Event Sourcing and CQRS as a Basis for MSApp Design

By Daniil Mikulik (dpmikulik@edu.hse.ru / mikulik086420@gmail.com)

In the realm of modern software development, microservice architecture (MSA) has emerged as a paradigm shift, offering flexibility, scalability, and resilience in deploying large-scale applications. Within this architecture, Command Query Responsibility Segregation (CQRS) and Event Sourcing (ES) have gained traction as patterns that can enhance the design and functionality of microservices. Visual representation of the Event Sourcing and CQRS patterns is described in figure 1. This essay explores whether CQRS and ES should be universally applied as foundational aspects of microservice applications (MSApps) architecture. The investigation will assess their potential benefits, challenges, and applicability, drawing insights from extensive literature and research in the field.

Fig.1 Event Sourcing and CQRS patterns implemented together

Microservice architecture advocates building applications as a collection of loosely coupled services, each responsible for a distinct function [1]. This approach enhances flexibility, fault isolation, and scalability, and allows development teams to work in parallel [2]. Despite its advantages, MSA introduces complexity, particularly in data management and system coordination. CQRS is a pattern where commands (write operations) and queries (read operations) are handled separately, allowing for optimized handling of each type of operation [3]. According to Greg Young, CQRS can improve system performance, scalability, and maintainability by decoupling data modification from retrieval processes [4]. Martin Fowler adds that by segregating these operations, systems can better align with distinct business logic requirements and scale independently [5].

Event Sourcing complements CQRS by persisting the state as a sequence of events rather than traditional data models [6]. Each event represents a change in state, allowing the entire application state to be reconstructed from these events. This approach not only aids audit logging but also facilitates debugging and enhances the ability to reprocess data events if needed [7]. Despite these advantages, the combined use of CQRS and ES is not without challenges. Nascimento and Santos discuss the increased architectural complexity and the need for robust event handling mechanisms [8]. Thakkar and Desai highlight the trade-offs between consistency and availability, which must be carefully managed [9].

To assess the attainability of adopting CQRS and ES patterns as foundational architecture approaches for MSApps, this essay synthesizes findings from diverse research studies, industry experiences, expert analyses and personal opinion. The methodology involves reviewing existing literature on microservices architecture and CQRS/ES patterns while evaluating practical applications through case studies of successful implementations across various domains.

First of all I should describe here key objectives of all architectural patterns which are important to be understood:

Microservice Architecture

Microservices architecture divides an application into independent services responsible for specific business capabilities [8]. This approach facilitates scalability and development agility but requires sophisticated coordination mechanisms to manage inter-service communication effectively.

CQRS Architecture

CQRS differentiates between command and query responsibilities, allowing optimized performance for each type of operation [9]. This separation provides benefits in performance, security, and scalability for complex applications that experience high demand [10].

General comparison of Design Patterns

When compared to other architectural patterns like layered architecture or CRUD-based models: Complexity: While CQRS and ES introduce additional complexity due to their architectural demands, they provide greater flexibility in handling diverse workloads.

• Operational Overhead: The operational overhead is higher with CQRS and ES due to the need for event storage mechanisms and separate read/write databases.
• Data Management: These patterns offer superior data management capabilities through immutable event logs compared to traditional models where data is often overwritten.

According to the previous research results on CQRS and ES applications in MSApps the following implications were highlighted:

Advantages of CQRS Applications in Microservices

In microservices environments, CQRS can provide unique advantages:

• Performance Optimization: By separating queries from commands, systems can scale read and write operations independently. For instance, high-performance point trading systems have successfully implemented this separation to enhance throughput [11].
• Enhanced Data Management: CQRS often pairs well with ES to create a real-time event-driven architecture. Systems can store events as immutable sequences that ensure consistency while facilitating audit trails [12].

