Intro to Message Oriented Middleware

Learn about MOM

what is MOM

decouple the request that’s being made from the component that resolves that request. this way, we structure around the messages and not the components

direct approach

button maintains a list of objects to call when pressed, a listener pattern is used to observe the press and actuate the appropriate method

general approach

build an interface where there is no direct relationship between interface and action

benefits of general approach 1

more specialised components which can be changed without affecting others. interface components to handle user interaction and action components to affect the database

benefits of general approach 2

dynamically add new components

decoupling

effect is not closely coupled with cause - events are sensed and reflected in the system and components react to those events

drawback of general approach 1

the behavior of the system comes from its composition

drawback of general approach 2

increased complexity but also increased flexibility

viewpoint of how components operate 1

object, component or service oriented - focus on the bits that do the work or hold the state

viewpoint of how components operate 2

event or message oriented - focus on the interactions between the components

event

packet of data saying “this just happened”, e.g., mouse click, transaction

what does an event carry

event type, timestamp, source, payload

design decision 1

set of event types - static, dynamic, extensible

design decision 2

payload - what, constrained, free form

how does a component produce an event

create the data structure and submit it to the middleware

asynchrony in event systems 1

producer and handler do not synchronise

asynchrony in event systems 2

client knows neither the identity nor number of components who will consume the event

asynchrony in event systems 3

client doesn’t know whether event has been consumed, and if so whether it was successful or not

asynchrony in event systems 4

client may not know that one event is processed before another