**Design a Digital Wallet System**

### Introduction

In this document, we will explore the design of a digital wallet system. The goal is to understand the requirements, challenges, and architectural decisions involved in building such a system.

### Requirements

#### Functional Requirements

The core functionalities that the system must provide include:

* User registration and authentication
* Payment processing (credit/debit cards, cryptocurrencies)
* Transaction history and management
* Wallet balance and transaction updates
* Secure storage of user data and transactions

Specific use cases or scenarios involve:

* Users creating digital wallets for online shopping and services
* Merchants accepting digital payments for their products and services
* Banks and financial institutions integrating with the system to provide account linking and transfers

#### Non-Functional Requirements

In addition to functional requirements, the system must also consider non-functional attributes such as:

* Performance: handle a large number of transactions per second
* Scalability: handle increased load without significant performance degradation
* Reliability: maintain high uptime and availability
* Security: protect user data and transactions from unauthorized access or tampering

### High-Level Architecture

The system architecture is composed of the following key components:

* **User Frontend**: a web-based interface for users to create, manage, and interact with their digital wallets
* **API Gateway**: handles incoming API requests and routes them to the appropriate services
* **Payment Processor**: responsible for processing transactions and communicating with payment gateways
* **Wallet Service**: manages user wallet data, including balance, transactions, and history
* **Database**: stores user data, transaction history, and other system metadata

[High-Level Architecture Diagram]

```
          +---------------+
          |  User Frontend  |
          +---------------+
                  |
                  | (API requests)
                  v
          +---------------+
          |  API Gateway    |
          +---------------+
                  |
                  | (route requests)
                  v
          +---------------+
          |  Payment Processor  |
          +---------------+
                  |
                  | (process transactions)
                  v
          +---------------+
          |  Wallet Service   |
          +---------------+
                  |
                  | (manage wallet data)
                  v
          +---------------+
          |  Database        |
          +---------------+
```

### Database Schema

The database schema consists of the following tables and relationships:

* **users**:
	+ user_id (primary key)
	+ username
	+ password_hash
* **wallets**:
	+ wallet_id (primary key)
	+ user_id (foreign key referencing users)
	+ balance
	+ transaction_history
* **transactions**:
	+ transaction_id (primary key)
	+ wallet_id (foreign key referencing wallets)
	+ timestamp
	+ amount
	+ type (credit/debit)

Indexing strategies include:

* Index on `users.user_id` for efficient lookup and retrieval of user data
* Index on `wallets.wallet_id` for efficient lookup and retrieval of wallet data

### API Design

The system provides the following key endpoints:

* **POST /register**: creates a new user account
* **POST /login**: authenticates an existing user account
* **GET /wallets/{wallet_id}**: retrieves a specific wallet's balance and transaction history
* **POST /transactions**: initiates a new transaction
* **GET /transactions**: retrieves a list of recent transactions

Example request and response:

```
POST /register
{
  "username": "johnDoe",
  "password": "password123"
}

200 OK
{
  "user_id": 1,
  "username": "johnDoe"
}
```

OpenAPI specification for the API endpoints is included below.

```yaml
openapi: 3.0.2
info:
  title: Digital Wallet System API
  description: API endpoints for managing digital wallets and transactions
  version: 1.0.0

paths:
  /register:
    post:
      summary: Create a new user account
      requestBody:
        content:
          application/json:
            schema:
              type: object
              required:
                - username
                - password
              properties:
                username:
                  type: string
                password:
                  type: string
      responses:
        200:
          description: User account created successfully
          content:
            application/json:
              schema:
                type: object
                properties:
                  user_id:
                    type: integer
                  username:
                    type: string

  /login:
    post:
      summary: Authenticate an existing user account
      requestBody:
        content:
          application/json:
            schema:
              type: object
              required:
                - username
                - password
              properties:
                username:
                  type: string
                password:
                  type: string
      responses:
        200:
          description: User authenticated successfully
          content:
            application/json:
              schema:
                type: object

  /wallets/{wallet_id}:
    get:
      summary: Retrieve a specific wallet's balance and transaction history
      parameters:
        - in: path
          name: wallet_id
          required: true
          schema:
            type: integer
      responses:
        200:
          description: Wallet data retrieved successfully
          content:
            application/json:
              schema:
                type: object

  /transactions:
    post:
      summary: Initiate a new transaction
      requestBody:
        content:
          application/json:
            schema:
              type: object
              required:
                - wallet_id
                - amount
                - type
              properties:
                wallet_id:
                  type: integer
                amount:
                  type: number
                type:
                  type: string
      responses:
        201:
          description: Transaction initiated successfully
          content:
            application/json:
              schema:
                type: object

    get:
      summary: Retrieve a list of recent transactions
      responses:
        200:
          description: Transactions retrieved successfully
          content:
            application/json:
              schema:
                type: array
                items:
                  type: object
```

### System Flow

The system flow involves the following steps:

1. User registration and authentication through the user frontend
2. Wallet creation and initialization through the wallet service
3. Transaction initiation and processing through the payment processor
4. Transaction history retrieval and display through the wallet service

[System Flow Diagram]

```
          +---------------+
          |  User Frontend  |
          +---------------+
                  |
                  | (register/login)
                  v
          +---------------+
          |  Wallet Service  |
          +---------------+
                  |
                  | (create/init wallets)
                  v
          +---------------+
          |  Payment Processor  |
          +---------------+
                  |
                  | (process transactions)
                  v
          +---------------+
          |  Transaction History  |
          +---------------+
```

### Conclusion

In this blog post, we've explored the design and architecture of a digital wallet system. We've covered topics such as database schema, API design, and system flow. This post aimed to provide a beginner-friendly introduction to the topic, highlighting key concepts and best practices for designing and building a scalable and secure digital wallet system.