What is pub/sub messaging? A simple explainer.

Definition

Pub/sub is shorthand for publish/subscribe messaging, an asynchronous communication method in which messages are exchanged between applications without knowing the identity of the sender or recipient.

Overview

Four core concepts make up the pub/sub model:

• Topic – An intermediary channel that maintains a list of subscribers to relay messages to that are received from publishers
• Message – Serialized messages sent to a topic by a publisher which has no knowledge of the subscribers
• Publisher – The application that publishes a message to a topic
• Subscriber – An application that registers itself with the desired topic in order to receive the appropriate messages

Advantages and disadvantages of pub/sub

As with all technology, using pub/sub messaging comes with advantages and disadvantages. The two primary advantages are loose coupling and scalability.

Loose coupling

Publishers are never aware of the existence of subscribers so that both systems can operate independently of each other. This methodology removes service dependencies that are present in traditional coupling. For example, a client generally cannot send a message to a server if the server process is not running. With pub/sub, the client is no longer concerned whether or not processes are running on the server.

Scalability

Pub/sub messaging can scale to volumes beyond the capability of a single traditional data center. This level of scalability is primarily due to parallel operations, message caching, tree-based routing, and multiple other features built into the pub/sub model.

Scalability does have a limit though. Increasing the number of nodes and messages also increases the chances of experiencing a load surge or slowdown. On top of that, the advantages of the pub/sub model can sometimes be overshadowed by the message delivery issues it experiences, such as:

• A publisher may only deliver messages for a certain period of time regardless of whether the message was received or not.
• Since the publisher does not have a window into the subscriber it will always assume that the appropriate subscriber is listening. If the subscriber isn’t listening and misses an important message it can be disastrous for production systems.

How Pub/Sub Works

In the overview we covered how a publisher sends a message to a topic and how the topic forwards the message to the appropriate subscriber. From a topology point of view it is a simple process.

When it comes to coding the publish or the subscribe process the model can be a bit more confusing. Consider the following Java code which is used to create a topic.

Topic createTopic(String topicName) throws IOException {
  String topic = getTopic(topicName); // adds project name and resource type
  Pubsub.Projects.Topics topics = pubsub.projects().topics();
  ListTopicsResponse list = topics.list(project).execute();
  if (list.getTopics() == null || !list.getTopics().contains(new Topic().setName(topic))) {
      return topics.create(topic, new Topic()).execute();
  } else {
      return new Topic().setName(topic);
  }
}

Cloud or edge providers often simplify this code. Google Cloud, for example, has simplified topic creation into a single line of code.

gcloud beta pubsub topics create topicName

Examples of Pub/Sub

Publish/subscribe messaging has a multitude of use cases, some of which include:

• Balancing workloads
• Asynchronous workflows
• Event notifications
• Data streaming

Faye

Faye is an open source system based on pub/sub messaging. The code below shows you how to start a server, create a client, and send messages using Faye.

var http = require('http'),
    faye = require('faye');

var server = http.createServer(),
    bayeux = new faye.NodeAdapter({mount: '/'});

bayeux.attach(server);
server.listen(8000);

var client = new Faye.Client('http://localhost:8000/');

client.subscribe('/messages', function(message) {
  alert('Got a message: ' + message.text);
});

client.publish('/messages', {
  text: 'Hello world'
});

Faye is much more straightforward than standard JavaScript and it was created specifically for Node.js and Ruby servers. It’s often used for online instant messaging which is a use case for pub/sub that most people experience on a daily basis.

Pub/sub on the edge

As with a lot of edge computing benefits, pub/sub thrives with the speed that comes with being on the edge. A publisher must send a message to a topic, sometimes located far away in the physical world, that a subscriber is listening to. To travel across a room, a message may need to travel halfway around the world through Internet exchange points and physical distance always creates latency.

On the edge that message can travel two to four times faster across the room or even across the world.

Key Takeaways

• Publish/subscribe messaging is when a publisher sends a message to a topic and the message is forwarded to a subscriber.

• The concept of pub/sub is easy to understand but every coding and programming language handles it differently, making it a little more challenging to learn across all platforms.

• On the edge, message delivery times can be two to four times faster by using a network backbone and multiple points of presence.