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Understanding Event Sourcing with Marten

Event sourcing is a design pattern in which changes to the state of an application are stored as a sequence of events. Instead of recording just the current state, event sourcing involves storing the history of all changes. This approach brings several benefits, such as auditability, complex state rebuilding, and more straightforward event replay. Marten, as a document database and event store for .NET applications backed by PostgreSQL, leverages these benefits, providing a robust infrastructure for event-sourced systems.

In this resource hub, we aim to demystify event sourcing concepts and illustrate how they can be effectively implemented using Marten. Whether you're just starting with event sourcing or looking to refine your understanding, this guide will provide the foundational knowledge and resources to deepen your expertise.

What is Event Sourcing?

Event sourcing is a paradigm shift from traditional data storage methods. It involves capturing all changes to an application state as a sequence of events, which are stored in an event store. These events are immutable, providing a reliable audit log of the system's history. The current state of the application can be derived by replaying these events. This approach offers numerous advantages:

  • Auditability: Every state change is recorded, allowing you to understand the series of actions that led to the current state.
  • Replayability: Events can be replayed to rebuild state, migrate event schemas, or implement event-driven architectures.
  • Flexibility in Querying: As events are stored, you can materialize views that best suit the query needs without affecting the write model.
  • Robustness: Storing events immutably and deriving state ensures the integrity of your application's history.

In the next sections, we'll explore the core concepts of event sourcing, how Marten harnesses this pattern to offer powerful capabilities for .NET applications, and provide valuable resources for you to deepen your understanding and skills in this domain.

Core Concepts of Event Sourcing

Event sourcing is not just a technical choice; it's a strategic approach to handling data and state changes in complex systems. To fully leverage the power of event sourcing with Marten, it's crucial to grasp the fundamental concepts that underpin this pattern.

Events as the Source of Truth

In event-sourced systems, events are the primary source of truth. An event represents a fact that has happened in the past. Each event is immutable and is stored sequentially in an event store. These events collectively represent the entire history of your application's state changes.

Aggregates and Event Streams

Aggregates are clusters of domain objects that can be treated as a single unit. An aggregate can be thought of as a consistency boundary where transactions are atomic and consistent. Each aggregate has an associated event stream, which is the sequence of events related to that aggregate.


Projections are read models created from events. They are representations of data built from the event stream, tailored to the specific needs of the query side of the application. Projections can be updated by listening to the stream of events and reacting accordingly.


Snapshots are occasional, full-state captures of an aggregate at a specific point in time. They are used to improve performance by reducing the number of events that must be replayed to reconstruct the current state of an aggregate.


Remember, snapshots in event-sourced systems are primarily a performance optimization tool. It's a "keep it simple, stupid" (KISS) principle—don't introduce snapshots until you actually encounter performance issues that necessitate them.

Marten's Role in Event Sourcing

Marten provides a seamless way to integrate event sourcing into your .NET applications by offering robust infrastructure for storing and querying events. With Marten, you can:

  • Store Events Efficiently: Marten uses PostgreSQL's advanced capabilities to store events in a highly efficient manner.
  • Build Projections: Marten supports creating projections from your event streams, allowing you to generate read-optimized views of your data.
  • Snapshot Management: Marten allows for easy creation and management of snapshots, reducing the overhead of rebuilding state from a large series of events.

In the next section, we'll explore a curated list of resources that will help you deepen your understanding of event sourcing and its practical implementation with Marten.

Grasping these core concepts is vital for effectively implementing and leveraging the power of event sourcing in your applications. Stay tuned for the final section, where we'll provide a comprehensive list of resources for you to explore and learn from.

Advanced Scenarios in Marten: Contributing to Complex Implementations

In this section, we delve into advanced scenarios that showcase the robust capabilities of Marten, particularly when dealing with intricate requirements in event-sourced systems. Marten's flexibility allows for the implementation of complex scenarios, such as ensuring unique constraints like email uniqueness or optimizing search capabilities through indexing strategies. We encourage contributions to this chapter, as sharing real-world scenarios and solutions can significantly benefit the Marten community.

Implementing a Unique Email Requirement

In an event-sourced system utilizing Marten, adopting the Command Query Responsibility Segregation (CQRS) pattern is typical, where the separation of read and write operations enhances the system's scalability and maintainability.

