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Rules to Better Modular Monoliths - 3 Rules

A Modular Monolith is an architectural style in software development that emphasizes modularity within a monolithic application structure.

  1. Do you know the Modular Monolithic architecture?

    A Modular Monolith is a software architecture pattern that combines elements of both monolithic and modular architectures. In this approach, the application is built as a single, unified codebase like a traditional monolith, but it is designed and organized in a modular manner, allowing for logical separation of different components or modules within the codebase.

    The Modular Monolith architecture is the “goldilocks” approach that combines the modularity of microservices with the simplicity of traditional Monoliths

    • Steve “Ardalis” Smith

    Modular Monolith characteristics

    modular monolith
    Figure: Modular Monolith architecture

    • Single Host/Process
    • Single Deployment
    • Loosely coupled modules that each have their own

      • Domain
      • Application
      • Infrastructure
      • Presentation (API or UI)
    • Each module represents a business capability or domain
    • Each module should be as highly cohesive and loosely coupled with other modules
    • Each module manages it's own data and persistence

    ✅ Advantages

    • Simplicity in Deployment - Since it's a monolith, the deployment is typically simpler than distributed systems like microservices
    • Ease of Development - Developers can work on separate modules without significantly affecting other parts of the application
    • Performance - Inter-module communication is often faster and more reliable than inter-service communication in distributed architectures

    ❌ Challenges

    • Scalability - While more scalable than a traditional monolith, it may not scale as effectively as microservices
    • Modular Discipline - Maintaining strict modularity can be challenging as the application grows and evolves

    A Modular Monolith offers a balance between the simplicity and coherence of a monolith and the modularity and maintainability of more distributed architectures. It is particularly useful for certain kinds of applications and organizational contexts.

    Modular Monolith compared to other architectures

    Trade-OffsLayered / CAMicroservicesModular Monolith
  2. Do you use MassTransit to build reliable distributed applications?

    When building distributed applications messaging is a common pattern to use. Often we might take a hard dependency on a specific messaging technology, such as Azure Service Bus or RabbitMQ. This can make it difficult to change messaging technologies in the future. Good architecture is about making decisions that make things easy to change in future. This is where MassTransit comes in.

    MassTransit is a popular open-source .NET library that makes it easy to build distributed applications using messaging without tying you to one specific messaging technology.

    .NET Messaging Libraries

    There are several .NET messaging libraries that all abstract the underlying transport. These include:

    There are also the service bus specific libraries:

    Advantages of using MassTransit

    ✅ Open-source and free to use

    ✅ Enables swapping of messaging transports by providing a common abstraction layer

    ✅ Supports multiple messaging concepts:

    • Point-to-Point
    • Publish/Subscribe
    • Request/Response

    ✅ Supports multiple messaging transports:

    • In-Memory
    • RabbitMQ
    • Azure Service Bus
    • Amazon SQS
    • ActiveMQ
    • Kafka
    • gRPC
    • SQL/DB

    ✅ Supports complex messaging patterns such as Sagas


    Scenario 1 - Modular Monolith

    A Modular Monolith architecture requires all modules to be running in a single process. MassTransit can be used to facilitate in-memory communication between modules in the same process.

    This allows us to send events between modules and also request data from other modules.

    Scenario 2 - Azure Hosted Microservices

    When building microservices in Azure, it's common to use Azure Service Bus as the messaging transport. With minimal changes, MassTransit can be used to send messages to and from Azure Service Bus instead of using the In-Memory transport.

    Scenario 3 - Locally Developing Microservices

    When developing microservices locally, it's common to use containers for each service. However, some of the cloud based messaging services (e.g. Azure Service Bus) are not able to be run in a container locally. In this scenario, we can easily switch from using the Azure Service Bus transport to Containerized RabbitMQ transport

    Demo Code

    If you're interested in seeing MassTransit in action, check out

  3. Do you know how to choose the best software architecture for your system?

    Choosing the right software architecture for your system is crucial for its success and maintainability. Making the wrong choice can lead to increased complexity, difficulty in scaling, and higher costs.

