMessage implementation that MassTransit can serialise.
  2. If DestinationAddress is set, the message is sent to that endpoint via ISendEndpointProvider.GetSendEndpoint.
  3. Otherwise, the message is published via IPublishEndpoint.Publish, allowing MassTransit's topology to route it.
  4. When MapAttributesToHeaders is true (or null, which defaults to true), CloudEvent attributes are written to the outgoing message headers so consumers can inspect them without deserialising the body.

Typed channel​

Use AddMassTransit<TEvent>() to register a channel that receives only events whose data class is TEvent. At construction time the typed channel (MassTransitPublishChannel<TEvent>) merges the general MassTransitPublishOptions with the type-specific MassTransitPublishOptions<TEvent>: non-null typed values win; null values fall back to the base defaults.

builder.Services
    .AddEventPublisher()
    // Default: publish (fan-out) with header mapping
    .AddMassTransit(opts =>
    {
        opts.MapAttributesToHeaders = true;
    })
    // OrderPlaced events are sent to a specific endpoint
    .AddMassTransit<OrderPlaced>(opts =>
    {
        opts.DestinationAddress = new Uri("queue:order-placed");
        // MapAttributesToHeaders inherited from base
    });

From configuration:

builder.Services
    .AddEventPublisher()
    .AddMassTransit("Events:MassTransit")
    .AddMassTransit<OrderPlaced>("Events:MassTransit:Orders");
{
  "Events": {
    "MassTransit": {
      "MapAttributesToHeaders": true,
      "Orders": {
        "DestinationAddress": "queue:order-placed"
      }
    }
  }
}

See Typed Channels for the full merge semantics and further examples.

Per-delivery options​

Pass a MassTransitPublishOptions instance as the second argument to PublishAsync to override individual properties for a single publish call. Only the properties you set (non-null) replace the channel default — all others fall back to the values configured at registration time.

// Resolve the concrete channel directly from DI.
var channel = serviceProvider.GetRequiredService<MassTransitPublishChannel>();

// Send this event directly to a specific queue,
// while still inheriting MapAttributesToHeaders from the channel default.
await channel.PublishAsync(@event, new MassTransitPublishOptions
{
    DestinationAddress = new Uri("queue:priority-orders"),
});