Why people use message/event buses in their code? [closed]

I am considering using a In memory Event Bus for my regular java code and my rationale is as follows

Each object in the system can listen to each message, and I think it's bad, it breaks the Encapsulation principle (each object knows about everything)

I am not sure if this is really true, I class needs to register with the event bus to start with, similar to observer pattern, Once a class has registered with the Event Bus, only the methods which have the appropriate signature and annotation are notified.

and Single Responsibility principle (eg when some object needs to a new type of message, event bus often needs to be changed for example to add a new Listener class or a new method in the Listener class).

I totally disagree with

event bus often needs to be changed

The event bus is never changed

I agree with

add a new Listener class or a new method in the Listener class

How does this break SRP ?, I can have a BookEventListener which subscribes to all events pertaining to my Book Entity, and yes I can add methods to this class but still this class is cohesive ...

Why I plan to use it ? It helps me model the "when" of my domain ....

Usually we hear some thing like send a mail "when" book is purchased

we go write down

book.purchase();
sendEmail()

Then we are told add a audit log when a book is purchased , we go to the above snippet

book.purchase();
sendEmail();
**auditBook();**

Right there OCP violated

I Prefer

book.purchase();
EventBus.raiseEvent(bookPurchasedEvent);

Then keep adding handlers as needed Open for Extension Closed for Modification

Thanks

Some people like it because it is the embodiment of the Facade pattern or Mediator pattern. It centralizes cross-cutting activities like logging, alerting, monitoring, security, etc.

Some people don't like it because it is often a Singleton point of failure. Everyone has to know about it.

I use it heavily in JavaScript. There can be so many various widgets that all need to do some sort of action whenever something else happens -- there is no real hierarchy of ownership of objects. Instead of passing references of every object to every object, or just making every object global, when something significant happens inside a particular widget, I can just publish "/thisWidget/somethingHappened" -- instead of filling that widget with all kinds of code specific to the API of other widgets. The I have a single class that contains all the "wiring", or "plubming" as they like to call it in the Java Spring framework. This class contains references to all of my widgets, and has all of the code for what happens after each various event fires.

It is centralized, easy to access and maintain, and if one thing changes or I want a new process to occur on a specific event, I don't have to search through every single class/object/widget to try to find out where something is being handled. I can just go to my "operator" class -- the one that handles all the "wiring" when a particular event happens, and see every repercussion of that event. In this system, every individual widget is completely API agnostic of the other widgets. It simply publishes what has happened to it or what it is doing.

I'm having trouble understanding what you're really asking in your question. You give an example of a simple event bus which is actually just Observable with a different name, then you say;

For these reasons I think, that for most software, Observer pattern is better than event bus. What do you think about event bus, does it make any good sense for typical applications?

..but given your example, they are the same. This makes me wonder if you have ever used something like a Enterprise Service Bus. At a base level an ESB logically does the same thing as the observer pattern, but commercial products add much, much more. Its like an event bus on steroids. They are complicated software products and offer;

Message pickup
Generate events by listening to various endpoints. The endpoint can be a listener (such as a HTTP server), a messaging system (such as JMS), a database or pretty much anything else you want.

Message routing
Take your event and send it to one/many endpoint. Routing can be pretty smart, the bus might route the message depending on the message type, the message contents or any other criteria. Routing can be intelligent and dynamic.

Message Transformation
Transforms your message into another format, this can be as simnple as from XML to JSON or from a row on a database table to a HTTP request. Transformation can occur within the data itself, for example swapping date formats.

Data Enrichment
Adds or modifies data in your message by calling services along the way. For example if a message has a postcode in it the bus might use a postcode lookup service to add in address data.

..and lots, lots more. When you start looking into the details you can really start to see why people use these things.