From Monolith to Distributed Monolith
How to Avoid Distributed Monolith Anti-pattern Architectural Design and Design Truly Modular and Scalable Microservices
Many software applications today are built using a monolithic architecture, which means that they are composed of a single, cohesive unit that handles all the business logic, data access, and user interface. While this approach has some advantages, such as simplicity and ease of deployment, it also has some drawbacks, such as lack of scalability, flexibility, and maintainability. As the application grows in size and complexity, it becomes harder to modify, test, and deploy the monolith.
To overcome these challenges, some developers have adopted a microservices architecture, which means that they break down the application into smaller, independent services that communicate with each other through well-defined interfaces. Each service is responsible for a specific functionality and can be developed, tested, and deployed independently. This approach has some benefits, such as scalability, agility, and resilience, but it also has some trade-offs, such as increased complexity, network latency, and operational overhead.
Another alternative is a modular monolithic architecture, which means that the application is still a single unit, but it is composed of loosely coupled modules that can be plugged in and out as needed. Each module is a self-contained piece of code that provides a specific functionality and exposes a clear interface. This approach has some advantages, such as simplicity and flexibility, but it also has some disadvantages, such as performance and compatibility issues.
However, none of these architectures is a silver bullet, and each one has its own pitfalls and challenges. Sometimes, developers may start with a monolithic architecture and try to migrate to a microservices or modular architecture, but end up with a distributed monolithic architecture, which is the worst of both worlds. A distributed monolith is an application that is split into multiple services or modules, but they are still tightly coupled and dependent on each other. This means that the application loses the benefits of both the monolithic and the microservices or modular architectures, and suffers from the drawbacks of both. A distributed monolith is hard to scale, modify, test, and deploy, and it is also prone to errors, failures, and inconsistencies. Therefore, developers should be careful when choosing and implementing an architecture for their applications, and avoid creating a distributed monolith.
Monolithic architecture
Monolithic architecture refers to a traditional software design pattern where an entire application is developed as a single, indivisible unit. In a monolithic application, all the components, such as the user interface, business logic, and data access layer, are tightly integrated and interconnected. This contrasts with other architectural styles, like microservices or service-oriented architecture (SOA), where an application is decomposed into smaller, independent services.
Key characteristics of a monolithic architecture include:
Single Codebase: The entire application is developed and maintained within a single codebase. This can make it easier to manage in some cases but might also lead to challenges as the codebase grows.
Tight Integration: Components of the application are tightly coupled, meaning that changes to one part of the application can have ripple effects on other parts.
Single Deployment Unit: The entire application is deployed as a single unit. When updates or changes are made, the entire application needs to be redeployed.
Scalability Challenges: Scaling a monolithic application can be challenging, as scaling typically involves replicating the entire application, even if only a specific component needs additional resources.
Development and Testing: Development, testing, and debugging are often done within the context of the entire application, making it easier to manage in some cases but potentially more complex as the application grows.
Some examples of frameworks that follow a monolithic architecture are:
Ruby on Rails: A web development framework that uses the Model-View-Controller (MVC) pattern and provides a full-stack solution for building web applications.
Django: A web development framework that uses the Model-Template-View (MTV) pattern and provides a full-stack solution for building web applications.
Spring Boot: A Java-based framework that simplifies the creation of stand-alone, production-grade applications that can run on embedded servers.
These frameworks are monolithic because they bundle all the components and modules of the application together in a single code base, which makes deployment, development, and performance easier. However, they also have some disadvantages, such as tight coupling, shared memory, monolithic deployment, and centralized control flow. These can make the application less secure, less stable, and less flexible in the face of changing requirements or scaling needs.
While monolithic architecture has been a common and successful approach for many applications, it does have limitations, especially as applications become more complex and need to scale rapidly. In recent years, there has been a shift towards more modular and distributed architectures, such as microservices or modular monolithic architectures, to address some of the challenges posed by monolithic applications.
A monolithic architecture is illustrated in Figure 1:
A typical monolithic architecture follows the Model-View-Controller (MVC) pattern as used in Ruby on Rails or the Model-Template-View (MTV) pattern as used in Django. Figure 1 simplifies a monolithic architecture with three services, libraries, database and database schema for further discussion in the following sections.
