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imperfect_notes/pragy/LLD 1.md

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+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
+
+-- --
+
+Example
+-------
+
+Website - webpage, application code, DB
+Fetch data, persist
+
+**HLD**
+- ![186867fa.png](:storage/f62e8f9f-4c32-453b-95d6-8e01da320ef2/186867fa.png =400x)
+- 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
+    ```python
+        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)
+
+-- --
+
+**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
+
+-- --
+
+Example:
+
+Template / Blueprint for a Bird
+-------------------------------
+
+```python
+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.
+
+-- --
+
+Constructing an instance / object
+---------------------------------
+
+- invoke the constructor of the class
+- 
+- ```python
+
+```