PUBLISH-

SUBSCRIBE

PATTERN

B Y -

R U T V I K B

A P A T

INTENTIn Software Architecture, PUBLISH-

SUBSCRIBE Pattern is a message pattern – a network oriented architectural pattern – which describes how two different parts of a message passing system connect and communicate with each other.

MORE ON IT FURTHER..

MOTIVATION/PROBLEM

How can an application in an integration architecture only send messages to the applications that are interested in receiving the messages without knowing the identities of the receivers?

GENERAL STRUCTURE

EXAMPLE

THREE ESSENTIAL ELEMENTS/COMPONENTS

The first two are obvious.

• PUBLISHER• SUBSCRIBER• COMMUNICATION

INFRASTRUCTURE

SO,Q: WHAT DOES A

PUBLISHER DO?A: Easy, it publishes

messages to the communication infrastructure.

OKAY. SO,Q: WHAT DOES A

SUBSRIBER DO?A: Easy, it subscribes to a

category of messages.

HMMM. SO,Q: WHAT IS A COMMUNICATION

INFRASTRUCTURE?A: • It receives messages from

publishers.• It maintains the subscribers'

subscriptions to transport the messages to the respective subscribers.

APPLICABILITYIn the pub/sub model, subscribers typically receive only a subset of the total

messages published. The process of selecting messages for reception and processing is called filtering.

There are two common forms of filtering: topic-based and content-based.

In a topic-based system, messages are published to "topics" or named logical channels. Subscribers in a topic-based system will receive all messages published to the topics to which they subscribe, and all subscribers to a topic will receive the same messages. The publisher is responsible for defining the classes of messages to which subscribers can subscribe.

In a content-based system, messages are only delivered to a subscriber if the attributes or content of those messages match constraints defined by the subscriber. The subscriber is responsible for classifying the messages.

Some systems support a hybrid of the two; publishers post messages to a topic while subscribers register content-based subscriptions to one or more topics.

CONSEQUENCESBenefits• Loose coupling. The publisher is not aware of the number of subscribers, of

the identities of the subscribers, or of the message types that the subscribers are subscribed to.

• Improved security. The communication infrastructure transports the published messages only to the applications that are subscribed to the corresponding topic. Specific applications can exchange messages directly, excluding other applications from the message exchange.

• Improved testability. Topics usually reduce the number of messages that are required for testing.

Liabilities• Increased complexity. Publish/Subscribe requires you to address the

following:You have to design a message classification scheme for topic

implementation.You have to implement the subscription mechanism.You have to modify the publisher and the subscribers.

• Increased maintenance effort. Managing topics requires maintenance work. Organizations that maintain many topics usually have formal procedures for their use.

• Decreased performance. Subscription management adds overhead. This overhead increases the latency of message exchange, and this latency decreases performance.

THANK YOU