I keep studying and trying Reactive Style of coding using Reactor and RxJava. I do understand that reactive coding makes better utilization of CPU compared to single threaded execution.

Is there any concrete comparison between reactive programming vs imperative programming in web based applications?

How much is the performance gain, throughput I achieve by using reactive programming over non-reactive programming?

Also what are the advantages and disadvantages of Reactive Programming?

Reactive Programming Advantages/Disadvantages

Spring Webflux, using project Reactor is very promising in terms of using your resources in a more efficient way. It's far from obvious how to calculate the resources required to support a certain number of users, however.

When building a standard web application (not Reactive) which will run on a Tomcat, you can simply define the number of webthreads required to support your users. This does not seem to count for Spring Reactive Web applications, however. A small number of threads is used, and less memory is used, but where in the past you could just define "X webthreads available per server" this doesn't seem possible to do. Making our sysops and devops friends a bit unhappy.

What's a solution for this issue? Because it's hard to "sell" a Reactive solution, when it's one that's not very deterministic when it could break down.

Defining the maximum number of concurrent users for a Spring Reactive Web application on a Netty Server

I started to use java reactor in my app due to better resource utilization thoughts (I have lot of external async integrations) but I came to the question, what is better way to write reactive applications in java? Should I use pure functional style or OOP or i can mixin both of two? I am also interested in good resources that can be helpful in learning the right reactive style. 

Java Reactive Programming: Something Old, Something New

Over time some concepts can fade away but later return as new ones as they are applied in the new environment. The same thing has happened with reactive programming. The term has been introduced to the IT world back in the 1960-70s, and nowadays, it came back to be applied in the development of mobile and web applications What is reactive programming? What are the benefits of reactive programming in Java apps? Let’s dive deep into this topic, study the examples, compare it to imperative programming, and highlight the advantages of this type of programming for apps as well as for business.What Is Reactive Programming Java?| Let’s Put It Simple

The term “Reactive Programming” is not a buzzword anymore, and over the years, it has acquired a number of meanings that might seem a little bit messy and unclear. We go with the most simple one that clearly defines the concept.Reactive Programming (RP) is a programming model that is designed to cope with asynchronous events (data streams) and the specific act of producing a change, in other words, it means that modifications are implemented to the execution environment in an effective ceratin order. Take a look at the sequences of events in real life in order to have a full understanding of the reactive programming Java paradigm. For instance, Ann wants to have a party on a Friday night. She has to invite her friends, clean the apartment, and cook dinner. What options do we have here?1. Ann finishes her work and calls her friends. Then she heads to the supermarket to buy food, waits in line to pay, and goes home. Once at home, she starts cooking. When dinner is ready, she cleans the apartment, gusts arrive, and most likely wait till Ann finishes all the work. The party starts later.This is a description of a sync approach to the problem (request), as all the tasks are completed one after another. As a result, it will take a long time for the party to start.2. Ann finishes her work and sends invitations to her friends. Then she orders food delivery online and heads home to clean the apartment. By the time the apartment is clean, food is delivered. Ann is having dinner without her friends.This is an example of the async approach.3. Ann finishes her work and sends invitations to her friends. She orders food delivery online and heads home to clean the apartment. She finishes cleaning and gets the delivery. But this time she waits for her friends to arrive and start the party.The last option is a description of the reactive approach. That is to say. The RP is when you wait for async actions to complete (make a change) to proceed with further steps.For better understanding, think of the experiment at your chemistry class. You have to combine two or more elements in order to get a certain reaction. Similar processes lay behind the reactive paradigm  

concept.Reactive Programming Java: Example Reactive programming Java paradigm concept is clear, but where, when, and how is it used?The RP is an effective solution for the applications that are high-load or multiple-user. This approach suits:Chats and social networks;

It can also be used for such components in the apps as:Real-time data streaming;

Server code that serves highly interactive UI elements. For instance, Netflix developers use Microsoft’s open-source reactive programming library Rx (Reactive Extensions) for their web application. Rx allows them to build complex and asynchronous programs much easier. Web applications like Netflix are reactive as code reacts to different events (mouse clicks, keypresses, async data arrival from the server, etc.). Thanks to the Rx library, developers can design evens as a collection of data and not as a series of callbacks. Now when events are modeled as collections, developers can make changes with events in the same way they transform in-memory collections. Events are pushed asynchronously from producer to consumer, and the consumer reacts to the available data as it comes.Java Reactive Programming Performance| How Is It  Done? The RP provides higher performance for Java apps and has lower memory requirements. It is achieved by avoiding blocking calls that always lead to process and context switches in the operating system.Now when the requirements for web apps have become much higher, the number of users has increased as well as a number of requests, it is better to use the RP approach in the application. In RP, the program flow is seen as a sequence of events that occur asynchronously.Java reactive programming consists of the four main building blocks:Observables (the data source/stream that cam emit a number of values, one or none; it can also emit errors and be infinite or finite); Subscribers (the ones who subscribe to Observables, they use the data provided, receive the errors and completion events from the Observable); Operators (used to create, transform, filter, and combine Observables); Schedulers (allows to easily add threading to our Observables and Subscribers). When using reactive programming, the spine of your application will consist of data streams that are going to convey events, calls, messages, even failures. The RP allows you to observe the streams and react when a value is emitted. In code, the developer creates streams of anything that can change or happen (click events, cache events, HTTP requests, etc.); thus, the application becomes asynchronous. To implement the RP modules into the app and compose async, event-driven programs using an observable sequence, use Rx (Reactive Extension) that gives you a great toolbox. Now you can combine, filter, merge, create, and transform data streams that are called Observables.Today you can find a number of libraries (Rx, Spring, etc.) that enable reactive programming implementation into your app with different tools and implementations.

Right way to write reactive apps in java? 

I am new to project reactor, while I am studying the project reactor I observed one thing, always project rector logs get print as the main tread. if reactor runs without caring the thread, how that happens? and how I get verified reactive codes run on different different threads? 

  as default Reactor runs base on the thread that you subscribed.

 if you subscribe from your own thread, the reactor runs base on that thread. which means that not every time the main thread. you can run this test code to verify that. here is the flux created on the main thread. and that flux subscribed from another thread that was newly created. after running the application, see the logs. the logs will prove it. the logs have been run on the new thread that we newly created.

  public void testTheThread() throws InterruptedException {
        //the flux created on the main thread.
        Flux<String> stringFlux = Flux
                .fromArray(new String[]{"a", "b"})
                .map(String::toUpperCase)
                .log();

        //the subscriber runs on another new thread. [my-new-thread]
        Thread newThread = new Thread(() -> {

            stringFlux.subscribe(s -> {
                System.out.println("String is  = " + s);
            });
        });
        newThread.setName("my-new-thread");
        newThread.start();
        //sleep the main thread until get the data from my-new-thread
        //otherwise the log will not be printed.
        Thread.sleep(1000);
    }

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