Lecture_Notes/imperfect_notes/pragy/LLD 1.md

LLD 1

• What is the Lowest Level?
  • Code
  • Not feasible to communicate at this level
  • Large systems can have hundreds of thousands of LOC
• if the interviewer asks you to design amazon, he doesn't expect you to write code. He first expects the HLD/LLD
• High Level Design
  • very high level view
  • still allows you to see that there are some components and the way things operate
• Low level design
  • closer look, but still higher than code.
  • behavior / skeleton, not implementation

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Example

Website - webpage, application code, DB Fetch data, persist

HLD

• 
• doesn't worry about how the data is fetched, what the DB schema is, what tables exist, what the tech stack is

LLD

• not the actual code. Higher level than code
• Consider a function that adds two numbers

      def add(a: int, b: int) -> int: ...
  

  LLD just tells you that the function takes 2 args and returns the sum. Doesn't bother about the actual implementation
• LLD talks about entities (classes), their interactions, and their properties (attributes) and behavior (functions)

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Procedural Code

• top-down
• no obvious segregation of responsibility
• no one-to-one mapping to real world entities

Object Oriented

• map any real world entity to a code entity (class)
• list down the properties/attributes of each entity
• list down the actions/behavior/methods of each entity
• list down the interactions b/w various entities

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Example:

Template / Blueprint for a Bird

class Bird:
    weight: float
    height: float
    color: Color
    
    def fly(): ...

This template does not signify one particular bird. It describes a class of a Bird. A specific bird will be represented by an instance of this class.

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Constructing an instance / object

• invoke the constructor of the class