What Is a Monolithic Application?

Monolithic applications are built as a single unit that includes all of the different components of the application, including APIs, services, databases, load balancers and so forth. Monolithic applications are designed to be self-contained: All of the aforementioned components must be present in order to deploy or compile the application. You can think of monolithic applications as the traditional way of building software.

All of the components are built into a single application, so codebases for monolithic applications tend to grow exponentially, which can result in difficulty maintaining and managing them. 

Monolithic Applications: 4 Key Components

• Client-side user interface: The user interface is the part of the application that allows users to communicate and interact with the app. 
• Business logic: This is a crucial part of the application that stands between the user interface and the database. It represents the real-world business rules that ultimately decide how the app processes data. 
• Data access layer: Between the business logic and the database, monolithic apps include what is known as a data access layer. This layer provides different methods the application uses to access the data stored in the underlying database. 
• Database: Monolithic applications typically have a single database that stores all the information that the application needs in order to function.

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How Does a Monolithic Application Work?

Monolithic applications are designed to perform every single function needed to complete a certain task. This includes everything from obtaining a user’s input to processing and storing complex data in a database. Let’s take an e-commerce monolithic application as an example.

Users interact with the application via browser and mobile clients. All network traffic generated by the client goes to either the server or a load balancer. We use a load balancer if the application needs to scale by creating multiple instances of the monolith. In the former case, the application handles traffic directly. In the latter, the load balancer handles and distributes the load between the application’s multiple instances. 

The application could contain several services, including a catalog service, an order service and a payment service. The logic contained in these services are the application’s business logic since the way these services work defines how the business operates. The services are in charge of persisting the data received from the user by way of the data access layer, which is in charge of taking the appropriate actions to ensure the data is properly stored in the database. By using a data access layer, the business layer is agnostic to the implementation details of the database. 

As to the database used to store the data, whether you’re using a relational or non-relational database depends entirely on the needs of the application, although for an e-commerce app, a relational database is preferable. 

Advantages of a Monolithic Application

• Simplicity: The main advantage monolithic apps have is that they’re easy to develop, test and debug. Monolithic architecture is the traditional way to build applications and they don’t require knowledge of complex architectural patterns. 
• Initial Development Speed: A small team (or even a single developer) can build a monolithic application from scratch fairly easily. For this reason, monolithic applications are ideal for small startups that lack a big budget, or for developing minimum viable products (MVPs). Bear in mind that monolithic architecture is quite limited regarding scalability, so as the application grows, it will become harder to both add new features while also maintaining the existing codebase. 
• Easy Deployment: Because monolithic applications are self contained and every single part of the codebase exists in the same place, deployment is quite simple. There are no dependencies to other applications. 

Disadvantages of a Monolithic Application

• Poor Scalability: The high level of coupling in monolithic applications is a great challenge to the application’s scalability. Scaling involves setting up multiple instances of the application behind a load balancer so it’s impossible to scale only a part of the application to meet different resource requirements across the app. Also, any change to a monolith affects the entire application; the whole system needs to be redeployed with every update.
• Reliability: Since the entire codebase is coupled and highly dependent, bugs and crashes can affect the whole system and even shut it down completely. 
• Coupling: The software elements that comprise the application are usually tightly coupled so they depend on each other. This means adding new features to the codebase can be difficult. Implementing new technologies, like migrating to a new database or using a new programming language, can also pose a challenge.
• Complexity: Since monoliths consist of a single codebase, the code becomes increasingly complex as the application grows and you implement new features. As complexity increases, so does maintenance cost. 

Monolithic Applications and Microservices: What’s the Difference?

Microservices consist of dividing a large application into many smaller pieces (also called services). Services are completely independent so they’re easier to maintain and modify. In other words, changes to one service won’t affect the others. This independence also enables each team to choose whichever stack suits them best to develop each service, as opposed to using the same stack for the entire application. The loose coupling between services also makes the application as a whole more reliable; even if one of the services crashes, the rest should be able to continue operating.

Monolithic Application vs. Modular Monolith Architecture

When many software architects want to move away from monolithic architecture, they decide to make the jump to microservices, but this may not always be the right choice. Despite the many benefits that microservices have regarding scalability, flexibility, reliability and more, building such a system is generally quite complex and requires investing a lot of time, not to mention having highly skilled and knowledgeable professionals.

Modular monolithic applications can be a good stepping stone toward the migration to microservices, or even as a midpoint between traditional monoliths and microservices. The modular monolith architecture involves dividing the application into several modules, each of which is independent from the others. Each module will contain its own business logic and can contain their own user interface and data source. Instead of communicating directly, these modules will interact through public APIs, which means other modules won’t have direct access to their internal functions. This low coupling between modules makes the application easier to modify and maintain, and you can even deploy modules independently if necessary, as opposed to having to deploy the entire codebase as a single unit.