LLD for refer or referal code service

Below is LLD (Low Level Design) for Referral / Refer Code Service suitable for .NET / Java / Node / Microservice / MVC architecture.
This design is production-level and can be used in interview + real project.

1. Requirements (Understand first)

Functional Requirements

• User can generate referral code
• User can share referral code
• New user can signup using referral code
• Referrer gets reward / points / discount
• Referral code should be unique
• One user can use only one referral code
• Referral code expiry support (optional)
• Track referral history

Non-Functional Requirements

• High performance
• Unique code generation
• No duplicate reward
• Secure validation
• Scalable

2. High Level Flow

User A registers
   ↓
Referral Code generated
   ↓
User B signup using code
   ↓
Code validated
   ↓
Referral record created
   ↓
Reward added

3. Database Design (Tables)

3.1 Users

Users
------
Id (PK)
Name
Email
Password
ReferralCode
CreatedDate

3.2 ReferralCodes

ReferralCodes
--------------
Id (PK)
UserId (FK)
Code
IsActive
ExpiryDate
CreatedDate

3.3 Referrals

Referrals
-----------
Id (PK)
ReferrerUserId
ReferredUserId
ReferralCode
RewardGiven
CreatedDate

3.4 Rewards

Rewards
---------
Id
UserId
Points
Type
CreatedDate

4. Class Design (LLD)

4.1 ReferralService

ReferralService
---------------
GenerateCode(userId)
ValidateCode(code)
ApplyReferral(code, newUserId)
GiveReward(referrerId)
GetReferralHistory(userId)

4.2 CodeGenerator

CodeGenerator
-------------
GenerateUniqueCode()

Example logic

REF + UserId + Random

Example

REF1023AB12

4.3 ReferralRepository

ReferralRepository
-------------------
CreateCode()
GetByCode()
SaveReferral()
GetUserReferrals()

4.4 RewardService

RewardService
--------------
AddPoints(userId)
AddCoupon(userId)
AddCashback(userId)

5. Sequence Flow (LLD)

Signup with referral

Controller
   ↓
ReferralService.ValidateCode()
   ↓
ReferralRepository.GetCode()
   ↓
ReferralService.ApplyReferral()
   ↓
RewardService.AddPoints()
   ↓
Save Referral

6. API Design

Generate Code

POST /api/referral/generate

Response

{
  code: "REF123ABC"
}

Apply Referral

POST /api/referral/apply

Request

{
  userId: 10,
  code: "REF123ABC"
}

Get History

GET /api/referral/history/{userId}

7. Code Example (.NET style)

Interface

public interface IReferralService
{
    string GenerateCode(int userId);
    bool ApplyReferral(string code, int newUserId);
}

Implementation

public class ReferralService : IReferralService
{
    public string GenerateCode(int userId)
    {
        return "REF" + userId + Guid.NewGuid().ToString("N").Substring(0,4);
    }

    public bool ApplyReferral(string code, int newUserId)
    {
        var referrer = repo.GetByCode(code);

        if(referrer == null)
            return false;

        repo.SaveReferral(referrer.UserId, newUserId);

        rewardService.AddPoints(referrer.UserId);

        return true;
    }
}

8. Important Interview Points

• Idempotent reward
• Unique code
• Concurrency handling
• Transaction required
• Cache for validation
• Rate limit
• Expiry support
• Fraud prevention

9. Advanced Design (Senior Level)

Add:

• Redis cache for code
• Message Queue for reward
• Event driven reward

Microservice

ReferralService → Event → RewardService