Rules to Better Modular Monoliths - 2 Rules

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

• 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-Offs	Layered / CA	Microservices	Modular Monolith
Modularity	❌	✅	✅
Cost	$	$$$	$
Scalability	❌	✅	❌
Simplicity	✅	❌	✅

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:

• MassTransit (recommended)
• NServiceBus
• Rebus

There are also the service bus specific libraries:

• Azure Service Bus(not recommended)
• Amazon SQS(not recommended)
• RabbitMQ(not recommended)
• (and more)

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

Scenarios

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 github.com/danielmackay/dotnet-mass-transit