Communicating Between Bounded Contexts

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

Introduction

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. 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.

Example

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
Messages:
- Name: IPatientUpdated
  Properties:
  - Name: PatientId
    Type: guid
BoundedContexts:
- SolutionName: Ordering
  Port: 1200
  DbContext:
   ContextName: OrderingDbContext
   DatabaseName: OrderingDbContext
   Provider: SqlServer
  Entities:
  - Name: Patient
    Properties:
    - Name: ExternalId
      Type: string
      CanFilter: true
      CanSort: true
    - Name: InternalId
      Type: string
      CanFilter: true
      CanSort: true
    - Name: FirstName
      Type: string
      CanFilter: true
      CanSort: true
    - Name: LastName
      Type: string
      CanFilter: true
      CanSort: true
    - Name: Dob
      Type: datetimeoffset?
      CanFilter: true
      CanSort: true
  Environments:
  - EnvironmentName: Development
    BrokerSettings:
      Host: localhost
      VirtualHost: /
      Username: guest
      Password: guest
  Bus:
    AddBus: true
  Producers:
  - EndpointRegistrationMethodName: PatientUpdatedEndpoint
    ExchangeName: patient-updated
    MessageName: IPatientUpdated
    ExchangeType: fanout
    ProducerName: PatientUpdated
  SwaggerConfig:
    Title: MySwaggerDoc
    Description: This is my swagger doc
    SwaggerEndpointName: "v1"
    AddSwaggerComments: true
    ApiContact: 
      Name: Paul
      Email: paul@test.com
      Url: https://www.thisispaulswebsite.com
- SolutionName: Billing
  Port: 1205
  DbContext:
   ContextName: BillingDbContext
   DatabaseName: BillingDbContext
   Provider: Postgres
  Entities:
  - Name: Patient
    Properties:
    - Name: ExternalId
      Type: string
      CanFilter: true
      CanSort: true
    - Name: InternalId
      Type: string
      CanFilter: true
      CanSort: true
    - Name: FirstName
      Type: string
      CanFilter: true
      CanSort: true
    - Name: LastName
      Type: string
      CanFilter: true
      CanSort: true
    - Name: Dob
      Type: datetimeoffset?
      CanFilter: true
      CanSort: true
  Environments:
    - EnvironmentName: Development
      BrokerSettings:
        Host: localhost
        VirtualHost: /
        Username: guest
        Password: guest
  Bus:
    AddBus: true
  Consumers:
  - EndpointRegistrationMethodName: SyncPatientEndpoint
    ConsumerName: SyncPatient
    ExchangeName: patient-updated
    MessageName: IPatientUpdated
    QueueName: sync-billing-patient
    ExchangeType: fanout
    UsesDb: true
  SwaggerConfig:
    Title: MySwaggerDoc
    Description: This is my swagger doc
    SwaggerEndpointName: "v1"
    AddSwaggerComments: true
    ApiContact: 
      Name: Paul
      Email: paul@test.com
      Url: https://www.thisispaulswebsite.com

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
  {
      request.PatientId
  };
  await _publishEndpoint.Publish<IPatientUpdated>(message);

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

namespace Ordering.WebApi.Features.Patients
{
    using Ordering.Core.Entities;
    using Ordering.Core.Dtos.Patient;
    using Ordering.Core.Exceptions;
    using Ordering.Infrastructure.Contexts;
    using Ordering.WebApi.Features.Patients.Validators;
    using AutoMapper;
    using AutoMapper.QueryableExtensions;
    using MediatR;
    using Microsoft.EntityFrameworkCore;
    using System;
    using System.Threading;
    using System.Threading.Tasks;
    using System.Collections.Generic;
    using MassTransit;
    using Messages;

    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 OrderingDbContext _db;
            private readonly IMapper _mapper;

            public Handler(OrderingDbContext db, IMapper mapper, IPublishEndpoint publishEndpoint)
            {
                _publishEndpoint = publishEndpoint;
                _mapper = mapper;
                _db = db;
            }

            public async Task<bool> Handle(UpdatePatientCommand request, CancellationToken cancellationToken)
            {
                var recordToUpdate = await _db.Patients
                    .FirstOrDefaultAsync(p => p.PatientId == request.PatientId);

                if (recordToUpdate == null)
                {
                    // log error
                    throw new KeyNotFoundException();
                }

                _mapper.Map(request.PatientToUpdate, recordToUpdate);
                var saveSuccessful = await _db.SaveChangesAsync() > 0;

                if (!saveSuccessful)
                {
                    // add log
                    throw new Exception("Unable to save the requested changes. Please check the logs for more information.");
                }

                var message = new
                {
                    request.PatientId
                };
                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.

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