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Sunday, April 9, 2023

What is Event Sourcing Design Pattern in Microservices? Event Sourcing and CQRS Architecture

Hello guys, In the world of microservices architecture, Event Sourcing Design Pattern is gaining increasing popularity as a way to manage data and processes across multiple services. This pattern, which is based on the idea of storing events rather than just the current state of an object, allows developers to build scalable, fault-tolerant, and event-driven systems. In the past, we have learned about Database Per Microservices, SAGA, CQRS, API Gateway, and Circuit-breaker pattern and in this article we will learn about the Event Sourcing Design Pattern in Microservices Architecture. Event Sourcing is a design pattern that is commonly used in microservice architectures to manage and store changes in the state of a system. As I said, It is based on the concept of capturing and storing all changes (events) made to the system as a sequence of events, rather than just the current state of the system. 

With Event Sourcing, every change or update made to the system is recorded as an event, and these events are stored in an event log or event store. This allows for a historical record of all changes made to the system, providing a reliable and auditable source of truth for the system's state.

In this article, we will explore why Event Sourcing is used in Microservice architectures and the benefits it can provide, including scalability, fault tolerance, auditability, and system evolution. Along with SAGA, and CQRS, Event Sourcing is also an important Microservices design pattern and also used in distributed transaction management



What is Event Sourcing Design Pattern?

The Event Sourcing Design Pattern is a way of modeling a system by storing all changes to the system as a sequence of events. Instead of persisting the current state of an object, Event Sourcing stores the history of all changes made to an object in the form of an immutable sequence of events. These events can be used to rebuild the current state of the object at any point in time.

For example, let's say we have an online shopping application, and a user places an order for a product. In a traditional system, we might update the state of the order with the new information, such as the order status and shipping address.

In Event Sourcing, instead of updating the current state, we would create an event that captures the fact that an order was placed. This event would contain all the relevant information about the order, such as the product, quantity, and shipping address.

This event would then be stored in an event store, which is a database specifically designed for storing events. Each event is assigned a unique identifier and is appended to the end of the event stream. To rebuild the current state of the order, we simply replay all the events in the stream in the order they occurred.



Benefits of Event Sourcing Design Pattern

One of the primary benefits of Event Sourcing is that it provides a complete audit trail of all changes made to an object. This is particularly useful in systems that require a high degree of data integrity and auditability, such as financial systems. Since events are immutable and cannot be modified once they are stored, it is easy to verify the integrity of the data.

Another benefit of Event Sourcing is that it makes it easy to implement temporal queries. Temporal queries are queries that involve data at a specific point in time, such as "what was the state of the order on March 1st?". With Event Sourcing, we can easily replay the events up to a specific point in time to answer these types of queries.

Event Sourcing also makes it easy to implement complex business logic. Since all changes to an object are captured as events, it is easy to implement complex business rules that involve multiple objects. For example, if we wanted to implement a rule that requires a customer to have a certain credit score before placing an order, we can simply listen for the "customer updated" event and perform the necessary checks.

Challenges of Event Sourcing Design Pattern

While Event Sourcing has many benefits, it is not without its challenges. One of the primary challenges is the increased complexity of the system. Since all changes to an object are captured as events, the event stream can become quite large, which can make it difficult to manage and query.

Another challenge is the need to implement event handlers for each event. In a traditional system, we might update the state of an object directly in response to a user action. With Event Sourcing, we need to implement an event handler that listens for the corresponding event and updates the state of the object accordingly.




Eventual consistency is another challenge with Event Sourcing. Since events are stored asynchronously and processed asynchronously, it is possible for the system to be in an inconsistent state for a brief period of time. This can be mitigated by implementing compensating transactions or by implementing a quorum-based system.

It is also worth noting that Event Sourcing can be used in conjunction with other patterns, such as Command Query Responsibility Segregation (CQRS) and Domain Driven Design (DDD), to build more complex and scalable systems.

CQRS is a pattern that separates the write model (commands) from the read model (queries) and uses different data stores for each. By combining CQRS with Event Sourcing, we can use the event store as the write model and a separate read model database for querying data. This can improve scalability and performance by allowing us to optimize each data store for its specific use case.

Domain Driven Design is a methodology that focuses on building software that models the domain it operates in. By combining Event Sourcing with DDD, we can create a system that is more closely aligned with the business domain and easier to reason about. For example, we can model events that correspond to business events, such as "customer placed an order" or "product was shipped".

In conclusion, Event Sourcing is a powerful pattern that provides many benefits in a microservices architecture. However, it is not without its challenges, and careful consideration should be given to determine if it is the right choice for a particular system. By combining Event Sourcing with other patterns such as CQRS and DDD, we can build more complex and scalable systems that are closely aligned with the business domain.

What is Event Sourcing Design Pattern in Microservices?



Conclusion

The Event Sourcing Design Pattern is a powerful way to manage data and processes in a microservices architecture. By storing all changes to an object as a sequence of events, it provides a complete audit trail of all changes made to an object and makes it easy to implement temporal queries and complex business logic.

However, it is not without its challenges, such as increased complexity, the need to implement event handlers for each event, and eventual consistency. It is important to carefully consider the trade-offs and determine if Event Sourcing is the right choice for a particular system.

In addition, it is important to design the event schema carefully to ensure that it is extensible and flexible enough to handle future changes. This requires careful consideration of the domain model and the types of events that might occur.

Overall, Event Sourcing is a powerful tool for managing data and processes in a microservices architecture. It provides many benefits and challenges, and careful consideration should be given to determine if it is the right choice for a particular system.

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Thanks for reading this article so far. If you like this Event Sourcing design pattern and when and how to use it then please share them with your friends and colleagues. If you have any questions, feedback, or other fee courses to add to this list, please feel free to suggest.

P. S. - If you want to learn more about Microservice Architecture and solutions from scratch and looking for free resources then I highly recommend you to check out my post about 7 free Microservice courses. It contains free Udemy and Coursera and courses to learn Microservice architecture from scratch.  

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