As modern applications evolve, developers and organizations face a crucial decision: whether to adopt a monolithic, modular monolith, or microservices architecture. Each approach has its own strengths and weaknesses.

This article explores the fundamental concepts of monolithic, modular monolith, and microservices architectures, comparing their characteristics, benefits, and challenges to help you make an informed decision.

What is Monolithic Architecture?

A monolithic architecture is a software design pattern where an entire application is built as a single, unified unit. All components, including the user interface, business logic, and database interactions, are tightly integrated into a single codebase and deployed together.

Key Characteristics of Monolithic Architecture:

1. Single Codebase – The entire application is built and maintained as one unit.
2. Unified Deployment – The application is deployed as a single executable or package. The build and release pipelines would contain stages to build entire application, run all tests and deploy whole application. Partial application deployment is not possible.
3. Shared Database – A single database is used to store all application data.
4. Tightly Coupled Components – All modules interact directly within the same codebase.
5. Centralized Scaling – The entire application must be scaled together rather than individual components.

Advantages of Monolithic Architecture

✅ Simplicity – Easier to develop, test, and deploy compared to microservices. All internal execution paths ( i.e. execution paths within the same application) can be traced easily from the code. =

✅ Faster Development – Single codebase allows for rapid development as compared to Microservices architecture.

✅ Easier Debugging – Logs and errors are centralized, making debugging straightforward. Also, you can run the whole application together locally in debugging mode and debug each and every line from the execution path. This helps in reducing the troubleshooting time needed when any issue is reported.

✅ Better Performance – Less network latency compared to microservices since all components communicate within the same process. There is no extra HTTP calls or there is no Azure Service Bus or RabbitMQ.

✅ Lower Operational Overhead – No need for service orchestration or complex infrastructure management.

Challenges of Monolithic Architecture

• No Partial Scalability – The entire application must be scaled as a whole. So if application has some specific parts which has more usage than the other parts, then there is no option to use higher coputing power for the heavily used part. You need to scale up the whole system, thus it may result in inefficient use of infrastructural resources.
• Always Whole Application Deployment – Even a minor change requires redeploying the entire application. So for deploying small changes, the deployment process may seem longer.
• Technology Lock-in – Difficult to adopt new technologies. Because it is one big application, it is hard to take a step by step approach to upgrade technology – only whole application at once should be migrated.
• Harder to Maintain Over Time – As the application grows, the codebase can become large and may be come difficult to manage.

What is Modular Monolith Architecture?

A modular monolith is an architectural style that maintains a monolithic deployment while enforcing modularity within the codebase. This means that the application consists of independent modules that interact through well-defined interfaces but are still deployed as a single unit. The database maybe shared, or there may be multiple databases as well.

Key Characteristics of Modular Monolith:

1. Modular Code Structure – The application is divided into distinct, reusable modules. Those modules are abstracted behind appropriate interfaces.
2. Single Deployment Unit – The entire application is still deployed together.
3. Clear Boundaries – Modules have well-defined interfaces to communicate while minimizing coupling.
4. Shared Database – Modules can share a database, but well-defined boundaries prevent direct table access across modules.
5. Easier Transition to Microservices – A modular monolith can be gradually refactored into microservices if needed.

Advantages of Modular Monolith Architecture

✅ Maintains Simplicity – Retains the benefits of monolithic development while improving the overall structure. Like Monoliths, here as well, all the code is at one place.

✅ Improved Maintainability – The application is divided into multiple modules, each having clearly distinguished responsibility. These clear module boundaries make it easier to manage and refactor code.

✅ Better Scalability Than Monoliths – Here, although it is monolith, the modular structure allows extracting a part of application and converting into a microservice. So, if some parts of application are used heavily than others, they can be extracted as separate service and thus it can scale independent of rest of the application.

✅ Faster Deployment Than Microservices – No need for complex service orchestration.

✅ Smooth Transition to Microservices – Well-structured modules can be extracted as independent services over time.

Challenges of Modular Monolith Architecture

• Still a Single Deployment Unit – A failure in one module can still impact the entire application.
• Requires Strong Architectural Discipline – Developers must enforce module boundaries to avoid tight coupling. These boundaries can be enforced via code reviews, coding standards and guidelines for teams.
• Scaling Limitations – Though better than a traditional monolith, a modular monolith still has constraints on independent scaling.

Can we scale modules independently without converting to Microservices ?

Well, technically it is possible ! Ultimately if your server side code is in form of RESTful APIs, each module will have its own URL. So we can:

• Deploy the server side modules on multiple servers (full application). For simplicity, let’s assume there are only two server, one with higher configurations and other with lower.
• Then we can identify the URLs for modules which need higher computing resources.
• Then we can setup load balancer, which will perform filtering based on incoming request URLs. It will allow incoming requests to reach to high end server if they are for modules which need higher end computing resources. Otherwise the requests would be routed to other server with relatively lower computing power.

