Event Sourcing

Introduction to Event-Sourced applications

If you've ever designed or developed large software applications, you may have noticed the scalability constraint of using a database as the single source of truth within your system. Whether you use a RDBMS such as Oracle or PostgresSQL, or a noSQL database like MongoDB, you inherently run into the issue where your system is not able to scale due to the limitations of your database.

For those who are unaware, event sourcing has emerged as a pivotal software pattern for building robust, scalable, and maintainable applications. Event-sourced applications radically shift how we perceive and handle data changes by capturing every state change as a sequence of events. This approach not only preserves the history of all changes but also opens up new avenues for auditing, troubleshooting, and deriving insights from the recorded events.

Understanding event sourcing is essential for developers aiming to create applications that require a high degree of reliability and traceability. By leveraging an event sourcing system (such as Axon Server), developers can reconstruct the state of an application at any point in time, offering unparalleled flexibility and control. In this article, we'll delve into the core concepts of event sourcing, explore the types of problems it solves, and understand the distinctions and similarities between event streaming and event sourcing within an event-driven architecture.

What are Event Sourced applications?

Simply stated, an event-sourced application is designed around the event-sourcing pattern, where all changes to the application state are stored as a sequence of immutable events. Instead of persisting only the current state, which is what developers typically do using a database, the system records every event that leads to the current state. This means that the application’s entire history is preserved, enabling it to recreate any previous state by replaying the events from the event store. You can think of an event store as a specialized database for event sourcing.

Now let’s consider a banking application as a practical example. In a traditional system, an account's current balance is stored and updated directly. However, in an event-sourced application, every transaction - deposits, withdrawals, transfers - is recorded as an event. The account balance is not stored explicitly; instead, it is derived by replaying all relevant events. This approach ensures that every financial operation is accounted for, providing a transparent and auditable trail of activities.

By adopting event sourcing, applications gain significant advantages in scalability, fault tolerance, and data integrity. The event store acts as the single source of truth, and since events are immutable, they provide a reliable foundation for the application's state management.

Learn how to create your first Event-Driven application in Java with this tutorial

What are the types of problems that Event Sourcing solves?

Event sourcing addresses several critical challenges in modern software development, particularly those related to maintaining consistency, ensuring data integrity, and facilitating complex transactions. One of the primary problems it solves is the difficulty of accurately reconstructing historical states of an application. As previously stated, by leveraging an event store, developers can replay events to recreate past states, which is invaluable for debugging, auditing, and compliance purposes.

In the context of our banking scenario, imagine the need to investigate a dispute over a transaction that occurred months ago. Due to the nature of event sourcing, every transaction is an event and is preserved. This allows the bank to trace the exact sequence of actions that led to the current account balance. This not only aids in resolving customer issues but also satisfies regulatory requirements for financial record-keeping.

Event sourcing also tackles the problem of data recovery after failures. Since all events are stored persistently and durably, the system can be restored to a consistent state by replaying events from the event store. This eliminates the risk of data loss and ensures that the application can recover gracefully from unforeseen disruptions.

Furthermore, event sourcing facilitates scalability in distributed systems. By decoupling the write model (event generation) from the read model (state representation), applications can handle high volumes of data and user requests more efficiently, enhancing overall performance in large-scale deployments.

Event Streaming and Event Sourcing: How are they similar?

Event streaming and event sourcing are closely related concepts that revolve around using events to drive application behavior. Both paradigms involve emitting events that represent changes within the system and rely on those events to inform other components or services about state changes.

In both event streaming architecture and event sourcing, events are propagated through an event bus or message broker, such as Apache Kafka or RabbitMQ. This infrastructure enables real-time data flow between different parts of the system, promoting responsiveness and scalability. For instance, in a banking application, when a customer makes a deposit, an event is generated and streamed to various services that might update account summaries, trigger notifications, or adjust risk assessments.

The similarity lies in the fact that both methodologies leverage events as a fundamental unit of communication and state change. They facilitate decoupled and asynchronous interactions between components, which is essential for building resilient and scalable microservices architectures.

Event Streaming and Event Sourcing: What's the difference?

Despite their similarities, event streaming and event sourcing serve different purposes within an application. Event streaming focuses on the continuous flow of event data across systems, enabling real-time processing and reactions to events as they occur. It is primarily concerned with distributing events to interested subscribers in an efficient and scalable manner. Pure event streaming is all about message routing, and there’s no form of data persistence.

Event sourcing, on the other hand, is an architectural pattern that involves both routing events to the proper destination as well as storing the events in an event store. Therefore, event-sourced systems use both an event bus to route the events and an event store to persist the events. Let’s analyze them further.

The two key components for Event Sourcing: The message bus and the event store

As previously alluded to, the successful implementation of event sourcing hinges on two critical components: the event bus and the event store. The event bus serves as the conduit for transmitting events between different parts of the system. It ensures that events generated by the application are dispatched to all interested subscribers promptly and reliably.

For example, when a banking customer initiates a fund transfer, an event representing this action is placed on the event bus. Services subscribed to this event—such as those responsible for account balances—receive the event and perform their respective functions. The event bus facilitates this decoupled communication, enhancing scalability and flexibility.

The event store acts as the persistent database for events where all events are stored immutably. Unlike traditional databases, which are designed to store the current state of a system with INSERT and UPDATE statements, the event store records every event that has occurred. This comprehensive collection of events allows the application to reconstruct states, audit changes, and perform temporal queries.

Together, the event bus and event store form the foundation of an event-sourced system. The event bus handles communication and distribution, while the event store ensures persistence and historical accuracy. Their synergy enables developers to build applications that are both dynamic and reliable.

Final thoughts

Event sourcing presents a powerful approach to application development, particularly for systems that demand high reliability, transparency, and scalability. By embracing the event sourcing pattern, developers can create applications that not only handle current operations efficiently but also maintain a complete and accurate history of all state changes.

In today's data-driven world, the ability to reconstruct and analyze past states is invaluable. Whether it's for auditing purposes, debugging, or gaining insights into user behavior, event sourcing provides the mechanisms to meet these needs effectively.

In conclusion, event-sourced applications represent a significant advancement in how we design systems that are both resilient and adaptable. By understanding and leveraging the event sourcing pattern, developers can build applications that meet the demands of modern enterprises, delivering reliable and insightful solutions that stand the test of time.