Design Payment Tracking system - Splitwise

Requirements

1. Creating Expenses

   1. Any user can log an expense, only a single user can log an expense
   2. The expense can be shared between multiple users
   3. The expense can be shared using percentage weights
   4. Subset of group users can be invloved in an expense

2. Group summary -> who owes how much to whom?

3. User summary -> How much the user has paid to whom

4. Settling of expenses can be done in a group level

5. Edit expenses is allowed

6. Minimize payment transactions is optional -> optimising this

7. Commenting on an expense

8. Creating users

9. Creating Groups

Extensible -> multiple users log an expense

assumptions

Groups and users are created -> userid1 and userid2, groupid1 Create a mock in-memory database Map<expenseId, Expense>

Actors involved

Enitites involved

Users Groups User many to many Groups userGroupRelationShip <userId, GroupId>

API Endpoints

1. Creating Expenses
   1. Any user can log an expense, only a single user can log an expense
   2. The expense can be shared between multiple users
   3. The expense can be shared using percentage weights
   4. Subset of group users can be invloved in an expense
2. Group summary -> who owes how much to whom?
3. User summary -> How much the user has paid to whom

HTTPMETHOD ROUTE REQ RESPONSE

//creating an expense
POST '/expense' {
  groupId: string;
  payers: [
    {
      userId: string;
      amount: number;
    }
  ],
  payees: [
    {
      userId: string;
      percentage?: number;
    }
  ],
} {
expenseId: string,
}

//user summary ->
GET '/user/:userid/summary' {}{
  userid: string;
  summary: {
  //   haveRecievedFrom?: [{
  //     userId: string;
  //     amount: number;
  //   }]
  //   haveOwedTo?: [{
  //     userId: string;
  //     amount: number;
  //   }]
    totalAmountOwed: number;
    totalAmountRecieved: number;
  }
}

Models

Amount {
  amount: number;
}

User {
  userId: string;
}

Group {
  groupId: string;
  expenses: Array<Expense>;
  // balance: Balance;
}

Expense {
  expenseId: string;
  payers: Array<{
    userId: string;
    amount: Amount;
  }>;
  payees: Array<{
    userId: string;
    amonut: Amount;
    percentage?: number;
  }>

}

Balance {
  userId: Array<{
    userId: string;
    amount: Amount;
  }>
}

GroupVsUserRelationship {
  groupId: string;
  userId: string;
}

npm init -y
npm i express dotenv typescript @tsconfig/node18 compression body-parser cors module-alias tstl
npm i -D typescript @types/express @types/node @types/compression @types/cors concurrently nodemon
npx tsc --init