Microservices architecture
Microservices architecture is an architectural style that structures an application as a collection of small, independent, and loosely coupled services. In a microservices architecture, each service is designed to perform a specific business function and can be developed, deployed, and scaled independently of the other services. The goal is to break down a large and complex application into smaller, more manageable components that can be developed and maintained by separate teams.
Key characteristics of microservices architecture include:
Service Independence: Microservices are self-contained and encapsulate specific business capabilities. Each service operates independently and can be developed and deployed without affecting the entire system.
Loose Coupling: Services communicate with each other through well-defined APIs (Application Programming Interfaces) and protocols, such as HTTP/REST or messaging. This loose coupling allows for flexibility, as changes in one service do not necessarily impact others.
Autonomous Development and Deployment: Microservices can be developed, tested, and deployed independently of each other. This autonomy allows development teams to work on different services simultaneously, promoting parallel development.
Technology Diversity: Each microservice can be developed using different programming languages, frameworks, and databases based on the specific requirements of that service. This technology diversity allows teams to choose the best tools for the job.
Scalability: Individual microservices can be scaled independently based on demand. This enables efficient resource utilization, as only the necessary services need to be scaled.
Resilience: Since services are independent, the failure of one service does not necessarily lead to the failure of the entire system. Microservices architectures are designed to be resilient, with services capable of functioning independently even if others are experiencing issues.
Decentralized Data Management: Each microservice manages its own data, and communication between services is typically performed through APIs. This decentralization allows for better data isolation and encapsulation.
Continuous Delivery and Deployment: Microservices support continuous integration, continuous delivery, and continuous deployment practices. This allows for faster and more frequent releases, facilitating agile development methodologies.
Ease of Scaling: Microservices make it easier to scale specific components of an application that experience increased demand. This contrasts with monolithic architectures, where the entire application is typically scaled.
Polyglot Persistence: Each microservice can use the most suitable database technology for its specific requirements. This approach is known as polyglot persistence, and it allows for better alignment with the data storage needs of each service.
While microservices architecture offers numerous benefits, it also introduces challenges, such as increased complexity in managing distributed systems, inter-service communication, and potential data consistency issues. Organizations need to carefully consider their specific requirements and the trade-offs associated with adopting a microservices architecture.
The monolithic architecture shown in Figure 1 could be decomposed to a microservices architecture illustrated in Figure 2.
Modular monolith
Modular monolithic architecture is an approach that combines the principles of modularity within the context of a monolithic application. In a traditional monolithic architecture, an entire application is developed as a single, tightly integrated unit. However, a modular monolithic architecture introduces the concept of breaking down the monolith into smaller, more manageable and loosely coupled modules.
Key characteristics of modular monolithic architecture include:
Module Organization: The monolith is organized into modules, where each module represents a distinct and cohesive set of functionalities. Each module can encapsulate related components and features.
Separation of Concerns: Modules are designed to adhere to the separation of concerns principle, meaning that each module has a well-defined responsibility or concern. For example, you might have separate modules for user interface, business logic, data access, and other concerns.
Loose Coupling: While the application is still a monolith, efforts are made to reduce tight coupling between modules. Modules interact with each other through well-defined interfaces, and dependencies are managed in a way that allows for flexibility and maintainability.
Independent Development and Deployment: Modules can be developed and deployed independently of each other. This means that updates or changes to one module do not necessarily require the entire application to be redeployed. It promotes agility in development and maintenance.
Code Reusability: The modular structure encourages code reusability. Common functionalities can be encapsulated in modules and reused across different parts of the application.
Scalability: While not as inherently scalable as microservices architectures, modular monolithic applications can still benefit from some level of scalability. Modules can be scaled independently based on demand.
Facilitates Team Collaboration: Different development teams can work on different modules simultaneously, promoting parallel development. This can be particularly beneficial in larger projects with diverse development requirements.
It's important to note that while modular monolithic architecture introduces modularity within a monolith, it does not completely eliminate the challenges associated with monolithic applications, such as limited scalability and potential deployment complexities. It represents a pragmatic compromise for organizations that want to introduce modularity without fully transitioning to a microservices or other distributed architecture. As the application evolves, some organizations may consider further decomposition into microservices if the benefits of a more distributed architecture become more apparent.
The monolithic architecture shown in Figure 1 could also be decomposed to a modular monolithic architecture illustrated in Figure 3.
