Pub/Sub systems

Question 1

When discussing system characteristics of pub/sub systems, what is meant by heterogenous?

Answer 1

Heterogenous: Components in a distributed system that are not designed to interoperate can be made to work together.

Question 2

When discussing system characteristics of pub/sub systems, what is meant by asynchronous?

Answer 2

Notifications are sent asynchronously by event-generating publishers to subsrcibers who have expressed interest in them.

Question 3

Coupling and coupled check notes

Answer 3

Coupling and coupled check notes

Question 4

What are the three categories of subscription models?

Answer 4

Channel-based, topic-based and content-based

Question 5

What is a centralised model?

Answer 5

A centralised server acts as an event broker. A broker is an entity that is responsible for the matching. Matching entails 3 phases:

• -Subscription processing: Indexing and storing subscriptions.
• -Event processing: upon event arrival, access subscription indices and identify all matched subscriptions
• -Event delivery: deliver event to clients with matched subsciptions

Question 6

What is a distributed model?

Answer 6

Distributed pub/sub systems consist of a network of brokers. Each local broker has a matching engine. see notes for diagram

Question 7

Content-based Routing: An event e is said to match a subscription s…

Answer 7

• -if and only if all attribute-value predicates in s are satisfied by the values carried by e
• -done through an event-subscription matching engine

Question 8

What is event flooding?

Answer 8

Each event is broadcast from the publisher in the whole system.
The implementation is straightforward but very expensive. This solution has the highest message overhead with no memory overhead.

Question 9

What is subscription flooding?

Answer 9

Each subscription is copied on every broker for building locally complete subscription tables.
This approach suffers from a large memory overhead, but event diffusion is optimal. It is impractical in applications where subscriptions change frequently.

Question 10

What is filtering?

Answer 10

Brokers forward notifications through the network only where there is a path to a valid subscriber. More complex routing algorithms. More memory efficient.