Disadvantages of CQRS Applications in Microservices

• Increased Complexity: Implementing CQRS can introduce significant complexity to the system. This includes managing the flow of data between command and query models and potentially duplicating data for different models, which can complicate maintenance and development efforts.
• Eventual Consistency: CQRS can lead to eventual consistency issues, where query models may not immediately reflect the latest changes made by commands. This inconsistency requires careful handling and synchronization, which can complicate application logic and user experience [2]

When to Use CQRS

According to the described advantages CQRS and ES should be employed in scenarios where:

1. The application demands high performance with distinct read/write workloads.
2. Real-time data processing and auditing are crucial.
3. Business logic is complex enough to benefit from segregation [1].

When Not to Use CQRS

According to the described disadvantages, CQRS and ES may not be suitable for:

1. Simple applications with low complexity where traditional CRUD operations suffice.
2. Systems requiring strong consistency without compromises on eventual consistency [2].
3. Projects lacking resources to manage increased complexity and operational overhead effectively.

I fully agree with the findings above that suggest using CQRS combined with Event Sourcing can indeed appear as overhead for simple CRUD applications. In scenarios where applications do not require complex business logic or high performance differentiation between read and write operations, implementing these patterns may introduce unnecessary complexity without providing significant benefits. Also while CQRS and ES offer distinct advantages for designing scalable microservices applications, their adoption should be guided by specific business needs and application characteristics. The architectural complexity introduced by these patterns demands careful management to ensure alignment with business goals. Using CQRS principles, performance can significantly enhance scenarios with diverse workloads and operational requirements [9].

Implementing CQRS alongside ES can lead to substantial benefits in scalability and data management—particularly in environments demanding high performance coupled with intricate business logic handling. However, the trade-offs between added complexity versus tangible benefits necessitate a tailored approach. Organizations should ensure that adopting these patterns serves their intended purpose without introducing unnecessary overhead or operational challenges [8].

CQRS and ES emerge as powerful patterns for designing scalable and resilient MSApps. Their ability to optimize performance, manage data effectively, and align with complex business demands makes them an attractive choice for many applications. However, they should not be ubiquitously applied; rather, their use should be guided by specific application requirements and strategic objectives.

In summary:

1. These patterns are particularly suitable for applications requiring high performance with distinct read/write workloads or complex business logic.
2. They may not be ideal for simple applications or those requiring strict consistency without compromises on availability.

The analysis suggests that while CQRS and ES provide numerous benefits in microservices architecture, careful consideration must be given to their implementation based on project size, team expertise, and business needs.

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2. DZone. (2023). Microservices With CQRS and Event Sourcing. Available at: https://dzone.com/articles/microservices-with-cqrs-and-event-sourcing.
3. AxonIQ. (2024). Learn the CQRS and Event Sourcing Patterns. Available at: https://www.axoniq.io/concepts/cqrs-and-event-sourcing.
4. Young, G. (2010). CQRS: Command Query Responsibility Segregation. Available at: https://cqrs.nu/.
5. Fowler, M. (2011). CQRS: Command Query Responsibility Segregation. Available at: https://martinfowler.com/bliki/CQRS.html.
6. Nascimento, R.F., & Santos, M.A.C. (2021). Adopting CQRS in Microservices Architecture: Benefits and Challenges. Proceedings of the International Conference on Software Engineering.
7. Thakkar, A., & Desai, A. (2023). Data Access Pattern Recommendations for Microservices Based Applications. IEEE Xplore. Available at: https://ieeexplore.ieee.org/document/9866064.
8. Gupta, S., & Sharma, V. (2022). Microservice Patterns and Best Practices: Explore Patterns Like CQRS and Event Sourcing to Create Scalable, Maintainable, and Testable Microservices. AOPR Publications.
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10. Pandiya, D.K., & Charankar, N. (2023). Optimizing Performance and Scalability in Micro Services with CQRS Design. International Journal of Engineering Research and Technology.
11. Zhong, Y., Li, W., & Wang, J. (2019). Using Event Sourcing and CQRS to Build a High Performance Point Trading System. Proceedings of the 2019 5th International Conference on E-Business and Applications.
12. Fanshawe, P., Kljun, T., & Akre, K. (2023). Events Sourcing and Command Query Responsibility Segregation Based Microservices Architecture. SCIRP Journal.