Reactive Design is an architectural approach for building systems that are:

• Responsive
• Resilient
• Elastic
• Message-driven

Responsive means the system responds in a timely manner if at all possible. It focuses on providing rapid and consistent response times, establishing reliable upper bounds so they deliver a consistent quality of service.

Resilient means the system stays responsive in the face of failure. This applies not only to highly-available, mission-critical systems – any system that is not resilient will be unresponsive after a failure.

Elastic means the system stays responsive under varying workload. Reactive Systems can react to changes in the input rate by increasing or decreasing the resources allocated to service these inputs.

Message-driven means Reactive Systems rely on asynchronous message-passing to establish a boundary between components that ensures loose coupling, isolation, and location transparency.

Benefits of loose coupling include:

• Better isolation between components
• Improved scalability
• Easier to understand and modify individual components
• Increased flexibility in system architecture

Reactive Design handles failures by:

• Replicating critical components
• Containing failures within components
• Delegating failure handling to another component
• Recovering automatically when possible

Back-pressure is an important feedback mechanism that allows systems to gracefully respond to load rather than fail under it. It allows Reactive Systems to be more resilient by providing a way for components to manage overload scenarios.

Reactive Design improves scalability by:

• Using message-passing for loose coupling
• Allowing for easy distribution of components
• Supporting both vertical and horizontal scaling
• Providing elasticity to handle varying loads

The Actor Model is often used in Reactive Design because:

• It naturally supports message-driven architectures
• Actors provide isolation and encapsulation
• It enables location transparency
• Actors can be used to implement fault-tolerant systems