Communicating Between Bounded Contexts

Details on how to publish and consume messages using MassTransit to communicate between bounded contexts.


Wrapt projects are broken up into distinct boundaries called bounded contexts. With this kind of separation you may need to communicate between bounded contexts for whatever reason.

To do this, you can either call another boundary in process using a REST request from your feature or out of process by queuing up a message in a message broker. Generally, this would be one bounded context sending out a notification that an event occurred and one or more additional bounded contexts reacting to that notification to perform some action in response. Some examples might be:

  1. A OrderRequested notification being sent and and getting consumed by a BillOrder consumer as well as a ConfirmOrder consumer. You could even take this a step further and create a saga or routing slip to orchestrate the process.
  2. You update a Patient record in the Ordering bounded context and send out a PatientUpdated notification so that the Billing bounded context can update the associated patient information in their database as well.

💡 If you're not familiar with eventing and MassTransit, I've started a blog series on the topic that I recommend you check out.


Let's look at an example like #2 above and say that I have a project built with the below domain template. Of note for this example:

  • I have a producer in my Ordering bc as well as a matching consumer in my Billing bc
  • I also have a message in my domain to send between the two
  • I am using a fanout exchange as there is no particular routing key needed for this scenario
  • I'm using the AddBus property to make sure that I am adding my event bus to each bc
DomainName: VsaLab
  - Name: IPatientUpdated
      - Name: PatientId
        Type: guid
  - ProjectName: Ordering
    Port: 1200
      ContextName: OrderingDbContext
      DatabaseName: OrderingDatabase
      Provider: Postgres
      - Name: Patient
          - Name: ExternalId
            Type: string
          - Name: InternalId
            Type: string
          - Name: FirstName
            Type: string
          - Name: LastName
            Type: string
          - Name: Dob
            Type: datetimeoffset?
        Username: specialguest # optional, but here to show how you can change them if you want
        Password: specialguest # optional, but here to show how you can change them if you want
      AddBus: true
      - EndpointRegistrationMethodName: PatientUpdatedEndpoint
        ExchangeName: patient-updated
        MessageName: IPatientUpdated
        ExchangeType: fanout
        ProducerName: PatientUpdated

So the idea here is that, when we update a patient in Ordering we want to send out a notification that we did that for other bounded contexts to know about. So Craftsman scaffolded out a producer for us in VsaLab.Ordering.src.Ordering.WebApi.Features.Producers, but we want to emit the event when we update a patient, so let's move the event publishing to our UpdatePatient feature. To do this, I'm going to add an IPublishEndpoint move the publishing code to my UpdatePatient feature, and delete the producer that we don't need in this case.

The message publishing looks like this. All I'm doing here is adding the patient id from the command to an anonymous object to use as our message and then publishing that message.

  var message = new
  await _publishEndpoint.Publish<IPatientUpdated>(message);

When we're done, our UpdatePatient feature will look something like this:

namespace PatientManagement.Features.Patients;

using SharedKernel.Dtos.PatientManagement.Patient;
using SharedKernel.Exceptions;
using SharedKernel.Messages;
using PatientManagement.Databases;
using AutoMapper;
using MediatR;
using MassTransit;
using Microsoft.EntityFrameworkCore;
using System.Threading;
using System.Threading.Tasks;

public static class UpdatePatient
    public class UpdatePatientCommand : IRequest<bool>
        public Guid PatientId { get; set; }
        public PatientForUpdateDto PatientToUpdate { get; set; }

        public UpdatePatientCommand(Guid patient, PatientForUpdateDto patientToUpdate)
            PatientId = patient;
            PatientToUpdate = patientToUpdate;

    public class Handler : IRequestHandler<UpdatePatientCommand, bool>
        private readonly IPublishEndpoint _publishEndpoint;
        private readonly PatientManagementDbContext _db;
        private readonly IMapper _mapper;

        public Handler(PatientManagementDbContext db, IMapper mapper, IPublishEndpoint publishEndpoint)
            _publishEndpoint = publishEndpoint;
            _mapper = mapper;
            _db = db;

        public async Task<bool> Handle(UpdatePatientCommand request, CancellationToken cancellationToken)
            var patientToUpdate = await _db.Patients
                .FirstOrDefaultAsync(i => i.Id == request.Id, cancellationToken);

            if (patientToUpdate == null)
                throw new NotFoundException("Patient", request.Id);

            await _db.SaveChangesAsync(cancellationToken);

            var message = new
            await _publishEndpoint.Publish<IPatientUpdated>(message);

            return true;

Now, our consumer is already set up in our Billing bc, so if we set both projects as startup projects and put a breakpoint in our consumer, you should see it get hit! This completes the connection to communicate between the bounded contexts. From here, you can add whatever work needs to be done in the consumer. In this case, we are only sending the PatientId, so you'd probably need to make a call back to Ordering to get the entire set of patient information. Another option could be to add all of the patient properties to the message and save yourself a call back to Ordering for all the patient info.