PaymentGateway is a payment API application built on top of .Net 6 letting the consumer able to submit payments and then read the transactions status through different endpoints, one by applying the transactionId filter and another by applying the MerchantId filter.

The project structure follows the Clean Architecture design, separating layers into ring levels as shown below:

• Domain: It is the center of the application which is not referencing any other project. It contains the domain model, value objects and domain exceptions.
• Command.Application: It's responsible to submit the payment through commands & events into the database. It is also exposing interfaces implemented by the infrastructure. It's referencing the Domain layer and referenced by the Api & Infrastructure layers.
• Query.Application: It's responsible to query the transactions from database through the query handler. It is also exposing interfaces implemented by the infrastructure. It's referencing the Domain layer and referenced by the Api & Infrastructure layers.
• Infrastructure: It's exposing concrete infrastructure implementations as write & read repositories, telemetries...
• Api: It's exposing the Payments and Transactions Api, the first for write operations and the second for the read ones.

The PaymentGateway has been implemented following the CQRS pattern:

The main benefit is in handling high performance applications. The above design allows you to separate the load from reads and writes allowing you to deploy and scale each application independently based on their needs.

In order to build and run the application the following are some pre-requirements:

• .Net 6 installed in the hosting machine in order to be able to build and run the application from your favourite IDE.
• Docker installed in order to be able to run the application from container.

Both you run from IDE or from Docker you will be able to play with the application endpoints by using the Swagger UI as below

1. Run from IDE

In order to run the application from your favourite IDE, the Checkout.Api needs to be set as default starting project, then F5 or click start from UI as shown below for Visual Studio or Rider:

• Rider:

• Visual Studio:

The application should launch the browser at page https://localhost:5001/swagger/index.html

2. Run from Docker

In order to run the application from Docker, these are the steps to follow:

• Move to the main root where the Solution and the Dockerfile are placed.
• Run the command:

docker build -t checkout-api -f Dockerfile .

It should take some seconds to pull all the dependencies and then:

docker run -ti --rm -p 8080:80 checkout-api

The above command will run your application in interactive mode, mapping it to port 8080 so you should be able to navigate to the swagger page at http://localhost:8080/swagger/index.html and start playing with the endpoints.

Each endpoint exposes its request/response schema.

1. Submit one or more payments through the /api/beta/transactions endpoint. (use card numbers from Checkout Test card numbers for valid ones).
2. Use the transactionId from previous response to invoke the /api/beta/transactions/{id}/ and fetch the transaction detail.
3. Use the merchantId from the previous response to invoke the /api/beta/merchants/{id}/transactions and fetch all the transactions against the given merchant.
4. After playng around with above endpoints, you can try out and have a look at metrics recorder by Prometheus at the endpoint /api/beta/metrics

The application metrics have being recorded during the Api utilization by Prometheus and available at the endpoint /api/beta/metrics as shown below:

The fake validation has been implemented by running checks against the submitted amounts as the following table. Inspiration taken from Checkout documentation - Soft decline

• Metrics recording through `Prometheus'.
• Dockerization

• Add and manage Idempotency-key on payments post requests
• Using 2 storages or collection/tables, one for the append-only domain events and one for the current state of the domain or aggregate root