All You Want to Know about Publish-Subscribe Pattern

The Pub-Sub model, short for Publish-Subscribe, is a communication service that is widely integrated into serverless and microservices architectures to facilitate asynchronous service-to-service interactions. 

Essentially, the Pub-Sub model comprises the following elements:

• Publisher to transmit a message.
• Subscriber to receive the message through a message broker.
• Broker to process the messages

Within this article, you can study the fundamental principles underlying Pub-Sub messaging, which serves as a design pattern for implementing event-driven architectures and real-time messaging systems. There is general information about pub-sub, its advantages, disadvantages, potential applications, and more. 

Publish-Subscribe Model Fundamentals: Definition and Principles

There are a lot of Publish-Subscribe definitions on the Internet, but it still can be a struggle to understand how this model works.

Publish-Subscribe is a messaging pattern widely used in software architecture. In this model, publishers send messages without explicitly targeting specific receivers (subscribers). Instead, they classify the published messages into different classes, unaware of the subscribers that may exist. On the other hand, subscribers indicate their interest in one or more classes and receive only messages that align with their interests. They are not aware of the publishers involved, if any.

Let’s make it simpler.

Considering decoupled and microservices-based apps, effective communication between components and services plays a crucial role in ensuring the overall functionality of an application. 

Pub-Sub messaging tends to be a valuable solution in addressing this need, offering two essential advantages:

• Simplifying the creation of decoupled devices by providing a reliable communication method.
• Enabling the seamless development of event-driven architectures.

The Pub-Sub model enables the asynchronous broadcasting of messages across various sections of an application. Messages are published to a central topic or channel, and subscribers interested in receiving those messages can subscribe to the topic. The publisher doesn’t need to have knowledge of the subscribers.

At the center of this functionality lies a primary component known as a “Topic.” Publishers transmit messages to a topic, which promptly distributes the message to all subscribed entities – if possible. This key distinction sets the Publish-Subscribe model apart from traditional message brokers, where individual messages are batched in a queue (server) until explicitly requested by a user or service.

In the Publish-Subscribe pattern, messages are automatically pushed to all subscribing subscribers. Of course, it’s possible to set an exception for any user-defined subscriber.

This approach empowers the creation of event-driven services without the need for continual querying of a message queue.

The intrinsic design enables the communication to be largely separated from the application logic. For example, this shows how subscribers can access news by subscribing to a topic, not a device. For example, mutual authentication can be achieved in certain publish-subscribe protocols through the use of TLS, employing a certificate/private key, or by utilizing Simple Authentication and Security Layer (SASL) mechanisms.

Let’s create a simple picture:

• Message refers to the data being transmitted within the service.
• Topic represents a named entity that serves as a channel for a stream of messages.
• Subscription stands for a named entity that expresses interest in receiving messages related to a specific topic.
• Publisher (or producer) is responsible for generating messages and transferring (publishing) them to the messaging service, targeting a designated topic.
• Subscriber (or consumer) gets messages from the messaging service, specifically from a designated subscription.

Simply saying, Publish-Subscribe stands out as a more versatile and scalable design pattern when compared to older counterparts like message queuing and event brokers. The key factor behind this lies in Pub-Sub’s ability to enable seamless message flow between distinct system components without requiring these components to be aware of each other’s identities.

Pub-Sub Model Principles to Consider

Publish-Subscribe, as a widely applicable messaging pattern implemented across various programming languages and platforms, operates on several key principles:

