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What are the best practices for event sourcing in software design?

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Event sourcing is a software design technique that captures the state of a system as a sequence of events, rather than a snapshot of the current state. This allows for better scalability, performance, auditability, and resilience of the system, as well as enabling complex business logic and domain-driven design. In this article, you will learn some of the best practices for event sourcing in software design, such as how to model events, how to store and publish them, how to handle concurrency and conflicts, and how to test and debug your event-sourced system.

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1 Model events as facts

One of the key aspects of event sourcing is to model events as facts that have happened in the past, and that cannot be changed or deleted. Events should be named with a verb in the past tense, such as OrderPlaced or CustomerUpdated , and should contain all the relevant data that describe the event, such as orderId , customerId , or items . Events should also have a unique identifier, a timestamp, and a version number, to ensure their traceability and order.

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Arslan Raza

Mobile Application Developer | Flutter Developer
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Event sourcing best practices: 1. Immutable events for reliable history. 2. Use domain language for event names. 3. Structured event data with ID and timestamp. 4. Idempotent event processing. 5. Dedicated event store. 6. Separate command and query models. 7. Event versioning for evolution. 8. Publish events for notifications. 9. Aggregate roots for consistency. 10. Event replay for state rebuilding. 11. Thorough testing. 12. Clear event documentation.

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2 Store and publish events in a reliable way

Events are the source of truth for your system, so you need to store them in a reliable and durable way, such as a database, a file system, or a cloud service. You also need to publish them to other parts of your system or external services that need to react to them, such as other microservices, message queues, or event streams. You should use a consistent and standard format for your events, such as JSON or Avro, and ensure that they are validated and serialized correctly. You should also use an event store that supports transactions, snapshots, and projections, to optimize your event storage and retrieval.

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Volkan Nazmi Metin

CTO @ Papara | AWS Certified Solutions Architect
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Providing outbox patterns especially for systems requiring scalability on publisher side of the events. This way you can be more sure that the event stored only once. This can be handled on the consumer side also with identity but requires more resources to control. This is even more valuable when making transactional operations.

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3 Handle concurrency and conflicts with optimistic locking and compensating actions

One of the challenges of event sourcing is to handle concurrency and conflicts, especially when multiple users or services are trying to update the same aggregate or entity at the same time. A common solution is to use optimistic locking, which means that you check the version number of the last event before appending a new one, and reject the update if the version has changed. This way, you avoid overwriting or losing data. However, this may also result in failed or rejected operations, so you need to have a strategy to handle them gracefully, such as retrying, notifying the user, or performing a compensating action, such as reversing or correcting the event.

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4 Test and debug your event-sourced system with event replay and correlation

One of the benefits of event sourcing is that you can test and debug your system by replaying the events from the past and observing the state changes and outcomes. This can help you identify and fix errors, bugs, or inconsistencies in your system, as well as verify your business rules and logic. You can also use correlation identifiers, such as requestId , sessionId , or userId , to link events that belong to the same context or workflow, and trace them across your system. This can help you understand and analyze the behavior and performance of your system, as well as troubleshoot issues or anomalies.

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James White

Director of Engineering at Novum Home Loans
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You have to do a little thinking to make this work. If you want to replay within a live system that is difficult. Even if the events are idempotent, often that is achieved by checking if a value exists and continuing on. This makes it go through a different path, so basically a different test. If you put it into a test environment, that environment needs to be able to mirror the original environment before the event that you are able to test in order to hit the same paths. It is extremely useful, the ability to replay your events, but care has to be taken for it to work that way that you hope. One suggestion is when building the system consider within your logging that you want to replay events, so that the code path followed is visible.

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5 Apply domain-driven design principles and patterns to your event-sourced system

Event sourcing is a natural fit for domain-driven design (DDD), which is a software design approach that focuses on the business domain and its logic, rather than the technical details. DDD helps you to model your system as a collection of aggregates, entities, and value objects, that communicate through events and commands. DDD also provides you with principles and patterns, such as bounded contexts, ubiquitous language, event storming, or CQRS, that can help you to design, implement, and evolve your event-sourced system in a consistent and coherent way.

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James White

Director of Engineering at Novum Home Loans
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I believe learning DDD is one of the best investments that a developer can make in their education. Over the years, knowledge of DDD has been surprisingly common among many of the best developers that I've worked and collaborated with. I provides a good mental model to build architecture with a component up to a large system.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the best practices for event sourcing in software design?

1

2

3

4

5

6

1 Model events as facts

2 Store and publish events in a reliable way

3 Handle concurrency and conflicts with optimistic locking and compensating actions

4 Test and debug your event-sourced system with event replay and correlation

5 Apply domain-driven design principles and patterns to your event-sourced system

6 Here’s what else to consider

Software Design

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What are the best practices for event sourcing in software design?

1

2

3

4

5

6

1 Model events as facts

2 Store and publish events in a reliable way

3 Handle concurrency and conflicts with optimistic locking and compensating actions

4 Test and debug your event-sourced system with event replay and correlation

5 Apply domain-driven design principles and patterns to your event-sourced system

6 Here’s what else to consider

Software Design

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