Can you explain what the MassTransit library is and how it is used for message exchange in distributed systems?

MassTransit is a C# library for implementing message exchange in distributed systems. It simplifies the creation of applications that communicate via messages, using various message brokers (such as RabbitMQ, Azure Service Bus). MassTransit provides routing, queues, publishing and subscribing to events, as well as support for patterns like request-response.

Example usage: you create a service that publishes an event, and another service subscribes to this event and processes it. MassTransit handles the work with message transport and serialization.

// MassTransit configuration with RabbitMQ
var busControl = Bus.Factory.CreateUsingRabbitMq(cfg =>
{
    cfg.Host("rabbitmq://localhost");
    cfg.ReceiveEndpoint("order_queue", e =>
    {
        e.Consumer<OrderConsumer>();
    });
});

await busControl.StartAsync();

// Publishing a message
await busControl.Publish(new OrderSubmitted { OrderId = 123 });

Thus, MassTransit helps build fault-tolerant and scalable distributed applications in C#.