A sample .NET application featuring DDD, CQRS, event-sourcing, Docker, integration testing and a bit of SignalR.
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A sample .NET application featuring domain-driven design, CQRS, event-sourcing and a bit of SignalR. It resembles an actual bar. You can open a tab, order beverages, have them served and close the tab. It has a very simple (and very, very ugly) AngularJs interface so you can run it locally and start clicking around.


I've always been against the "traditional" way of doing CQRS with void command handlers that throw exceptions for simple business logic validations (eg. TabClosedException). This application started off as a quick test of how you could avoid that by having your handlers use the Either monad and it kind of grew to something that I think is a decentish example of the idea, so I'm sharing it here.

What to look for

One of the things that is [supposed to be] interesting is the TabCommandsHandler.cs.

Each handler is implemented as a chain of functions (using Optional.Async). Each function represents an operation that can either pass (continue the execution) or fail (return an Error to the consumer).

The chain itself contains all of the business validations such as checking whether the tab is closed, checking whether you're not serving beverages that haven't been ordered, etc.


public Task<Option<Unit, Error>> Handle(OrderBeverages request, CancellationToken cancellationToken) =>
    ValidateCommandIsNotEmpty(request).FlatMapAsync(command =>
    GetTabIfNotClosed(command.TabId, cancellationToken).FlatMapAsync(tab =>
    GetBeveragesIfInStock(command.MenuNumbers).MapAsync(beveragesToOrder =>
    PublishEvents(tab.Id, tab.OrderBeverages(beveragesToOrder)))));

public Task<Option<Unit, Error>> Handle(ServeBeverages request, CancellationToken cancellationToken) =>
    ValidateCommandIsNotEmpty(request).FlatMapAsync(command =>
    AssureAllBeveragesAreOutstanding(command, cancellationToken).FlatMapAsync(tab =>
    GetBeveragesIfInStock(command.MenuNumbers).MapAsync(beveragesToServe =>
    PublishEvents(tab.Id, tab.ServeBeverages(beveragesToServe)))));

Besides looking very neat and being easy to read (if we ignore the long ass FlatMapAsync calls), another benefit that this gives us is that is makes the testing part easier. Since there is one function call for every operation that can potentially fail, you can simply check if you have at least one integration test per line.

The application has a complete suite of integration tests that make it very clear what it's all about. (see TabTests.cs and BarTests.cs)


public async Task CanOpenTab(Guid tabId, string clientName)
    // Arrange
    var command = new OpenTab
        TabId = tabId,
        ClientName = clientName

    // Act
    var result = await _fixture.SendAsync(command);

    // Assert

    await AssertTabExists(
        t => t.Id == tabId &&
        t.ClientName == clientName &&
        t.IsOpen == true);

Getting Started

These instructions will get you a copy of the project up and running on your local machine for development and testing purposes.


If you have Docker installed you can just run docker-compose up.

If not, you'll need to have PostgreSql either installed locally or at least have some instance available to set up the connection strings.

You'll also need at least version 2.2 of the .NET Core SDK.


Note that you can point both the event-store and the relational connection to the same database

Using Docker

  1. Execute run-app.sh

Using Visual Studio

  1. Open the .sln file using Visual Studio
  2. Set up the connection strings inside Bar.Web/appsettings.json
  3. Execute Update-Database inside the Package Manager Console
  4. Run the application

Using the dotnet CLI

  1. Open the project folder inside your favorite editor
  2. Set up the connection strings inside Bar.Web/appsettings.json
  3. Execute dotnet ef database update inside the Bar.Web folder
  4. Execute dotnet run
  5. Go to https://localhost:5001 (or whatever port you're running it on)

You should see the "open tab" screen.

open tab

Running the tests

Note that you can point both the event-store and the relational connection to the same database

Using Docker

  1. Simply run run-tests.sh.

Using Visual Studio or the dotnet CLI

  1. Set up the connection strings inside Bar.Tests/appsettings.json to a valid database. (if you point it to an unexisting one, the app will create it for you)
  2. Either run them through the Test Explorer in Visual Studio or using dotnet test


If you feel like contributing, PRs are welcome!


This project is licensed under the MIT License.


Jimmy Bogard for his awesome MediatR library, all of the helpful articles and code samples. You're truly an inspiration :).

Nils Luck for Optional.