What Is Low-Level Design (LLD)? With Example

What Is Low-Level Design (LLD)? With Example

Low-Level Design (LLD) is the detailed design. It digs into each component so a developer can code without asking more questions.

What LLD Contains

• Database schemas: collections, fields, types, indexes, relationships.
• API contracts: endpoints, HTTP method, request body, response shape, status codes.
• Internal logic: what each handler does, step by step.
• Error cases: validation errors, not found, internal errors.
• Edge cases: empty states, duplicates, race conditions.

Example: DevTinder Signup LLD

Collection: User

• _id: ObjectId
• firstName: String, required
• lastName: String, required
• email: String, required, unique, lowercase
• passwordHash: String, required
• age: Number, min 18
• gender: String, enum ['male', 'female', 'other']
• photoUrl: String, default ''
• createdAt: Date, default Date.now
• updatedAt: Date, default Date.now

Index: unique on email.

Endpoint: POST /signup

• Request body: { firstName, lastName, email, password, age, gender }
• Validation: email format, password length >= 8, age >= 18, gender in enum.
• Logic:
  1. Validate body with zod. If invalid, return 400 with errors.
  2. Check if user with email exists. If yes, return 409.
  3. Hash password with bcrypt (10 rounds).
  4. Create user in MongoDB.
  5. Send welcome email via SES (async, do not block).
  6. Return 201 with { id, firstName, lastName, email }.
• Errors: 400 (validation), 409 (email exists), 500 (internal).

Edge cases:

• Email passed in uppercase. Normalize to lowercase before checking.
• Race condition: two signups with same email at the same time. The unique index saves you; one will fail with a duplicate key error.

LLD Goes Deep

For every API, you write the schema, the contract, the logic, the errors, and the edge cases. LLD is long. HLD is short.

Why LLD Matters

LLD means any developer can pick up the doc and code. No questions, no meetings. Handoffs work. Estimates work.

The Takeaway

LLD shows the detailed design: schemas, API contracts, internal logic, errors, and edge cases. It is enough for a developer to code without asking more questions. HLD is the big picture; LLD is the real spec.