We have seen how reliable systems are being realized through the service mesh concept. This is another approach for bringing forth reliable software systems. A reactive system is a new concept based on the widely circulated reactive manifesto. There are reactive programming models and techniques to build viable reactive systems. As described previously, any software system is comprised of multiple modules. Also, multiple components and applications need to interact with each other reliably to accomplish certain complex business functionality. In a reactive system, the individual systems are intelligent. However, the key differentiator is the interaction between the individual parts. That is, the ability to operate individually yet act in concert to achieve the intended outcome clearly differentiates reactive systems from others. A reactive system architecture allows multiple individual applications to co-exist and coalesce as a single unit and react to its surroundings adaptively. This means that they are able to scale up or down based on user and data loads, load balance, and act intelligently to be extremely sensitive and royally responsive.

It is possible to write an application in a reactive style using the proven reactive programming processes, patterns, and platforms. However, for working together to achieve evolving business needs quickly, it needs a lot more. In short, it is not that easy making a system reactive. Reactive systems are generally designed and built according to the tenets of the highly popular Reactive Manifesto. This manifesto document clearly prescribes and promotes the architecture that is responsive, resilient, elastic, and message driven. Increasingly, microservices and message-based service interactions become the widely used standard for having flexible, elastic, resilient, and loosely coupled systems. These characteristics, without an iota of doubt, are the central and core concepts of reactive systems.

Reactive programming is a subset of asynchronous programming. This is an emerging paradigm where the availability of new information (events and messages) drives the processing logic forward. Traditionally, some action gets activated and accomplished using threads of execution based on control and data flows. 

This unique programming style intrinsically supports decomposing the problem into multiple discrete steps, and each step can be executed in an asynchronous and non-blocking fashion. Then, those steps can be composed to produce a composite workflow possibly unbounded in its inputs or outputs. Asynchronous processing means the processing of incoming messages or events happen sometime in the future. The event creators and message senders need not wait for the processing and the execution to get done to proceed with their responsibilities. This is generally called non-blocking execution. The threads of execution need not compete for a shared resource to get things done immediately. If the resource is not available immediately, then the threads need not wait for the unavailable resource and instead continue with other tasks at hand, using their respective resources. The point is that they can do their work without any stoppage while waiting for appropriate resources for a particular task at a particular point in time. In other words, they do not prevent the thread of execution from performing other work until the current work is done. They can perform other useful work while the resource is being occupied.

In the future, software applications have to be sensitive and responsive. The futuristic and people-centric applications, therefore, have to be capable of receiving events to be adaptive. Event capturing, storing, and processing are becoming important for enterprise, embedded, and cloud applications. Reactive programming is emerging as an important concept for producing event-driven software applications. There are simple as well as complex events. Events are primarily being streamed continuously, and hence the event-processing feature is known as streaming analytics these days. There are several streaming analytics platforms, such as Spark Streams, Kafka Streams, Apache Flink, Storm, and so on, for extricating actionable insights out of streams.

In the increasingly event-driven world, EDAs and programming models acquire more market and mind shares. And thus reactive programming is a grandiose initiative to provide a standard solution for asynchronous stream processing with non-blocking back pressure. The key benefits of reactive programming include the increased utilization of computing resources on multi-core and multi-processor hardware. There are several competent event-driven programming libraries, middleware solutions, enabling frameworks, and architectures to carefully capture, cleanse, and crunch millions of events per second. The popular libraries for facilitating event-driven programming include Akka Streams, Reactor, RxJava, and Vert.x. 

Reactive programming versus reactive systems: There is a huge difference between reactive programming and reactive systems. As indicated previously, reactive programming is primarily event-driven. Reactive systems, on the other hand, are message-driven and focus on creating resilient and elastic software systems. Messages are the prime form of communication and collaboration. Distributed systems coordinate by sending, receiving, and processing messages. Messages are inherently directed, whereas events are not. Messages have a clear direction and destination. Events are facts for others to observe and act upon with confidence and clarity. Messaging is typically asynchronous with the sender and the reader is decoupled. In a message-driven system, addressable recipients wait for messages to arrive. In an event-driven system, consumers are integrated with sources of events and event stores.

In a reactive system, especially one that uses reactive programming, both events and messages will be present. Messages are a great tool for communication, whereas events are the best bet for unambiguously representing facts. Messages ought to be transmitted across the network and form the basis for communication in distributed systems. Messaging is being used to bridge event-driven systems across the network. Event-driven programming is therefore a simple model in a distributed computing environment. That is not the case with messaging in distributed computing environments. Messaging has to do a lot of things because there are several constraints and challenges in distributed computing. That is, messaging has to tackle things such as partial failures, failure detection, dropped/duplicated/reordered messages, eventual consistency, and managing multiple concurrent realities. These differences in semantics and applicability have intense implications in the application design, including things such as resilience, elasticity, mobility, location transparency, and management complexities of distributed systems.