Event Sourcing Architecture

Post by: syed hussain in All

Summary

Let’s kick things off with a quick overview in code. Event Sourcing isn’t as complex as it sounds. Imagine capturing every change to your system as a distinct event. You never overwrite state. Instead, you store each action (event) that led to the current state. To reconstruct the current state, you replay these events in sequence.

Here’s a basic pseudocode sample of how Event Sourcing might look in practice:

// Event store to log events
eventStore = []

// Example event: User changes email address
event = { 
    "eventType": "UserEmailChanged", 
    "data": { 
        "userId": "123", 
        "newEmail": "new@example.com" 
    },
    "timestamp": "2024-09-29T12:00:00Z"
}

// Save event in store
eventStore.append(event)

// To restore state, replay all events
user = {}
for event in eventStore:
    if event["eventType"] == "UserEmailChanged":
        user["email"] = event["data"]["newEmail"]

In this example, events are captured in the eventStore, and the system rebuilds the current state (the user’s email address) by replaying the events in order. No data is ever overwritten. Instead, each change is stored as an immutable record of what happened.

Now, let’s dive deeper into what Event Sourcing really means, how it works, why it’s useful, and how you can implement it.

What is Event Sourcing?

Event Sourcing is a design pattern where you persist the state of an entity as a sequence of events rather than storing the current state in a database. Each time something changes in your system, you don’t update the current state directly—instead, you record an event representing that change.

The idea is simple: when something happens (like a user registering, a payment being processed, or an item being added to a cart), an event is generated. This event is stored in an event log, and the current state of the system can always be derived by replaying the series of events in order.

In a traditional system, you’d probably store the current state of, say, a user in a database. You’d update that state whenever something changes, potentially overwriting the previous values. In Event Sourcing, no such overwriting happens. Instead, you append a new event to the event store.

Key Concepts:

  • Event: A record of something that happened in the system.
  • Event Store: A database or log that holds all events.
  • Replay: The process of rebuilding the current state by replaying past events.
  • Projection: A snapshot or view of the current state, derived from events.

Why Use Event Sourcing?

So why would you bother with Event Sourcing when you can just store the current state in a database like everyone else? There are several strong reasons to adopt this pattern, depending on your use case:

  1. Auditability: You get a full audit trail. Every action that led to the current state is recorded. This is invaluable in industries like finance, healthcare, or compliance-heavy environments, where you need to know exactly how something got to where it is.
  2. Debugging and troubleshooting: Since you can replay the exact sequence of events, you can easily trace bugs or anomalies. You can rewind your system to any point in time and see exactly what happened.
  3. Temporal queries: Event Sourcing lets you ask questions about past states. Want to know what the user’s email was three months ago? No problem—just replay the events up to that point in time.
  4. Scalability: Events are simple, lightweight, and immutable. Storing a sequence of events can be easier to scale than constantly updating a large dataset.
  5. Compensating actions: With Event Sourcing, it’s easy to handle compensation for errors. For instance, if you accidentally charge a customer twice, you can create a compensating event (refund) instead of rolling back or deleting data.

How Event Sourcing Works

Now let’s get into the nuts and bolts of how Event Sourcing works. At its core, it revolves around storing and managing events in an Event Store. Each event represents a single action in the system, and events are immutable. Once an event is logged, it can never be changed or deleted. New events simply append to the store.

Here’s how the flow typically works:

1. Capture events

Whenever something happens in your system, an event is generated. This could be anything from a user updating their profile, a payment being processed, or an order being placed. These events are usually defined as specific types, like UserRegistered, OrderPlaced, or ProductShipped.

Each event should include:

  • Event type: What happened (e.g., UserEmailChanged).
  • Data: Details about the event (e.g., user ID and new email address).
  • Timestamp: When the event occurred.

2. Store events in an Event Store

The event store holds all the events, typically in order of when they occurred. This is a special kind of database optimized for storing and retrieving events. Popular solutions include EventStoreDB, Apache Kafka, and Azure Event Hubs.

The event store ensures that:

  • Events are immutable.
  • Events are stored in a sequential order.
  • Events are persisted reliably.

