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Splitwise

Keep track of shared expenses and balances with housemates, trips, groups, friends, and family.

Assumptions

1. Simplified Currency Handling: Amounts are treated as integers in micro units (e.g., cents).
2. No Authentication: No security or authentication mechanisms are in place.
3. Open Transactions: Anyone can create transactions.
4. Clearing Balances: Clearing a user’s balance resets all +ve and -ve balances to zero.
5. Immediate Processing: Transactions are processed immediately.
6. Percentage Rounding: Percentages are rounded to the nearest natural number.

Deployment Instructions

• Backend: Live Deployment
• API Documentation: ReDoc
• YouTube Video: Overview

Installation Instructions

git clone git@github.com:subhrashisdas/splitwise.git
cd splitwise
pip install -r requirements.txt
export PYTHONPATH=$(pwd)
export DATABASE_URL=sqlite:///./splitwise.db
fastapi dev main.py

Data Model

erDiagram
    User {
        int id PK
        string email
    }

    Transaction {
        int id PK
        string description
        int total_amount
        datetime deleted_at
    }

    Ledger {
        int id PK
        int from_user_id FK
        int to_user_id FK
        int amount
        int transaction_id FK
        datetime created_at
    }

    User ||--o{ Ledger : "sends"
    User ||--o{ Ledger : "receives"
    Transaction ||--o{ Ledger : "includes"

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Deployment Checklist

1. Codebase: Single version-controlled codebase.
2. Dependencies: Managed via requirements.txt or similar.
3. Configuration: Stored in environment variables.
4. Backing Services: Configured via environment variables.
5. Build, Release, Run: Automated processes and versioned artifacts.
6. Processes: Stateless and concurrency-friendly.
7. Port Binding: Listen on a port; use service discovery if needed.
8. Concurrency: Managed with process managers; conduct load testing.
9. Disposability: Quick startup/shutdown and graceful shutdown.
10. Dev/Prod Parity: Maintain similar environments; automate and test thoroughly.
11. Logs: Centralized logging; avoid local log storage.
12. Admin Processes: Run as one-off processes; automate where possible.
13. Automatic Migrations: Manual migrations to avoid unintended schema changes.

Code Review

1. Core Logic for Transactions: Modular and predictable transaction handling.
2. Rounding Handling: Rounding errors assigned to the last user in percentage splits.
3. Payload Validation: Extensive validation to prevent incorrect/malicious inputs.
4. Update Logic: Soft-delete old transactions and create new ones to maintain history.
5. Database Flexibility: Supports MySQL, PostgreSQL, and SQLite.
6. Optimistic Amount Assignment: Handles all cases including partials.

Improvements

1. Cache User Balances: Improve performance with caching.
2. Currency Handling: Support multiple currency types.
3. Sharding and Scaling: Consider for scalability.
4. Enhanced Security: Implement authentication and authorization.
5. Asynchronous Processing: Handle high request volumes efficiently.
6. Proper Transaction Management: Ensure data integrity and avoid blocking operations.
7. Advanced Error Handling: Develop robust error handling and feedback.
8. User Interface: Build a user-friendly interface for transactions and balances.
9. Comprehensive Testing: Expand test coverage for reliability.
10. Knapsack Algorithm: Optimize ledger allocation.
11. Pagination: Implement pagination for list endpoints.

User Endpoints

Create a User

Endpoint: POST /users

Request:

curl --location 'https://sd-splitwise.vercel.app/users' \
--data-raw '{"email": "test_4@gmail.com"}'

Response:

{
  "id": 5,
  "email": "test_5@gmail.com"
}

Get User Balance

Endpoint: GET /users/{user_id}/balance

Request:

curl --location 'https://sd-splitwise.vercel.app/users/1/balance'

Response:

[
  { "user_id": 3, "total_amount": 9000 },
  { "user_id": 4, "total_amount": 9000 }
]

Clear User Balances

Endpoint: POST /users/{user_id}/clear

Request:

curl --location 'https://sd-splitwise.vercel.app/users/1/clear' \
--data '{}'

Transaction Endpoints

Create a Transaction

Endpoint: POST /transactions/

Request:

curl --location 'https://sd-splitwise.vercel.app/transactions/' \
--data '{
    "description": "Office supplies",
    "total_amount": 30000,
    "split_type": "uneven",
    "computation_type": "amount",
    "from_users": [{"user_id": 1, "value": 10000}, {"user_id": 2, "value": 20000}],
    "to_users": [{"user_id": 1, "value": 12000}, {"user_id": 3, "value": 15000}, {"user_id": 4, "value": 3000}]
}'

Response:

{
  "description": "Office supplies",
  "total_amount": 30000,
  "id": 13,
  "ledgers": [
    { "amount": 10000, "from_user_id": 1, "to_user_id": 1, "transaction_id": 13 },
    { "amount": 2000, "from_user_id": 2, "to_user_id": 1, "transaction_id": 13 },
    { "amount": 15000, "from_user_id": 2, "to_user_id": 3, "transaction_id": 13 },
    { "amount": 3000, "from_user_id": 2, "to_user_id": 4, "transaction_id": 13 }
  ]
}

Create a Transaction with Even Split

Endpoint: POST /transactions/

Request:

curl --location 'https://sd-splitwise.vercel.app/transactions/' \
--data '{
    "description": "Office supplies",
    "total_amount": 30000,
    "split_type": "even",
    "computation_type": "amount",
    "from_users": [{"user_id": 1}, {"user_id": 2}],
    "to_users": [{"user_id": 1}, {"user_id": 3}]
}'

Response:

Response will depend on transaction processing logic.

Create a Transaction with Percentage Split

Endpoint: POST /transactions/

Request:

curl --location 'https://sd-splitwise.vercel.app/transactions/' \
--data '{
    "description": "Office supplies",
    "total_amount": 30000,
    "split_type": "uneven",
    "computation_type": "percentage",
    "from_users": [{"user_id": 1, "value": 60}, {"user_id": 2, "value": 40}],
    "to_users": [{"user_id": 3, "value": 30}, {"user_id": 4, "value": 70}]
}'

Response:

Response will depend on transaction processing logic.

Update a Transaction

Endpoint: PUT /transactions/{transaction_id}

Request:

curl --location --request PUT 'https://sd-splitwise.vercel.app/transactions/2' \
--data '{
    "description": "Office supplies",
    "total_amount": 30000,
    "split_type": "uneven",
    "computation_type": "amount",
    "from_users": [{"user_id": 1, "value": 10000}, {"user_id": 2, "value": 20000}],
    "to_users": [{"user_id": 1, "value": 12000}, {"user_id": 3, "value": 15000}, {"user_id": 4, "value": 3000}]
}'

Response:

{
  "description": "Office supplies",
  "total_amount": 30000,
  "id": 2,
  "ledgers": [
    { "amount": 10000, "from_user_id": 1, "to_user_id": 1, "transaction_id": 2 },
    { "amount": 20000, "from_user_id": 2, "to_user_id": 1, "transaction_id": 2 },
    { "amount": 12000, "from_user_id": 1, "to_user_id": 1, "transaction_id": 2 },
    { "amount": 15000, "from_user_id": 2, "to_user_id": 3, "transaction_id": 2 },
    { "amount": 3000, "from_user_id": 2, "to_user_id": 4, "transaction_id": 2 }
  ]
}

License

© 2024 Subhrashis Das. All rights reserved.