1. Architecture: MassTransit is a .NET-based distributed application framework that uses Service Bus architecture, enabling communication between distributed systems. Mosquitto, on the other hand, is a lightweight open-source MQTT message broker designed for IoT applications, following a publish-subscribe messaging pattern.
2. Language Support: MassTransit is primarily used with .NET languages like C# and supports communication between systems built on the .NET framework. In contrast, Mosquitto is implemented in C and can be utilized in various programming languages like Java, Python, and JavaScript.
3. Protocol: MassTransit uses messaging protocols like AMQP and RabbitMQ to handle communication between services, offering features like message serialization, routing, and delivery. Mosquitto focuses on the MQTT protocol, which is lightweight and ideal for IoT devices due to its low bandwidth usage and efficient message delivery.
4. Ease of Use: MassTransit provides a higher level of abstraction for developers with its declarative configuration model and built-in features for handling message exchange. Mosquitto offers a simpler setup and configuration process, making it easy to deploy and manage in IoT environments where resource constraints may be a concern.
5. Scalability: MassTransit supports scaling out by deploying multiple instances of services and utilizing tools like RabbitMQ for load balancing and high availability. Mosquitto is designed to be lightweight and efficient for constrained environments, making it less complex to scale horizontally compared to MassTransit, which may require additional configurations for large-scale deployments.

In Summary, MassTransit and Mosquitto differ in architecture, language support, protocol usage, ease of use, and scalability, catering to distinct requirements in distributed applications and IoT environments.