Software Developers need to learn Low-Level Design not only to crack the Interviews but also to build modular, extensible, reusable, and maintainable software. It is the thought process that developers need to DEVELOP to effectively build the software from a set of requirements.

Here is the list of things that a software developer needs to learn for LLD Interviews:

• Object-Oriented Language: The general expectation in the machine coding round (or) LLD round is that candidate writes Object Oriented Code. As the name suggests, Object-Oriented Languages revolve around everything about Objects. The candidate should identify the DIFFERENT entities that need to be created from the problem statement and model the classes as per the required entities along with choosing the right set of data structures depending on the use case.  Learning any Object-Oriented Language is a must as the candidate needs to write code using that language. Some of the popular Object-Oriented Languages are Java, C#, C++, and Python.
• Object-Oriented Principles: Knowing an Object-Oriented Language itself is not sufficient for writing extensible and maintainable code. Many Object-Oriented principles are designed to make the software extensible and maintainable. Some of the well-known principles are :
  • YAGNI: You Ain't Gonna Need It is a practice in software development that states that features should only be added when required and thus help trim away excess and inefficiency development to FACILITATE the desired increased frequency of releases)
  • DRY: Don't Repeat Yourself is a principle that states that don't repeat the code again and again. If there is a code that is duplicated then whenever a change needs to be made, the change has to be done in both places. So the developer has to remember all the places where the code is duplicated which are difficult and error-prone. Thus this principle states that the code should never be duplicated and has to refactor into its method or class and all other places need to use this method/class instead of duplicating the code.
  • SOLID: These are a set of 5 principles. Single Responsibility, Open-Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion.

Apart from these, there are well-known principles such as favouring composition over inheritance, Encapsulating what varies, Striving for loosely coupled classes, etc.

• UML Diagrams: UML is an acronym that stands for Unified Modeling Language and is a standard for modelling, visualizing, and documenting the artefacts of software SYSTEMS. It is not a programming language but a tool to visually depict different types of diagrams that can be useful for building the software.
  • There are 14 different types of UML Diagrams and these 14 diagrams are classified into 2 high-level groups.
    •  Structural Diagrams: They represent the static view of the system. Class Diagram, Composite Structure Diagram, Object Diagram, Component Diagram, Deployment Diagram, Profile Diagram, and Package Diagram are part of Structural Diagrams.
    •  Behavioural Diagrams: They represent the dynamic view of the system. Activity Diagram, State Machine Diagram, Use Case Diagram, Interaction Overview Diagram, Timing Diagram, Sequence Diagram, and Communication Diagram are part of Behavioral Diagrams.

Class Diagram & Use Case Diagram are generally used in LLD rounds.

• Design Patterns: Design Patterns are typical solutions to common problems in software design. "Don't reinvent the wheel" is a well-known phrase in software engineering. If a problem is already solved, don't solve the same problem to get the same solution. Instead, use the solution to the problem. Similarly, there are a lot of common problems in software design that are solved and are given as a toolkit called Design Patterns. Design Patterns are not specific to a particular language. They are the general solutions that specify how a design problem can be solved and hence can be implemented in any language. These design patterns are proven and tested and hence make the development process smooth and easy.
• Practice LLD Questions: Try to practice as many low-level design interview questions as possible. Solving different problems gives different perspectives and thereby improves design skills. There is no right solution for design problems as opposed to data structures and algorithmic problems. So solving different problems gives different ideas and thus develops the overall thought process for solving any design problem.