What is Microservices Architecture?

Microservices, or the Microservices Architecture, is an architectural style where an application is structured as a collection of small, independent services that communicate with each other via APIs. Each service is designed to perform a specific function and can be developed, deployed, and scaled independently.

Key Characteristics of Microservices:

1. Independent Deployment – Each microservice can be deployed separately without affecting the entire application. This is very important criteria and if it is needed for business reasons, then only Microservices architecture is generally useful. I mean, if you anyway want to deploy all services together, then that you can do with Modular monolith as well.
2. Single Responsibility Principle (SRP) – Each service is responsible for a single distinct functionality.
3. Loosely Coupled Services – Microservices interact through APIs, reducing dependencies between them.
4. Scalability – Individual microservices can be scaled as needed.
5. Resilience – Failure in one service does not necessarily bring down the entire application. This is one important advantage that we get with Microservices. If one part of application fails, it would not necessarily mean that your whole application would not be accessible.

Advantages of Microservices

✅ Scalability – Services can be scaled independently based on demand.

✅ Faster Development – Teams can work on different microservices simultaneously. As long as tasks are independent and there is no dependency on data from any other microservice, the development team should be able to continue their development indepedently.

✅ Flexibility – Different microservices can use different technologies. Generally, in real world, no department would want different services written in different technologies, for many reasons. But just in case, if it is needed for any weird reasons, it is possible with Microservices architecture.

✅ Resilience – If one service fails, the rest of the system remains functional.

✅ Easier Maintenance – Code is more manageable and less complex.

Challenges of Microservices

• Complex Deployment – Requires orchestration tools like Kubernetes, docker, helm charts, etc. If your team is not expert in these tools, then it may result in some learning curve.
• Complex Debugging / Troubleshooting – Monoliths can be run on single machine easily. Running all application (which is Microservices based) on your machine may be a big headache, if each service has its own database and other dependencies. Although it should be possible with docker images, it is not straightforward and it may be time-consuming. Also, if proper logging ins not in place it is very hard to figure out what is happening while troubleshooting the issue. Bottomline, you should always review logging to ensure you get appropriate info from logs if any issue is reported. This is generally true, but in case of Microservices, you may end up spending a lot of time in troubleshooting if logs are not present.
• Inter-Service Communication Overhead – More network calls can impact performance. Also, sometimes inter-service communication is synchronous and sometimes it is asynchronous. If it is asynchronous, then it may be hard to figure out what went wrong and when it went wrong if events are not properly logged.
• Data Consistency – Managing transactions across multiple services is difficult. Many times, the microservices keep local copy of data (i.e. view cache) in order to function properly. And these local copies are updated based on some asynchronous mechanism. But if that asynchronous mechanism is silently failing without showing any alerts in application, there is high chance that user may see stale data in some parts of application.
• Security Risks – More services mean more surface area for attackers – meaning more possibility of security attacks.
• Increased Operational Overheads – Deployments and Debugging is complex, it is already discussed in first two points. In addition to that, some regular activities like package upgrade or upgrading to latest technology can be very challenging and time-consuming. In case of Monoliths, for example, package upgrade can be very easy activity. Just upgrade packages in one go and commit. But in case of Microservices, teams are working independently. So they may prioritize based on their backlog. Different versions of package may fail inter-service communication so analysis may be needed to ensure that services are not failing. Also, if same package is referred in multiple services, separate efforts are needed to upgrade the same package in all of those services if teams are independent or technologies are different. These are just two examples of how simple looking activities from monolith world can grow in size when you go for Microservices.

Monolithic vs. Modular Monolith vs. Microservices: A Comparison

Feature	Monolithic Architecture	Modular Monolith	Microservices Architecture
Deployment	Entire application deployed together	Entire application deployed together	Each service deployed independently
Scalability	Entire application must be scaled together	Some modules can be optimized	Individual services can scale independently
Development	Easier for small teams, but can become complex	Structured with clear module boundaries	Teams can work on separate services independently
Technology Stack	Usually a single technology for the entire app	Mostly single technology but modular	Different services can use different technologies
Maintenance	Becomes challenging as the application grows	More maintainable due to module separation	Easier to update and maintain modular services

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Conclusion

Choosing between a monolithic, modular monolith, or microservices architecture depends on your project’s requirements. A modular monolith offers a middle ground between the simplicity of monoliths and the flexibility of microservices, making it a great choice for teams looking for scalability without the operational complexity of microservices.

Nothing is silver bullet. Every approach has its own challenges. Microservice is not the future of development for sure. It is suitable for some types of project but does not necessarily mean that every product should be developed by using Microservices pattern. Same argument can be done for monoliths and modular monoliths.

Generally, it may be good idea to start with modular monolith and evaluate periodically about if you really want Microservices.

Are you working with a monolithic, modular monolith, or microservices application? Do you share same thoughts ? Share your experiences / thoughts in the comments!