How do you migrate from a traditional CRUD architecture to event sourcing?

It's important to consider that event-based systems (like using event sourcing instead of CRUD) are significantly more complex to design, implement and maintain. For enterprise large systems, the number of possible states of the system and the side effects of events can grow to a point that troubleshooting and making changes may be really laborious. Before spending time doing a full architecture design, domain design and reviewing every single entity and query of your system, assess if you system does need or benefit from event sourcing at all. Writing a CRUD for a system that has a not-so-complex business logic is faster, cheaper and easier to maintain than going full fledged into event-based systems.

Some specific aspects and motivations that may lead you to consider event sourcing instead of CRUD: 1 - Does your system require extreme scale? 2 - Does your system has multiple access patterns and consumer which makes it difficult to model entities in a traditional CRUD-based database? 3 - Does changes in your entities need to be propagated to multiple other entities and other data stores? 4 - What's the level of consistency, auditing and data durability you have? 5 - Is your system fine with eventual consistency? There other aspects, but these should give a good start on assessing the system for event sourcing.

Moving from a tradicional CRUD arch to events involves a strategic towards caputing and processing domain events. It requires careful considerations of data consistency, event modeling and adopting event-driven patterns.

Analyze the data model and get an understandings on how data flows through CRUD operations (Create, Read, Update, Delete). Then identify pain points. You can look for limitations in the CRUD approach such as can it handle complex historical data or real-time updates? To give a practical scenario, imagine a SaaS application managing user accounts. A CRUD system might struggle to track user activity history or preferences effectively.

While implementing a CQRS application using the Axon framework, some of the key learnings were: 1. All domain commands should be directed towards the Aggregate to ensure that business invariants and rules are enforced at all times. 2. It should be possible to reconstruct the aggregate by replaying the events. This will allow the construction of a point-in-time snapshot of the aggregate and will also be helpful in spinning up new domain services in the future. 3. The event should closely resemble the facts that occurred in the underlying business domain and should not be a mere technical event; for example, CustomerAcquiredEvent, CustomerChurnedEvent, and CustomerWonbackEvent are more meaningful than CustomerStatusUpdatedEvent.

Be mindful of your aggregates. Try to keep these to a minimal when designing these parts of the event sourcing system. Treat them similar to how you treat transactions on a regular CRUD-based system.

Identify and define the events that describe the changes to your domain model. Each event should capture a meaningful change to the system.Define a schema for your events, including event type, timestamp, and payload.

Implementing a log-first data store to leverage event sourcing is a great option, however it's not necessarily the best option. The even sourcing is also achievable by other set of technologies, such as using event buses and streaming hubs to distribute events across systems that are comprised of multiple microservices and different data stores for each of them.

Choose a reliable event store to persist all data changes as a series of immutable events. These events become the single source of truth for the system's state. Then create projections that represent different data views required by your application. These projections materialize a snapshot of the current state based on the event stream. To give practical scenario, your event store could capture events like "User Created," "User Updated Profile," and "User Submitted Feedback." Projections can then present user data for the front-end application or analytics dashboards.

A gradual migration strategy is often the most effective approach: - Start by implementing event sourcing for new features or modules. - Gradually refactor existing CRUD-based components to use event sourcing. - Maintain both CRUD and event-sourced versions of certain functionality during the transition period. - Use dual-write techniques to update both the old CRUD system and the new event store simultaneously. - Implement a reverse projection to recreate the current state from events for testing and validation purposes.

When integrating with your existing systems, you can do a gradual migration. You need to consider a phased approach where new functionalities utilize event sourcing, while older parts using CRUD remain functional. This minimizes disruption and allows for a smooth transition. You can also design APIs that allow communication between your existing CRUD system and the new event-sourced components. For example, the front-end engineer can continue using existing APIs to fetch user data for display. However, new features capturing user behavior can be implemented with Event Sourcing, feeding into future analytics dashboards.

Code reviews and performance optimization is important. You can gradually refactor existing code to interact with the event store and leverage projections. Further, you can monitor system performance and optimize queries on your event store and projections for efficiency. For an example, the front-end engineer can refactor user profile update functionalities to publish events to the event store, notifying other parts of the system about the changes. Performance optimizations can ensure retrieving event data for analytics remains efficient as data volume grows.

Update your application to handle commands that generate events. Commands should represent actions that change the state.Implement event handlers or projectors to apply events and update read models (projections) that are used for querying.

Deploy the changes in a controlled manner. You might start with less critical components or services.Implement monitoring to track performance and identify issues with the event store, projections, and overall system behaviour.Collect feedback from users and developers to identify any issues or areas for improvement.

While moving from a CRUD based system to an Event Sources system is majorly an architectural concern, it is equally important to consider what implications it has on the end user experience of the product. Event sourced systems are typically "eventually consistent" systems and that may confuse end users if they are not aware of it. In a future article, we will talk about the impact of migrating from CRUD systems to Event Source systems has on User Experience and which sort of applications are better suited for Event Sourcing vs. CRUD.

Transitioning to Event Sourcing is best approached as a team effort. Collaborate with Back-End developers, system architects, and data scientists to ensure a seamless integration and leverage the full potential of this architecture. Utilize tools and frameworks that simplify this process for your team.