Integration PatternsPublish/Subscriber Messaging

How can the sender broadcast an event to all interested receivers?How can we eavesdrop the messages without affecting current flow?

Observer Pattern

Observer Pattern• Observer pattern is a simple pub/sub model in programming • Defines a one-to-many dependency between objects so that when one object

changes state, all its dependents are notified and updated automatically.• Observer

• Gets notified of the change

• Subject• Maintains a list of its dependents, called observers• Have methods to add and remove observers  • Notifies them automatically of any state changes by calling one of their(observers)

methods

Publisher/Subscriber Model• JMS implements Pub-Sub model with the help of Topic• Message Flow

• Subscriber registers them with a topic they are interested• Publisher sends a message to topic• Each registered subscribers would get a copy of message• When message has been received by all subscribers topic deletes the message• Topic will retain the message if any of the registered subscriber is not

available at a time• Topic should send message once inactive subscriber becomes active

Simple Flow

Complex Flow

Advance features• More than one producer can publish messages to a topic• More than one subscriber can consume messages from a topic• Subscribers retrieve all messages published to a topic unless they use

selectors to filter out messages or messages expire before they are consumed• Durable subscribers can be active or inactive. The broker retains messages

for them while they are inactive• Publishers and subscribers can be added and deleted dynamically at runtime,

thus allowing the messaging system to expand or contract as needed

Important Notes• Subscribing to Topic should be restricted by security policies• Messages are published to a topic in the order

• Order in which they are consumed is not guaranteed• Message expiration time, message priority and Selector

• Always set message expiry time• Accumulating messages for inactive consumers for long duration would run topic out

of space

• Publishers and subscribers have a timing dependency• A subscriber can consume only messages published after it has created the

subscription

Share Market Stock Quote Service

Stock Exchange

Topic

Stock Quote Servic

e

Stock Quote Receiver

Manufacturing Industries Stock Quote

Receiver

Advantages• Adds modularity

• Each component has clear responsibility

• Abstraction for Senders and receivers• Highly scalable

• Publishers and subscribers can be added and deleted dynamically at runtime

• Eavesdrop on a message channel without disturbing the existing message flow• Helpful in debugging applications

References

1. https://docs.oracle.com/cd/E19587-01/821-0028/6nl41ccqd/index.html

2. https://dzone.com/articles/design-patterns-uncovered

Thank you