[Part 3] Six modern software architecture styles: Event-Driven

This blog post series reviews six common architectural styles used in distributed systems and talks about how to choose the best one for your use case.

This blog post focuses on the Event-Driven architectural style, however we also recommend checking the other posts in the series:

(1) Monolithic
(2) Microservices
(3) Event-Driven
(4) Serverless
(5) Edge Computing
(6) Peer-to-Peer

Event-Driven Architectural Style

One-liner Recap:

Your components or services communicate through events.

Background:

“With hybrid cloud-native implementation and microservices adoption, EDA gets a new focus by helping to address the loose coupling requirements of microservices and avoid complex communication integration. The adoption of the publish/subscribe communication model, event sourcing, Command Query Responsibility Segregation (CQRS), and Saga patterns help to implement microservices that support high scalability and resiliency for cloud deployment.”

- Jerome Boyer, Distinguished Engineer, IBM Cloud and Cognitive Software

Event-Driven Architecture is not a new concept; it has been around for decades, in fact Gartner proclaimed it the “next big thing” almost 20 years ago when faster networks, general-purpose event management software tools and the emergence of standards for event processing were paving the way for its adoption.

It’s important to underline that EDA is a complementary architecture for Microservices and SOA: it originates from the need for modern digital businesses to respond immediately to events as they occur, providing users with up-to-date information and rapid feedback.

Traditional architectures usually use a command-based communication pattern: they deal with data as batches of information to be inserted and updated (at some defined interval) and respond to user-initiated requests rather than new information.

In an Event-Driven Architecture events are used as a means of communication between software components: each component responds to events that are relevant to its function, and events are used to trigger actions in other components. In summary EDA enables real-time, asynchronous, multicast, fine-grained, and filtered communication, both for predictable and unpredictable events.

Beyond real-time monitoring and responsiveness, this architectural style addresses various use cases, including data replication, parallel processing, and ensuring redundancy during service outages.

In recent years, the rise of cloud-native architectures, container-based workloads, and serverless computing has underscored EDA's value in enhancing the resilience, agility, reactivity, and scalability of modern applications.

Weaknesses:

According to a survey conducted by Solace in 2021 Event-Driven Architecture is in widespread use for 72% of global businesses. However, only 13% of organizations have reached the “gold standard of Event-Driven Architecture maturity”. Communicating using events seems like a straightforward idea, but it’s deceptively simple.

Implementing how events are identified, captured, and produced based on specific triggers or conditions requires careful consideration. When you evaluate the solutions for event generation, communication, error handling, consumption, event deployment, governance, etc. you realize that there’s much more complexity than you might have initially thought.

This type of architectural style also presents other challenges:

Trade offs in data consistency. When multiple components subscribe to and process events independently, ensuring data consistency across all services and databases can become complex. Implementing mechanisms like event sourcing and distributed transactions may be necessary to maintain data integrity in such scenarios.

Complex documentation and maintainability. As with any distributed, highly decoupled application, you need a proper design to understand how the components interact with each other. Maintaining a clear, up-to-date system documentation becomes crucial but can be difficult to achieve in practice. Developers often grapple with the need for effective solutions to document the intricate architecture, making comprehensive documentation and visualization tools essential.

Difficult testing and debugging. With the distributed and decoupled nature of event-driven applications it’s very hard to test the system as a whole and, likewise, hard to trace an event from source to destination. Since you don’t have a statement of the overall behavior of the system (i.e. you can’t just check the code to see how the events will be consumed), you need to watch the path that the individual event takes to understand what is occurring.

Traditional debugging approaches struggle with event-driven systems because events cascade asynchronously through multiple components, often triggering additional events in response.

Full stack session recordings address this by capturing the complete event chain: from the initial user action through every event emission, queue message, and service response. Instead of manually correlating timestamps across distributed logs to reconstruct what happened, developers can replay the entire event flow in sequence, seeing exactly which events fired, in what order, and how each component reacted. This visibility is particularly valuable when debugging race conditions, event ordering issues, or unexpected side effects from event handlers (problems that are notoriously difficult to reproduce and diagnose in event-driven architectures).

Strengths:

The main benefits of the Event-Driven Architecture are the business outcomes that come from improved responsiveness and agility: the more quickly you can get information about events where they need to be, the more effectively your business can seize opportunities to delight users, shift production, and re-allocate resources.

EDA's decoupled nature allows for independent component updates without impacting the entire system, and higher resiliency to service failures.

Another significant advantage lies in scalability: because events are broadcast to multiple components of the system, it is possible to process large volumes of data and transactions in parallel. This makes it easier to handle high traffic and spikes in demand.

Real-world use cases:

Leading companies like Heineken, Schwarz Group, Unilever, and Wix have embraced the Event-Driven Architecture to revolutionize their operations. The applications are numerous from sharing and democratizing data across applications, to connecting IoT devices for data ingestion/analytics, to event-enabling microservices.