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PaymentGateway

An in-process implementation of a payment gateway using Event Sourcing, CQRS, .NET Core, Streamstone, MediatR and Azure Table Storage.

Requirements

To play with the API, you either need an Azure Table Storage account or to install Microsoft Azure Storage Emulator. This application has been tested with Microsoft Azure Storage Emulator v5.9 and Visual Studio 2019 Professional.

If you wish to move from the emulator to a real Azure Table Storage, just update the connection string in the appsettings.json file from the PaymentGateway.Api project:

"Storage": {
    "ConnectionString": "UseDevelopmentStorage=true"
}

Setup

Run the API project PaymentGateway.Api and go to the swagger URL if it doesn't open automatically: http://localhost:55666/swagger/index.html

Make a POST request to the infrastructure route :

That will set up the required infrastructure to execute integration tests and play with the Api.

Event store

The event store is managed by the Streamstone library. You can browse the event store with Microsoft Azure Storage Explorer, within the table named eventstore :

Projections

Projections are managed through event handlers using the MediatR library.

You can browse the payment projection view with Microsoft Azure Storage Explorer, within the table named payments :

FAQ

Why ValueObjects can't have a public constructor?

I've chosen to embed value object within events. Events are immutable and must always be deserializable to load an aggregate root. On the opposite, business rules regarding validation of value objects may change over time. Therefore, if we put validation in a public constructor of a value object, we could end up not being able to deserialize older events due to validation violation in embedded value objects.

There are many possibilities to overcome this problem:

  • Don't use value object in events. I'm not a huge fan of this, because that means that the aggregate root must know how to build a value object. If the later one has a lot of parameters, that becomes easily cumbersome.
  • Use a binary serializer instead of JSON. Using a binary serializer like Protobuf or Avro would deserialize an event without calling any constructor. And it's faster. But I didn't want to add the complexity of managing proto files in this demo project.
  • Move the validation outside the constructor, and use a static factory. That's what I've decided to do in this project.