What Is an Event Bus?

In event-driven architecture (EDA), an event bus facilitates real-time communication about event data between event publishers and event subscribers.

Events are changes in the state of a system. An event is something that happened that can trigger other things to happen. An ecommerce transaction, a prospect’s request for information, a data breach, a faltering piece of equipment, a change in stock price, and a customer’s click on a URL on a website are all examples of events. For enterprises, getting notified of events as fast as possible is key to driving efficiency, productivity, and competitiveness. To accomplish this, IT architects may choose to implement event-driven architecture.

EDA is a design pattern for software and hardware that enables systems to quickly detect, process, and respond to a wide range of events. Event-driven architecture includes:

• Event publishers. An event publisher, or event producer, is a system that issues a notification as soon as an event occurs. Event publishers may be user interfaces that transmit data about user events, IoT devices that communicate changes in temperature or movement, APIs that communicate payment completions, databases that signify changes in database records, and workflow engines that produce events when tasks, workflows, or milestones are completed.
• Event buses. An event bus is middleware that serves as an intermediary between the event publishers and consumers. An event bus receives event messages in real time from event publishers and makes the data available to event consumers who have subscribed to receive information from a certain event channel or event class. (A message queue, event broker, or event stream may also fill this function.)
• Event consumers. Also known as event subscribers or event listeners, event consumers receive information about events, process it, and perform tasks or actions based on it. The actions of an event consumer may include updating a database, sending an email, issuing an alert, placing an order, triggering a workflow, or producing a new event.

Event-driven architecture and event buses offer several benefits for enterprise IT systems.

With event buses as an intermediary, EDA systems enable asynchronous messaging and loose coupling of applications. Systems that publish events do not need to know where they are sending information, and event consumers do not need to know where event messages are originating. This offers a significant advantage over “request/response” architecture, where one application requests information from another, and then waits for the response. 

This asynchronous messaging means that publishers and consumers don’t need to wait for responses before acting on information or moving onto the next task. The event producers don’t need to know about the event consumers, and the event consumers don’t need to know about the event producers. The producers simply signify that something happened, and the consumers listen for events that they care about. This loose coupling makes the system more resilient to failure. Consequently, event-driven architecture allows various components to scale independently when needed, and it enables systems to handle massive amounts of data with greater speed and real-time processing.

An event bus is typically used in a publish–subscribe model. In this approach to EDA, event buses receive messages from publishers, translate or transform the information if needed, keep messages organized and ordered, route messages to subscribers for consumption, and then delete messages once they are consumed.

Here’s how the process works:

• To receive event data, event consumers are configured to receive information from event buses about a specific event, type of event, or class of events.
• When the event bus receives a message from an event publisher, it routes the data to all appropriate subscribers. Enterprises may also configure buses to keep data for a period of time, enabling consumers to get data on their own schedule.

Event buses enable the components in event-driven architecture to:

• Communicate without dependencies. This makes it easier to add or modify components without causing downtime or affecting the overall system.
• Scale horizontally. Event bus architecture makes it possible to deploy multiple instances of the same component simultaneously, enabling parallel processing to enhance system performance.
• Handle asynchronous communication. By facilitating event messages across the system, event buses make asynchronous communication easier and enable components to trigger actions independently.

An event bus may be used to facilitate event messages for a wide range of applications.

• Ecommerce. Ecommerce platforms rely on event buses to communicate events such as “product ordered,” “payment received,” or “package shipped.” These events may lead event consumers to trigger actions such as creating invoices, sending email confirmations, or ordering more inventory.
• Microservices. The loosely coupled nature of event buses and EDA are ideal infrastructure for communication between microservices.
• IoT data collection. Event buses can receive massive amounts of event data from Internet of Things (IoT) devices and sensors that track environmental data or the movements of traffic and people.
• Change data capture. Event buses can facilitate events around changes in a database, such as when a customer’s CRM record is updated, a new registration is recorded, or an inventory item is out of stock.
• Integration. EDA and event buses promote integration of diverse technologies through asynchronous communication and loosely coupled relationships between systems.
• Real-time data processing. Event buses promote real-time databases by letting consumers know about events as soon as they happen.

Some of the most popular event bus services include Java Message Service (JMS), Apache Kafka, RabbitMQ, Microsoft Azure Event Grid, Google Cloud Pub/Sub, and AWS services like Amazon EventBridge.