    Here are some of the popular architectures and factors to consider when deciding the best fit for your project:

    Clean Architecture

    Clean Architecture emphasizes separation of concerns, making your system easier to maintain and scale. This architecture is designed to keep the business logic independent of the frameworks and tools, which helps in achieving a decoupled and testable codebase.

    See more on Rules to Better Clean Architecture.

    You can find our CA template on GitHub

    Vertical Slice Architecture

    Vertical Slice Architecture structures your system around features rather than technical layers. Each feature is implemented end-to-end, including UI/API, business logic, and data access. This approach improves maintainability and reduces the risk of breaking changes.

    This modular approach to software development can introduce inexperienced teams to the idea of shipping features as functional units with no shared knowledge of the domain entities, infrastructure layer, or application layer within another subsystem, further preparing them for future develeopment environments that may use Modular Monolith or Microservices Architecture.

    You can find our VSA template on GitHub

    Modular Monolith

    A Modular Monolith organizes the system into modules that encapsulate specific functionalities. While it runs as a single application, it retains some benefits of microservices, such as independent module development and testing. It’s a good middle-ground between a monolith and microservices.

    See more on Rules to Better Modular Monoliths.


    Microservices architecture involves splitting the application into small, independently deployable services. Each service focuses on a specific business capability and can be developed, deployed, and scaled independently. This approach is beneficial for complex and large-scale applications with multiple teams working on different parts.

    See more on Rules to Better Microservices.

    Architecture Decision Tree

    architecture decision tree v2
    Architecture Decision Tree

    It's important to keep in mind that these architectures are not mutually exclusive.

    Within a Modular Monolith Architecture, each module could be implemented using Clean Architecture or Vertical Slice Architecture. Similarly, a Microservices Architecture could use Clean Architecture or Vertical Slice Architecture within each service.

    Also, from a pragmatic point of view a combination of Modular Monolith and Microservices might provide the best of both worlds. The majority of the system could be implemented as a Modular Monolith, with a few key services implemented as Microservices to provide scalability and flexibility where needed.

    Factors to Consider

    • Are your requirements certain?
      If requirements are likely to change, Clean Architecture or Vertical Slice Architecture can offer more flexibility.
    • Do you have multiple domains?
      For applications with multiple domains, Modular Monoliths or Microservices can provide better separation and modularity.
    • Do you have many teams? If you have many teams, Microservices or Modular Monolith can help in reducing inter-team dependencies and allow parallel development.
    • Do you need independent deployments? If independent deployments are necessary, Microservices is the best choice due to its isolated nature.
    • Do you need independent scalability? Microservices allow each service to be scaled independently based on its specific needs, which can be more efficient and cost-effective.
    • Do you have DevOps maturity? Microservices require a mature DevOps culture to manage deployments, monitoring, and scaling effectively. Without this, the overhead can be overwhelming.
    • Is the team experienced? The complexity of Microservices can be challenging for less experienced teams. Vertical Slice Architecture although simple, has fewer guardrails when compared to Clean Architecture and can lead to a mess if not managed correctly. This leads to recommending Clean Architecture for less experienced teams that need more structure.


    Here are some practical scenarios to illustrate the decision-making process:

    Scenario 1: Startup with uncertain requirements

    You are building an MVP with a small team and expect the requirements to evolve rapidly.

    Choice: Clean Architecture or Vertical Slice Architecture - These architectures offer flexibility and are easier to refactor as requirements change.

    Scenario 2: Medium-sized business with limited DevOps maturity

    You have a mid-sized team, and your organization is still developing its DevOps practices.

    Choice: Modular Monolith - A Modular Monolith provides some modularity benefits without the full complexity of Microservices, making it easier to manage with limited DevOps capabilities.

    Scenario 3: Large enterprise with multiple domains and teams

    You are developing a large-scale application with multiple business domains and have several teams working in parallel.

    Choice: Microservices - Microservices allow independent development, deployment, and scaling, which suits large and complex applications.

    By carefully considering these factors and understanding the strengths and limitations of each architectural style, you can choose the best architecture for your system, ensuring a balance between flexibility, scalability, and maintainability.

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