In software engineering, a monolithic application describes a single-tiered software application in which the user interface and data access code are combined into a single program from a single platform.
A monolithic application is self-contained, and independent from other computing applications. The design philosophy is that the application is responsible not just for a particular task, but can perform every step needed to complete a particular function. Today, some personal finance applications are monolithic in the sense that they help the user carry out a complete task, end to end, and are "private data silos" rather than parts of a larger system of applications that work together. Word processors are an example of a monolithic application. These applications are sometimes associated with mainframe computers.
In software engineering, a monolithic application describes a software application which is designed without modularity. Modularity is desirable, in general, as it supports reuse of parts of the application logic and also facilitates maintenance by allowing repair or replacement of parts of the application without requiring wholesale replacement.
Modularity is achieved to various extents by different modularization approaches. Code based modularity allows developers to reuse and repair parts of the application, but development tools are required to perform these maintenance functions (e.g. the application may need to be recompiled). Object based modularity provides the application as a collection of separate executable files which may be independently maintained and replaced without redeploying the entire application (e.g. Microsoft "dll" files, Sun/UNIX "shared object" files). Some object messaging capabilities allow object based applications to be distributed across multiple computers (e.g. Microsoft COM+). Service Oriented Architectures use specific communication standard/protocols to communicate between modules.
The extent to which an application is described as monolithic is dependent upon perspective. Software that is not Service Oriented may be described as monolithic even though it is Object based and can be distributed.
The original use of the monolithic term described enormous main frame applications with no usable modularity, therefore resulting in un-maintainable systems and the "software crisis."
See also 
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