• Separation of Communication and Application Logic: Apps developed using the pub-sub pattern maintain a clear distinction between communication and application logic. As a result, the creation of robust and maintainable microservice components is possible.
• Publisher-Subscriber Decoupling: Publishers remain unaware of their subscribers, while subscribers only need knowledge of the topic channel to which they are subscribed. Publishers define the information associated with specific channels and transfer it, allowing for easy and asynchronous sharing among several apps.
• Bi-directional Communication and Multiplexing: Publishers can also act as subscribers due to bidirectional communication. Data streams can be multiplexed, enabling the development of interconnected systems that seamlessly integrate and maintain internal consistency.
• Event-Driven Architecture and Notifications: Intercommunication in Pub-Sub architecture is event-driven, eliminating the need for client-to-server polling techniques. This results in lower latency and enhanced scalability.
• Versatile Message Data Types: Published messages can encompass various data types, ranging from simple strings to complex objects representing text, sensor data, audio, video, or other digital content.
• Message Brokers for Distribution and Filtering: In many use cases, such as IoT, automation, network operations, or distributed cloud environments (e.g. Google Cloud), a message broker or messaging system uses as an intermediary layer. It handles the distribution and filtering of messages and provides a low-latency messaging service leveraging dedicated network infrastructure.

Top 5 Publish-Subscribe Model Benefits

A distributed microservices-based application developed using the Publish-Subscribe pattern brings benefits to the entire organization, from software architects to QA engineers. Here are the advantages of choosing Pub-Sub:

#1. Decoupled/Loosely Coupled Components

As it was mentioned, Publish-Subscribe enables the separation between communication and application logic, resulting in modularized, robust, and secure software components. As a result, code quality and maintainability are improved. 

Additionally, these components provide system-wide visibility, making it easy to understand the flow of the app.

#2. Real-Time Communication

Pub-Sub delivers messages to subscribers instantly through push-based delivery, making it suitable for near real-time communication requirements. The model eliminates the need for polling, reducing delivery latency.

#3. Ease of Development

The Publish-Subscribe pattern works with any language and technology, allowing easy integration with any supported message broker and programming language.

This model improves communication between components built using different languages, acting as a bridge for inter-component communication. It also simplifies integrations with external systems without creating additional functionality or security concerns.

#4. Increased Scalability & Reliability

This model provides an opportunity to add publishers and subscribers dynamically per request. The separation between communication and logic eases troubleshooting, as issues can be isolated to specific components.

The Pub-Sub model enhances application scalability by enabling changes in message broker architecture, filters, and users without impacting underlying components.

#5. Testability Improvements

Modularity in the application architecture allows for targeted testing of each module, streamlining the testing process. The Pub-Sub simplifies testing of data origin, destination, flow, corruption, formatting, and security-related issues.

As you can see, the Publish-Subscribe pattern empowers organizations by enabling decoupled and modularized components, real-time communication, simplified development and integration, scalability, reliability, and improved testability.

Pub-Sub Drawbacks to Handle Beforehand

While the Publish-Subscribe pattern is a powerful messaging service, it may not be the optimal choice for every requirement. Let’s briefly examine some limitations of this pattern:

#1. Unnecessary Complexity in Smaller Systems

Configuring and maintaining a Pub-Sub architecture requires effort and resources. In smaller systems where scalability and decoupling are not crucial factors, implementing Pub-Sub can introduce unnecessary complexity.

#2. Media Streaming

Pub-Sub is not well-suited for scenarios involving media streaming, such as audio or video transmission. These applications require smooth synchronous streaming between the sender and receiver, so there are better choices than the Pub-Sub model.

It’s important to consider these limitations and assess whether Pub-Sub aligns with the specific requirements of your system. Alternative messaging patterns or technologies may be more suitable for certain use cases.

When to Use the Publish-Subscribe Pattern

Having all the advantages and disadvantages listed, the question arises — when is the appropriate time to choose Pub-Sub messaging?

The Pub-Sub model finds relevance in various industries for ensuring real-time and distributed communication. One notable area benefiting from this pattern is automation.

Below you can find the most common use cases where Pub-Sub proves valuable:

#1. IoT (Internet of Things)

Pub-Sub enables reliable and efficient gathering and distribution of information in IoT environments. This pattern allows controlling nodes or servers can publish updates that are automatically delivered to subscribed IoT devices.

End-user IoT devices can also act as publishers, sending notifications and sensor information to the cloud for user notification.

Additionally, let’s discuss the MQTT client.

The Mosquitto Management Center (MMC) serves as the central hub for overseeing and controlling your Pro Mosquitto broker instances and clusters. It is a comprehensive web application with a user-friendly interface that encompasses vital facets of an IoT project. 

