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What are the benefits and challenges of using snapshots in event sourcing?

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Event sourcing is a pattern that captures the state of an application as a sequence of events, rather than storing it in a database. This allows for replaying, auditing, and scaling of the application logic. However, event sourcing also introduces some challenges for testing and debugging, especially when combined with CQRS (Command Query Responsibility Segregation), a pattern that separates the write and read models of the application. In this article, we will explore how snapshots can help or hinder the testing and debugging of event-sourced systems.

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1 What are snapshots?

Snapshots are a way of reducing the number of events that need to be replayed to restore the state of an aggregate, which is a logical unit of data in event sourcing. Snapshots store the current state of an aggregate at a certain point in time, and can be used to skip over the previous events when loading the aggregate. Snapshots can improve the performance and efficiency of event-sourced systems, but they also have some drawbacks.

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Yuriy Shkraba

Senior Full Stack Engineer | .Net | ReactJS | AngularJS | Azure Certified
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Snapshots are like taking a picture of something in your computer system to remember its current state. They help make things faster when you have lots of changes happening. The concept of snapshots in event sourcing is a crucial optimization technique that can significantly improve the performance and efficiency of event-sourced systems. Snapshots essentially capture the current state of an aggregate (a logical unit of data) at a specific point in time, allowing you to skip over the previous events when loading the aggregate

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2 How do snapshots affect testing?

Testing event-sourced systems can be difficult due to the various scenarios and outcomes that can result from applying events to the aggregates. Utilizing snapshots can make this process simpler by decreasing the setup and data that tests must manage. However, snapshots can also bring risks and complexities, such as becoming stale or inconsistent if the events or aggregate logic change over time without updating the snapshots. Additionally, they can mask bugs or errors in event handling or aggregate behavior by bypassing the replaying of events, as well as making tests less expressive and readable by obscuring the history and context of the aggregate state.

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3 How do snapshots affect debugging?

Debugging event-sourced systems can be difficult, as the debugger needs to trace the sequence of events and their effects on the aggregates. Snapshots can help or hinder the debugging process, depending on the situation. While they can speed up the process by reducing the time and resources needed to load and inspect the aggregates, they can also complicate it by making it harder to identify the root cause or source of a problem. Additionally, snapshots can interfere with the debugging process by preventing the debugger from stepping through or replaying the events and their effects on the aggregates. Ultimately, snapshots can provide a snapshot of the aggregate state at a certain point in time, which can be compared with the expected state or the current state.

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4 How to use snapshots effectively?

Snapshots can be a useful tool for testing and debugging event-sourced systems, but they must be used with caution and consideration. It is recommended to use snapshots sparingly and selectively, only for aggregates that have a large number of events or a complex state. Additionally, they should be used as an optimization rather than a requirement, and the system should always be able to work without them. Furthermore, snapshots should complement the system rather than substitute it, and should always be tested with and without them. Lastly, they should be treated as snapshots rather than sources of truth, and the events should always be stored and validated alongside the snapshots.

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Yuriy Shkraba

Senior Full Stack Engineer | .Net | ReactJS | AngularJS | Azure Certified
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To use snapshots effectively, start by determining when to create them based on the number of events or a schedule, ensuring they capture the current state of your system. Store these snapshots alongside your event data. Incorporate a version identifier within snapshots to accommodate changes in your system's structure over time. Once implemented, thoroughly test the snapshot creation and loading processes to confirm they function correctly. Continuously monitor snapshots to ensure they remain accurate and aligned with your system's evolving state. Nice to have the document of your snapshot strategy to maintain clarity among your team, and include snapshots in your backup and recovery plans to safeguard your data.

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5 What are the alternatives to snapshots?

Snapshots are not the only way to handle the difficulties of testing and debugging event-sourced systems. An alternative or supplement is event compression, which reduces the number of events by merging or discarding redundant or irrelevant events while still preserving the essential information and behavior of the system. Event versioning allows for evolving or changing the events or the aggregate logic over time, while maintaining backward compatibility and consistency. Event projection creates and updates read models from the events, which can be used for querying, reporting, or testing.

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What are the benefits and challenges of using snapshots in event sourcing?

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1 What are snapshots?

2 How do snapshots affect testing?

3 How do snapshots affect debugging?

4 How to use snapshots effectively?

5 What are the alternatives to snapshots?

Application Development

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