Pub/Sub Messaging Components and Usage in Cache

The Publish/Subscribe (Pub/Sub) messaging paradigm is used to exchange messages between multiple applications without any knowledge of the sending or receiving applications. The messages are exchanged via an intermediary channel, called a topic. As the name of the model indicates, the applications sending the message are considered publishers, and the recipients of the message are subscribers, as they subscribe to the channel they are interested in.

For example, a set of subscribers can be interested in information about the order shipment details so that they can process it to track order delivery. Hence, they will subscribe to a topic which relays messages for order details. Once the publisher publishes a message to the topic, the subscribers will be notified and receive the message containing order details to further process it at their end.

Since all Pub/Sub models require a channel for communication, NCache acts as a medium for topics so that the publisher publishes the message to the NCache topic, and subscribers receive the message via the topic on notification. The notification model is Poll-based Notification model. Using NCache as medium for topics ensures loose coupling within the model and allows berth to scale with abstraction. This also provides the added benefit of the topics being distributed.

Note

It is recommended to use a separate cache dedicated to Pub/Sub only. This is because of two features of NCache:

Eviction: If the cache is shared for messages and cache items, frequent eviction for cache items may also remove the messages before they are relayed to the subscriber.

State Transfer: State transfer adds a cost over each cache item operation. Since messages are frequently being published, relayed and expired, this recurrent activity can prove costly because it triggers state transfer.

The components in the figure are briefly described below:

• Topic
  A channel through which the publisher and subscriber communicate with each other via messages. A topic maintains the list of subscribers registered to it and the relays the published messages to the subscribers. Priority can be added to the topic w.r.t. the importance of it.

• Message
  The object in interest of the subscribers. Publishers will publish serializable messages irrespective of any subscriber’s knowledge. You can also add multiple messages and get them published in an order.

• Publisher
  Publishes the message to the topic and can optionally receive message delivery failure notification.

• Subscriber
  Registers itself against the desired topic so that it receives messages related to the topic.

Types of Subscriptions

You can implement the following types of subscriptions using NCache:

• Durable subscription
  • Shared vs Exclusive
  • Fixed Name vs Pattern-Based
• Non-Durable subscription
  • Exclusive
  • Fixed Name vs Pattern-Based

In This Section

Pub/Sub Subscription Types and Overview
Explains the different subscriptions under the Pub/Sub model in NCache.

Pub/Sub Topics Behaviour and Properties
Explains in depth the concept and code samples for topics in Pub/Sub model in NCache.

Pub/Sub Messages Behaviour and Properties
Explains in depth the concept of messages in Pub/Sub model of NCache and provides samples to create messages.

Pub/Sub Event Notifications
Explain Pub/Sub events to notify the publisher and subscriber about various events happening across the cache and applications.

Publish Messages to Topic
Provides sample code which creates a topic and publishes messages to it.

Subscribe for Topic Messages
Provides sample code to subscribe against a topic and receive messages of interest.

Monitoring Pub/Sub Topic Statistics
Describes the ways Pub/Sub statistics can be monitored through NCache Monitor, PerfMon and PowerShell cmdlets.