Distributed monolith
A Distributed Monolith is a term used to describe a system that exhibits characteristics of both a monolithic architecture and a distributed system. It refers to a situation where a monolithic application, which is traditionally developed as a single, tightly integrated unit, is deployed and operated in a distributed environment as shown in Figure 4.
Key features of a Distributed Monolith include
Complexity:
Increased Management Overhead: Managing a distributed system with multiple interconnected components is more complex than a single monolithic application. This requires additional tools, processes, and expertise to effectively monitor, deploy, and debug issues.
Inter-Service Communication Challenges: Communication between services can be complex, requiring careful design and implementation to avoid bottlenecks, latency issues, and distributed transaction complexities.
Testing and Debugging: Testing and debugging distributed systems can be challenging due to the interconnected nature of the components and potential distributed failures.
Reduced Scalability:
Horizontal Scaling Limitations: While components can be individually scaled horizontally, the overall application may not scale as effectively as a true microservices architecture. Tight coupling between services can create bottlenecks that limit scalability.
Shared Database Challenges: Scaling a shared database can be complex and expensive, potentially becoming a single point of failure for the entire application.
Other Drawbacks:
Tight Coupling: Despite being distributed, services may still be tightly coupled, making them difficult to independently deploy, update, or evolve. This can hamper agility and innovation.
Single Point of Failure: While distributed, the system might still rely on centralized components like a shared database or message broker, creating potential single points of failure.
Increased Operational Overhead: Monitoring, logging, and tracing across multiple distributed services can be more complex and require additional tools and expertise.
The term "Distributed Monolith" highlights the tension between the benefits of distribution (such as improved fault tolerance, scalability, and resource utilization) and the challenges associated with monolithic architectures (tight coupling, difficulty in independent development, etc.).
In some cases, a system may start as a monolith and, over time, due to scaling or operational requirements, be distributed across multiple nodes. This distribution, however, may not fully address the challenges and limitations associated with a monolithic architecture. In contrast, more modern architectures, like microservices or serverless, aim to provide better solutions to the challenges of large-scale distributed systems.
For example, Figure 4 appears to be a microservices architecture, but it doesn’t follow the principles of microservices architecture.
There are many design principles for microservices, but some of the most common ones are:
Single responsibility principle: Each microservice should have a clear and well-defined responsibility and focus on doing one thing well.
Model around business domain: Microservices should be aligned with the business needs and processes, and reflect the natural boundaries and contexts of the domain.
Isolate failure: Microservices should be resilient and fault-tolerant, and avoid cascading failures across the system. They should also have mechanisms for error handling, logging, and monitoring.
Infrastructure automation: Microservices should leverage automation tools and practices for development, testing, deployment, and configuration management. They should also use containerization and orchestration technologies to simplify and standardize the deployment process.
Deploy independently: Microservices should be able to be deployed, updated, and scaled independently of each other, without affecting the availability or functionality of the system. They should also have minimal or no downtime during deployment.
Summery
This article explores the pitfalls of transitioning from monolithic architectures to distributed monoliths, highlighting the anti-pattern aspects that can hinder scalability, agility, and maintainability. It emphasizes the importance of avoiding this trap and offers guidance on designing truly modular and scalable microservices.
Key Points:
Distributed monoliths: While seemingly attractive due to their familiarity, they retain tight coupling and shared dependencies, negating the benefits of microservices.
Anti-pattern characteristics: Tight coupling, shared databases, excessive communication, and inadequate boundaries lead to complexity, inflexibility, and scalability limitations.
True microservices principles: Clear boundaries, independent deployment, loose coupling, and event-driven communication are crucial for success.
Path to true microservices: A well-defined strategy, domain-driven design, and gradual, iterative adoption are essential for a smooth transition.
By understanding the pitfalls of distributed monoliths and embracing true microservices principles, you can achieve a more agile, scalable, and maintainable architecture for your applications.
References
Building Microservices, 2nd Edition, (Chapter 3, Splitting the Monolith), Sam Newman, O'Reilly Media, Inc., Jul 2021
Monolith to Microservices, (section The Distributed Monolith, Chapter 1: Just Enough Microservices), Sam Newman, O'Reilly Media, Inc., Nov 2019
Understand the Difference Between Monolith, MicroServices, and Distributed Monolith
Your Distributed Monoliths are secretly plotting against you | by João Vazao Vasques | Medium
Distributed Monoliths vs. Microservices: Which Are You Building?
Distributed Monoliths: Recognizing, Addressing, and Preventing