> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# HLD vs LLD Where This Course Lives
> Explains differences between high-level and low-level system design, focusing on HLD concepts, components, data flow, scaling, and contrasting with LLD implementation details.
In system design discussions you'll repeatedly encounter two complementary perspectives: High-Level Design (HLD) and Low-Level Design (LLD). Both matter, but they answer different questions and are used in different contexts.
High-level design is the zoomed-out blueprint. It identifies major components—application servers, databases, caches, queues, CDNs—and maps how requests flow between them. When interviewers ask you to "design Instagram," they usually expect an HLD: how requests travel, where data lives, how the system scales under load, where single points of failure exist, and how to recover.
Low-level design zooms into a single component or feature. It describes implementation details: function responsibilities, data structures, APIs, and edge-case handling. Examples of LLD interview prompts include "Design the Like feature" or "Implement an in-memory cache." These questions probe the detailed code structure, algorithms, and performance trade-offs.
This lesson focuses on High-Level Design—component interaction, system behavior under load, and scaling strategies. We deliberately avoid internal implementation details for each component in this course.
To make the difference concrete, consider a photo-sharing app:
* HLD: Show where the web/mobile clients connect, which frontends and backend services handle requests, where the image store and metadata DB live, any caches or message queues, and how to scale or replicate components.
* LLD: Cover the exact steps when a user taps "like": which API is called, how to enforce idempotency, how to update counters atomically, and how to design cache keys and TTLs.
Comparison at a glance:
| Aspect | High-Level Design (HLD) | Low-Level Design (LLD) |
| ------------------ | -------------------------------------------------------------- | ----------------------------------------------------------------------- |
| Scope | System architecture, major components, data flow | Component internals, functions, classes, data structures |
| Questions answered | "Where do requests go?" "How does the system scale?" | "What happens when this API is called?" "How to avoid double-counting?" |
| Artifacts | Architecture diagrams, component diagrams, deployment topology | Sequence diagrams, interface definitions, code snippets, pseudocode |
| Interview examples | "Design Instagram", "Design a scalable messaging system" | "Design the Like feature", "Implement an LRU cache" |
| Deliverables | Service boundary definitions, scaling and redundancy plan | API design, algorithms, concurrency control, optimizations |
Below is a short LLD-style example showing the kind of code-level detail we intentionally do not cover in this HLD-focused course. It illustrates cache lookups, database checks, and count updates for a "like" action. Note: this is illustrative only.
```javascript theme={null}
import { db, cache } from "./store";
async function handleLike(userId, photoId) {
if (await hasLiked(userId, photoId)) return;
await saveLike(userId, photoId);
await bumpCount(photoId);
}
async function hasLiked(userId, photoId) {
// Check cache first for speed
const key = `likes:${userId}:${photoId}`;
const cached = await cache.get(key);
if (cached !== null) return Boolean(cached);
// Fallback to the database
const exists = await db.likes.exists({ userId, photoId });
// Cache the result for a short time
await cache.set(key, exists ? 1 : 0, { ttl: 300 });
return exists;
}
```
Further reading and references
* [Kubernetes Basics](https://kubernetes.io/docs/concepts/overview/what-is-kubernetes/)
* [Designing Data-Intensive Applications (book)](https://dataintensive.net/)
* [System Design Primer (GitHub)](https://github.com/donnemartin/system-design-primer)