High-Level Design (HLD) vs Low-Level Design (LLD): What's the Difference

High-Level Design (HLD) vs Low-Level Design (LLD)

Before building a backend, you design it. Two design phases matter: HLD and LLD. Skipping either leads to confusion.

What Is High-Level Design (HLD)

HLD is the big picture. It shows the system at a glance:

• The major components (auth, user, feed, chat services).
• How they talk to each other.
• External services (database, cache, email provider).
• Tech stack choices.

HLD is a diagram anyone can understand in 30 seconds.

What Is Low-Level Design (LLD)

LLD is the detail. It digs into each component:

• The exact database schemas and fields.
• The API contracts (endpoints, request/response shapes).
• The internal logic and algorithms.
• Error handling and edge cases.

LLD is what a developer needs to write code without asking more questions.

HLD Example: DevTinder Auth Module

HLD says: "Auth module uses Express routes, bcrypt for hashing, JWT for tokens, and an httpOnly cookie for storage. User data lives in MongoDB." Just the outline.

LLD Example: DevTinder Auth Module

LLD says: "POST /signup with {email, password}. Validate email format and password length. Hash password with bcrypt (10 rounds). Create user document. Return 201 with user id. POST /login with {email, password}. Find user by email. Compare hashed password. If match, sign JWT with userId, expire in 7 days. Set cookie httpOnly, secure, sameSite lax. Return 200. POST /logout clears cookie. Return 200." Concrete contracts.

When to Write Each

Write HLD before LLD. HLD settles the architecture; LLD settles the implementation. For small projects, both can fit in one doc. For bigger projects, split them.

The Takeaway

HLD is the big picture: components, data flow, tech stack. LLD is the detail: schemas, API contracts, logic. Write HLD first, then LLD. Skipping either leads to confusion and rework.