Monolithic architecture is an all-in-one methodology that encapsulates all the required services as a single deployable artifact. It works on a single technology stack and is deployed and scaled as a single unit. Since there is only one technology stack to master, it is easy to deploy, scale, and set up a monitoring infrastructure for monolithic applications. Each team member works on one or more components of the system and follows the design principle of Separation of Concerns (SoC) (https://en.wikipedia.org/wiki/Separation_of_concerns). Such applications are also easier to refactor, debug, and test in a single standalone development environment.
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Applications based on monolithic architecture may consist of one or more deployable artifacts that are all deployed at the same time. Such a monolithic architecture is often referred to as a Distributed Monolith.
For example, a very common monolithic application is a word processing application; Microsoft Word is installed via a single deployable artifact and is entirely built on Microsoft .NET Framework (https://www.microsoft.com/net/). There are various components within word processing application, such as templates, import/export, spell-checker, and so on, that work together to help create a document and export it the format of choice.
Monolithic architecture applies not only to standalone applications, but also to client-server based applications that are provided as a service over the web. Such client-server based applications have a clearly defined multitier architecture that provides the relevant services to its end users via a user interface.
The user interface talks to application endpoints that can be programmed using well-defined interfaces.
A typical client-server application may adopt a three-tier architecture to separate the presentation, business logic, and persistence layer from each other, as shown in the preceding diagram. Components of each layer talk strictly to the components of the layer below them. For example, the components of the presentation layer may never talk to the persistence layer directly. If they need access to data, the request will be routed via the business logic layer that will not only move the data between the persistence layer and the presentation layer, but also do the required processing to serve the request. Adopting such a component-based layered architecture also helps in isolating the effect of change to only the components of dependent layers instead of the entire application. For example, changes to the components of the business logic layer may require a change in the dependent components of the presentation layer but components of the persistence layer may remain intact.
Even though a monolithic application is built on SoC, it is still a single application on a single technology stack that provides all required services to its users. Any change to such an application requires to be compatible with all the encapsulated services and underlying technology stack. In addition to that, it is not possible to scale each service independently. Any scaling requirement is met by deploying multiple instances of the entire system as a single unit. A team working on such a monolithic application scales over time and has to adapt to newer technologies as a whole, which is often challenging due to the rapidly changing technology landscape. If they do not change with the technology, the entire software becomes obsolete over time and is discarded due to incompatibility with newer software and hardware, or a shortage of talent.