3. Rebuild state by replaying events

To get the current state of an entity (like a user, an order, or an account), the system replays all events related to that entity. This means that you don’t need to store the current state directly; you can always rebuild it by applying the events in sequence.

For example, if you have an order that goes through multiple stages (placed, paid, shipped, delivered), the system can reconstruct the current state by replaying all the events from the time the order was created until now.

4. Projections: Optimizing for querying

While replaying events is great for reconstructing state, it’s not always efficient when you need to query the current state frequently. That’s where projections come in. A projection is a materialized view of the current state of the system, often stored in a separate database. You can update these projections whenever new events occur, allowing you to query the current state directly without replaying the entire event history every time.

Think of projections like a cache: they give you fast access to the latest state, but they aren’t the source of truth—the events are.

Event Sourcing vs. CRUD

If you’re used to working with traditional CRUD systems (Create, Read, Update, Delete), Event Sourcing might seem like a strange and overly complex alternative. So how does it compare?

  • CRUD: In a CRUD system, the database stores the current state. When something changes, the old state is overwritten, and you lose the history of how that state was reached.
  • Event Sourcing: With Event Sourcing, you store each change as an event. The current state is never stored directly but is derived by replaying all the events in order.

The key difference is that CRUD systems lose information with every update, while Event Sourcing preserves every action. If you need to know not just what the current state is but how it got that way, Event Sourcing is a better choice.

Handling Complex Scenarios in Event Sourcing

Dealing with Large Event Streams

In real-world systems, replaying events can get expensive if you have a huge number of events to replay. For example, a long-running entity like a bank account could have thousands of transactions (events) over the years. Replaying all those events every time you need the current balance would be inefficient.

To handle this, you can use snapshots. A snapshot is essentially a checkpoint—a saved version of the current state at a particular point in time. Instead of replaying all events from the beginning, you can start from the snapshot and replay only the events that happened after the snapshot was taken.

Event Versioning

As your system evolves, your event definitions might change. For instance, you might decide to add new fields to an event or change its structure. In traditional systems, this would be straightforward—you’d update the database schema. But in Event Sourcing, you can’t change past events—they’re immutable.

The solution is to version your events. This involves:

  • Keeping the old event structure for older events.
  • Introducing a new event version for future events.

Your system should be able to handle both versions and transform older events into the newer format when necessary.

Event Sourcing and CQRS

Event Sourcing is often used alongside Command Query Responsibility Segregation (CQRS), another architectural pattern. CQRS separates the operations that change data (commands) from those that read data (queries).

In a typical CQRS + Event Sourcing setup:

  • Commands modify the state by generating new events.
  • Queries read the current state from projections, which are built by replaying events.

The combination of these two patterns can lead to systems that are both scalable and highly responsive to complex business logic.

Common Pitfalls in Event Sourcing

Despite its benefits, Event Sourcing isn’t always a silver bullet. There are a few challenges and pitfalls to watch out for:

  1. Complexity: Event Sourcing introduces a significant amount of complexity compared to traditional CRUD systems. Replaying events, managing projections, and handling snapshots require careful design and implementation.
  2. Storage Overhead: Storing every event indefinitely can lead to a lot of data. You’ll need to manage storage carefully, especially if events are being generated frequently.
  3. Eventual Consistency: In an Event Sourcing system, the current state is often built asynchronously from events. This means that you can encounter situations where the system is temporarily inconsistent while waiting for events to be processed. It’s important to design your system to handle this kind of inconsistency gracefully.
  4. Versioning Headaches: As mentioned earlier, versioning events can get tricky. Over time, managing different versions of events can lead to bloated code and added complexity.

Conclusion

Event Sourcing is a powerful pattern that offers unique advantages over traditional state-based systems, particularly in terms of auditability, scalability, and traceability. By storing each change as an event, you get a complete history of your system’s behaviour, allowing you to reconstruct any state at any point in time.

That said, it’s not without its challenges. Event Sourcing can introduce significant complexity, and you’ll need to carefully manage issues like storage, replay performance, and event versioning. But for systems that need a robust, traceable, and scalable architecture, Event Sourcing can be an excellent choice.

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29 Feb 2024
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