Why people use message/event buses in their code?
I think that you have heard of message/event buses, it's the single place when all events in the system flow. Similar architectures are found in computer's motherboards and LAN networks. It's a good approach for motherboards and networks as it reduces the number of wires, but is it good for software development? We don't have such restrictions as electronics does.
The simplest implementation of message bus/event bus can be like:
class EventBus {
void addListener(EventBusListener l}{...}
void fireEvent(Event e) {...}
}
Posting events is done with bus.fireEvent(event), receiving messages is enabled by bus.addListener(listener). Such architectures are sometimes used for software development, for example MVP4G implements similar message bus for GWT.
Active projects:
- Google Guava EventBus
- MBassador by Benjamin Diedrichsen
- Mycila PubSub by Mathieu Carbou
- mvp4g Event Bus
- Simple Java Event Bus
Dormant/Dead projects:
- Sun/Oracle JavaBeans InfoBus
- https://eventbus.dev.java.net/ [Broken link]
It's just the popular Observer (Listener) pattern made 'globally' - 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) 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).
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?
EDIT: I'm not talking about 'big' enterprise solutions like ESB - they can be useful (what's more ESB offers much, much more than just an event bus). I'm asking about usefulness of using message bus in 'regular' Java code for object-to-object connection - some people do it, check the links above. Event bus is probably best solution for telephone-to-telephone communication or computer-to-computer communication because each telefone (or computer) in a network can typically talk to each other, and bus reduces the number of wires. But objects rarely talk to each other - how many collaborators one object can have - 3, 5?
Answer
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