How did the application appear? Was it a synchronous or asynchronous request? How did the request processing engine work?

A request could appear as a result of either a synchronous or asynchronous request, depending on the system architecture.

If it is a synchronous request, the client waits for a response from the server immediately after sending the request. In this case, the request processing engine is usually called directly in the same thread or through a controller that handles the request.

In the case of an asynchronous request, the request is placed in a message queue (e.g., Kafka, RabbitMQ), and the request processing engine operates as a separate service that listens to this queue, receives requests, and processes them independently of the client. This increases scalability and fault tolerance.

The request processing engine is typically implemented as a set of business logic that:

• Validates the request
• Performs necessary operations (e.g., calculation, data verification)
• Saves the result to a database
• Sends notifications or initiates subsequent processes

Example of asynchronous processing in Java using a queue:

// Receiving a request from the queue
public void onMessage(Message message) {
    Application application = deserialize(message);
    processApplication(application);
}

private void processApplication(Application app) {
    if (validate(app)) {
        saveToDatabase(app);
        notifyUser(app);
    }
}