In this lecture, we will explore how subscribers can listen to multiple channels using pattern-based subscriptions instead of subscribing to exact topic names. This allows greater flexibility and selective listening in real-time messaging systems.

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1. Exact Topic vs Pattern-Based Subscriptions

Exact Topic Subscription:

• Subscriber listens to a single topic.
• Only messages published to that topic are received.
• Example: Subscriber subscribes to "slack-room-1" Only receives messages published to "slack-room-1"

Pattern-Based Subscription:

• Subscriber listens to multiple topics that match a specific pattern.
• Patterns can include wildcards, prefixes, or regular expressions.
• Useful when a subscriber wants messages from a group of related topics.
• Example: Subscriber subscribes to "slack-room-*" Receives messages from "slack-room-1", "slack-room-2", "slack-room-3", etc.

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2. Example Use Case

• Subscriber 1: Interested only in "slack-room-1"
• Subscriber 2: Interested in all rooms starting with "slack-room-"

This allows one subscriber to filter messages precisely while another receives a broader range.

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3. Implementing Pattern-Based Subscription

Step 1: Define Subscription Pattern

• Use a string pattern to define the topics of interest.
• Example: String patternAllRooms = "slack-room-*"; String patternRoomOne = "slack-room-1";

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Step 2: Subscribe to Topics Using Patterns

• Subscribers can implement a reactive or lambda-based listener.
• Each message includes:
  1. The topic name
  2. The message content

subscriber.subscribe(patternAllRooms, (topic, message) -> {
    System.out.println("Subscriber 2 received from " + topic + ": " + message);
});

subscriber.subscribe(patternRoomOne, (topic, message) -> {
    System.out.println("Subscriber 1 received from " + topic + ": " + message);
});

• This implementation allows lambda expressions for simplicity.
• The listener interface typically has three parameters:
  1. Topic – the name of the channel the message came from.
  2. Message – the actual payload.
  3. Pattern – optional reference to the subscription pattern.

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Step 3: Publish Messages

• Using a CLI or programmatically, messages are published to specific topics:

publisher.publish("slack-room-1", "Hi everyone in Room 1!");
publisher.publish("slack-room-2", "Hello from Room 2!");

• Message Delivery Rules:
  • Subscriber 1 (exact match "slack-room-1") receives only Room 1 messages.
  • Subscriber 2 (pattern "slack-room-*") receives all messages from any room starting with "slack-room-".

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4. Behavior Overview

1. Exact match subscription:
   • Only receives messages for that topic.
   • Ignores all other topics.
2. Pattern subscription:
   • Receives messages for all topics matching the pattern.
   • Can include multiple channels without listing them individually.
3. Inactive subscribers:
   • If a subscriber is offline during message publication, it misses those messages.

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5. Example Flow

1. Subscriber Setup:
   • Subscriber 1 subscribes to "slack-room-1".
   • Subscriber 2 subscribes to "slack-room-*".
2. Publish Messages:
   • "slack-room-1": Hi everyone in Room 1!
     • Subscriber 1 → receives
     • Subscriber 2 → receives
   • "slack-room-2": Hello from Room 2!
     • Subscriber 1 → ignores
     • Subscriber 2 → receives
3. Offline Scenario:
   • Subscriber 1 goes offline.
   • A message is published to "slack-room-1".
   • Subscriber 1 does not receive the message.

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6. Key Points

• Pattern subscriptions allow dynamic and flexible message routing.
• Ideal for scenarios with many channels where a subscriber wants messages from a subset of topics.
• Works well with reactive streams and real-time messaging systems.
• Subscribers can implement lambda functions to handle messages efficiently.
• Offline subscribers will miss messages unless persistence is implemented.