These include security measures, management of user and access controls, monitoring and administration of brokers, REST APIs, visualization of topic trees, management of high-availability (HA) clusters, and more. 

#2. System Monitoring & Event Notifications

Pub-Sub architecture provides an opportunity to create topics for collecting system information and pushing it to visualization and notification. This is particularly useful in large-scale deployments, where messages can be categorized into different topics.

All servers or services can publish data to common topics, eliminating the need for separate notification pipelines. Maintenance or management functions can subscribe to topics, triggering actions like automatic server replacement upon error reports.

#3. Database Backup & Replication

Pub-Sub tends to be used in order to create a channel for pushing messages about completed backups or snapshots. This enables the migration or copying of backups to different regions or cloud storage if necessary.

Subscribed functions can trigger the migration process based on the received message.

#4. Log Management

The Publish-Subscribe model serves as an intermediary for aggregating and distributing logs. They can be collected from multiple locations and pushed to subscribed services like Google Services or stored across various destinations.

Logs can be filtered based on issues, audit trails, notifications, background tasks, etc., and directed to different subscribers, facilitating proper log management.

These are just a few examples highlighting the versatility of the Pub-Sub pattern in diverse scenarios where real-time and distributed communication is vital.

The Most Useful Pub-Sub Features to Integrate

The features of a Pub-Sub messaging system include:

• Push Delivery. The Pub-Sub model is used to promptly deliver independent event notifications to subscribers. Whenever messages are published on the chosen topic, subscribers get a notification, ensuring they receive timely updates about new messages.
• Multiple Delivery Protocols. Pub-Sub systems support various types of endpoints, like message queues, serverless functions, HTTP servers, or email addresses. This flexibility enables seamless integration with different communication channels.
• Fanout. The system can replicate and push a message to multiple endpoints when it is delivered to a topic. Fanout allows for parallel processing and independent notifications, improving scalability and performance.
• Filtering. Subscribers can define message filtering policies to receive only the notifications they are interested in. This feature eliminates the need to receive every message related to the topic, enhancing efficiency and relevance.
• Multiplexing. In certain scenarios, publishers can also act as subscribers. It is possible to multiplex message streams and construct interconnected systems that maintain internal consistency.

Overall, a Pub-Sub Messaging system offers powerful features to support efficient and flexible communication between publishers and subscribers, promoting scalability, modularity, and customized message delivery.

Publish-Subscribe Model: to Use or Not to Use?

To conclude all the explanations, there is a simple list of cases where it’s better to use the Pub-Sub pattern:

• When an application requires transferring information to a substantial number of consumers.
• When an app needs to establish communication with independently-developed applications or services that operate on diverse platforms, programming languages, and communication protocols.
• When an application needs to deliver information to consumers without necessitating real-time responses from them.
• When integrating systems are designed to support an eventual consistency model for their data.
• When an application needs to transmit information to multiple consumers who may have varying availability requirements or uptime schedules compared to the sender.

When this pattern might not be useful:

• In an application with a limited number of consumers, each requires distinct information from the producing application.
• When an application necessitates near real-time interaction with consumers, immediate responses are crucial.

The Bottom Line

The Pub-Sub messaging pattern serves as a robust and straightforward communication method. It tends to be a foundation for driving real-time distributed microservices-based applications. 

By managing the communication between internal and external components, Publish-Subscribe empowers the creation of scalable asynchronous message flows with minimal delivery delays. This pattern offers significant advantages over traditional message brokers, enhancing the efficiency and effectiveness of message transmission.

Author‘s bio: As an accomplished technology enthusiast, Laurenz Dallinger has a strong interest in digital transformation, with a particular focus on IoT. His commitment to advancing the field of device communication is evident in his work as a speaker and exhibitor at events like the Ars Electronica museum. As a valued member of the Cedalo team, the company behind the top MQTT broker, Laurenz is passionate about sharing his knowledge and ideas with developers seeking new and efficient ways of communicating with devices.