When addressing unique constraints, such as ensuring a unique email address:

  1. On the Write Side (Command Side):

    • Implement a unique index on an inline projection to maintain data integrity. This approach ensures that duplicate entries are prevented, adhering to the business rule that each email must be unique. It's important to note that the unique index is not applied directly on the events themselves but on an inline projection derived from these events.
    • The event store, serving as the write side, focuses solely on capturing and storing events. By applying the unique index on an inline projection rather than directly on the events, the event store's performance and integrity are preserved, and the system efficiently enforces the uniqueness constraint.
  2. On the Read Side (Query Side):

    • If the requirement involves searching entity streams by attributes like name and description, a full-text index is beneficial. However, to prevent the conflation of read and write concerns, consider implementing a separate read model.
    • This read model can possess the full-text index, optimizing search capabilities without impacting the performance of the event store.
    • The separation of concerns ensures that the event store remains dedicated to its primary role of storing events, while the read model efficiently handles query operations.
  3. Inline Projections for Read Model Consistency:

    • Inline projections are employed to maintain consistency between the read model and the write operations. These projections are performed directly on the write database, ensuring that the read model is updated in tandem with the write operations.
    • This approach can lead to faster reads, as the data is already prepared and indexed, aligning with the system's performance and consistency requirements.

In summary, for scenarios like implementing an unique email requirement, the following approach is advised:

  • Implement a unique index on an inline projection on the write side to ensure data integrity without overloading the event store with direct indexing.
  • Apply full-text indexes on a separate read model, optimizing search capabilities without burdening the event store.
  • Consider using inline projections to maintain consistency between the read and write models, especially if it aligns with the system's performance and consistency requirements.

Contributions to this chapter are highly valued. Sharing your implementation strategies, challenges, and solutions helps enrich the knowledge base of the Marten community, paving the way for more robust and innovative applications. If you have an advanced scenario or a unique solution you've implemented using Marten, we encourage you to share it with the community.

Valuable Resources for Learning Event Sourcing with Marten

To further your understanding of event sourcing and how to implement it effectively using Marten, we have compiled a list of resources. These resources range from foundational readings to more advanced discussions, offering insights for both beginners and experienced developers.


  1. Practical Microservices: Build Event-Driven Architectures with Event Sourcing and CQRS by Ethan Garofolo

    • Ethan Garofolo's book is an invaluable resource for understanding and implementing microservice architectures using event sourcing and CQRS (Command Query Responsibility Segregation). It offers practical guidance, patterns, and real-world examples, making it an essential read for developers looking to build scalable and maintainable event-driven systems.
  2. Versioning in an Event Sourced System by Greg Young

    • Greg Young dives into the complexities of managing versioning and schema changes in event-sourced systems, a critical aspect for maintaining long-lived applications.
  3. Hands-On Domain-Driven Design with .NET by Alexey Zimarev

    • Alexey Zimarev's book provides a hands-on approach to applying the principles of Domain-Driven Design within .NET applications. It's a practical guide that shows how DDD concepts can be implemented effectively, especially in systems that leverage event sourcing and CQRS.

Blogs and Articles

  1. Jeremy D. Miller's Blog

    • Jeremy D. Miller, the creator of Marten, shares insights, updates, and deep dives into the Marten library and its capabilities.
  2. Event Sourcing in .NET Core

    • Oskar Dudycz's repository is a treasure trove of examples, best practices, and guidance on implementing event sourcing in .NET Core applications.

    • This platform provides a plethora of articles, case studies, and tutorials focused on event-driven architecture and event sourcing, offering valuable perspectives and best practices.

Community and Support

  • Marten Discord Community

    • Join the Marten community on Discord to engage in discussions with other developers and contributors, ask questions, and share your experiences with Marten. It's a vibrant community where you can get support, discuss best practices, and stay updated on the latest developments.
  • GitHub Issues

    • Report issues, suggest features, or contribute to the Marten library directly on GitHub.

By exploring these resources, you'll gain a more profound knowledge of event sourcing and how to harness Marten's capabilities to build robust, scalable, and maintainable applications. The journey of mastering event sourcing is continuous, and these resources will serve as your guideposts along the way.

Released under the MIT License.