📧 Resilient Email Sending Service (JavaScript)

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📦 Folder Structure

• /src → Source code

  • /EmailService.js
  • /Provider1.js
  • /Provider2.js

• /test → Unit tests

  • /EmailService.test.js

• index.js → Entry/demo file to run the service

• package.json → Project metadata & scripts

• README.md → Project documentation

• .gitignore → Git ignore rules

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🚀 Features Implemented

• Retry logic with exponential backoff
  Retries failed, sends on the primary provider.

• Fallback mechanism
  Switches to a second provider if the first continues to fail.

• Idempotency
  Prevents sending the same email (by checking the unique id).

• Rate limiting
  Limits the number of emails sent per small window to avoid spamming.

• Status tracking
  Keeps a log of email attempts with status and timestamp.

• Unit tests
  Tests cover successful send, fallback, idempotency, rate limiting, and logs.

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🚀 Run locally

1️⃣ Clone the repository

git clone https://github.com/mehtashish/Email-Sending-Service.git
cd Email-Sending-Service

2️⃣ Install dependencies

npm install

▶ Running the demo

• Run the demo script:

npm start

What happens:

• Creates an EmailService instance -Sends email(s)
• Shows console logs and results

🔲 Server will run at: http://localhost:3000

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🧪 API Endpoints

• POST /email

  • Send an email.
  • Request body:

  {
  "id": "1",
  "to": "test@example.com",
  "subject": "Testing",
  "body": "This is a test email."
  }

• 📊 GET /status

  • View status logs (history of all send attempts, provider used, success/failure, timestamp).

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✅ Running Unit Tests

• Run tests:

npm test

Uses Jest to:

• Mock providers (simulate sending)
• Test key features: send, retry, fallback, idempotency, rate limiting, status tracking

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✅ Deployed live on Render

API Base URL:
https://email-sending-service-28gk.onrender.com

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🧪 How to test features

Feature	How to test
Retry & fallback	Temporarily force Provider1 to fail → POST /email and see it switch to Provider2
Idempotency	Send same email twice (same id) → second time returns { success: false, info: "Duplicate email" }
Rate limiting	Send >3 emails quickly → see { success: false, error: "Rate limit exceeded" }
Status tracking	GET /status → see list of all attempts with timestamp, success, provider

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🧰 How it works (high level)

• Tries the primary provider first.
• If it fails, it retries with exponential backoff (e.g., 1s → 2s).
• After retries, switches to the fallback provider.
• Uses Set to keep track of sent email IDs (idempotency).
• Implements simple rate limiting based on a small time window.
• Logs each attempt in a status log.

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🛠 Technologies Used

• Node.js (JavaScript)
• Jest (testing framework)

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✏ Ideas for future improvements

• Add a circuit breaker pattern
• Write logs to a file instead of console
• Add a simple queue system for high load
• Use real email providers instead of mocks