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Unlocking the Power of Pure Event-Driven Systems with Kalix

Hugh McKee.
Developer Advocate, Lightbend.
  • 28 September 2023,
  • 5 minute read

Event-driven architecture (EDA) has become an increasingly popular approach for building distributed, scalable applications. The core idea behind EDA is that events, which signify something happening in the system, are the backbone of the architecture. Components publish events when something occurs, and other components subscribe to the events they are interested in.

This loose coupling between event producers and consumers enables highly flexible and agile systems. Components can be added or removed without needing to rework the entire architecture. The event-driven approach also lends itself well to asynchronous and non-blocking workflows, which can improve performance and resilience.

Within the broad category of event-driven systems, there is a specific pattern called pure event-driven architecture. This post will explain what defines a pure event-driven system and how it differs from other forms of EDA. We'll look at the key technical characteristics of pure event-driven systems and the benefits and potential drawbacks of using this pattern.

By the end, you'll clearly understand what makes a system purely event-driven and when this architectural style is most appropriate. You'll also learn how pure event-driven systems contrast with other variations of event-driven design.

Defining Pure Event-Driven Systems

The key differentiator of a pure event-driven architecture is that events are the core building blocks of the entire system. All components are designed to create, send, receive, and react to events. This differs from some EDA implementations where events augment other patterns like microservices.

Some key technical characteristics of pure event-driven systems include:

  • Events as First-Class Citizens - Events contain all necessary info to trigger processing.
  • Loosely Coupled Components - Components interact solely through events.
  • Asynchronous and Non-Blocking - Components react to events asynchronously.
  • Event Notifications via Pub/Sub - Pub/sub system handles event routing and delivery.
  • Resilience and Scalability - Loose coupling enables high scalability and fault tolerance.

With this pure approach, the benefits of event-driven architecture can be more fully realized:

  • Flexibility - It is easy to add/change components by publishing new events.
  • Agility and Extensibility - Add features by reacting to new event types.
  • High Performance - Asynchronous and state optimization increase throughput.
  • Fault Tolerance - The system is resilient if components fail, as isolated.

Contrasting Pure Event-Driven with Other Forms

While a pure event-driven approach maximizes flexibility, decoupling, and scalability, many systems blend events with other architectural styles. It's helpful to contrast pure event-driven systems with these different forms.

Event-Driven with Shared State

Some event-driven systems maintain a shared state that components can read and update. For example, a user profile service emits events and stores user profiles in a shared database.

Adding this shared mutable state couples components together more tightly. Components may need to coordinate around locking or access to the shared state. This can reduce scalability benefits.

However, shared state can simplify some operations. The tradeoff is between loose coupling and simplicity of access to shared data.

Event-Driven on Top of Monoliths

In some architectures, an event mesh is layered on top of monolithic applications. The core system is still a monolith, but events add a communication layer.

This can improve extensibility by publishing events from the monolith. However, the monolith limits scalability gains from event-driven. The system is only as scalable as the monolith.

Hybrid Event-Based Systems

Many systems take a hybrid approach, using synchronous REST or RPC calls alongside events. This allows combining the benefits of events with request/response where needed.

However, it requires handling the coordination between event-driven and synchronous communication. The architecture is more complex overall.

The pure event-driven approach favors events exclusively for their advantages, like loose coupling. However, practical systems often integrate events with other styles that have tradeoffs.

Example Use Cases

Pure event-driven architecture is best suited for certain types of systems and use cases. Some examples include:

  • High-Volume Transaction Systems - Services processing large volumes of transactions like payments or orders can benefit from event-driven scalability.
  • IoT Systems - The decoupled nature of events suits Machine-to-machine communication like IoT devices sending telemetry.
  • Online Gaming Backends - Events help coordinate state across players and handle spikes in gaming traffic.
  • Cloud to Edge Systems - Events can transmit state changes from cloud to edge networks, which may have intermittent connectivity.
  • Data Pipelines - Breaking up data flows into discrete event steps enables parallelism and resilience.
  • User Activity Tracking - Recording user actions as events is highly scalable and provides an audit log.

Systems with needs like low latency, high throughput, and fault tolerance are ideal candidates for pure event-driven architecture. The decoupled and asynchronous nature of events addresses these types of challenges.

Enabling Pure Event-Driven with Kalix

Kalix is a platform designed specifically for building and running event-driven systems in the cloud. It provides a managed runtime and frameworks tailored for pure event-driven architectures.

For developers, Kalix aims to simplify the creation of decoupled, event-driven services:

  • The Kalix SDKs include libraries for publishing events and reacting to events using familiar languages like Java, Scala, JavaScript/TypeScript, Python, and other languages. This reduces boilerplate code.
  • The framework handles pub/sub event routing, retries, threading, and other cross-cutting concerns. Developers can focus on business logic.
  • Services integrate via events published to topics managed by the Kalix platform—no direct connections between services.
  • Distributed stateful service templates further encourage decoupled, scalable services.
  • The CLI streamlines packaging and deploying event-driven services to the Kalix platform.

At runtime, the Kalix platform provides:

  • A fast, scalable event platform that handles event streams cloud-natively.
  • Managed distributed pub/sub event flows that connect services using events.
  • Service discovery and routing that enables dynamic event subscriptions.
  • Instrumentation and analytics for monitoring event-driven systems.

By handling infrastructure complexities, Kalix enables teams to focus on pure event-driven principles in their system design. It brings the strengths of platforms like Kubernetes to event-driven architectures.


Pure event-driven represents the ideal for decoupled, scalable architectures using asynchronous events. While practical systems balance tradeoffs, understanding pure event-driven principles provides guidance.

Implementing a pure event-driven approach has often required significant development effort. But platforms like Kalix aim to change this by providing purpose-built runtimes and frameworks.

Kalix reduces the need for developers to build core infrastructure like brokers and delivery guarantees from scratch. Its SDKs, templates, and CLI allow teams to focus on application-specific business logic and events.

The platform handles cross-cutting event-driven concerns like service discovery, retries, and threading transparently, making creating scalable, resilient services that interact exclusively via events simpler.

By leveraging Kalix as a managed service optimized for event-driven systems, organizations can more quickly realize the benefits of pure event-driven architecture.

The strengths of cloud-native platforms and practices combine with the loose coupling of events. This intersection unlocks the next level of distributed, large-scale applications - where Pure Event-Driven shines.