Simple URL Availability Checker

A robust, TypeScript-based utility to monitor the availability of URLs and send alerts via Email or AWS SNS when availability changes.

Features

• Multi-protocol support: Checks both HTTP and HTTPS URLs.
• Configurable Intervals: Set sleep time between checks and alert intervals.
• Modular Alerting System:
  • Email: Send alerts via SMTP (Gmail, etc.).
  • AWS SNS: Publish alerts to an SNS topic.
• Structured Logging: Uses winston for clean, informative logs.
• TypeScript: Typed, maintainable codebase.

Algorithm

The checker operates on a simple state machine logic for each URL:

1. State Tracking: Each URL maintains a state of either normal (200 OK) or alerted (Down / Non-200).
2. Check Loop: The system fetches the URL.
   • If the response code is 200 and the current state is alerted, it transitions back to normal (Recovery).
   • If the response code is not 200 (or timeout) and the current state is normal, it transitions to alerted (Failure).
3. Alerting: Alerts are triggered only on state transitions.
   • To prevent spam, alerts are throttled by a configurable INTERVAL. If a transition happens but the last alert was sent too recently, the alert is skipped (but state is still updated).

Prerequisites

• Node.js (v14 or higher)
• npm or yarn

Installation

1. Clone the repository.
2. Install dependencies:

   npm install

Configuration

The application is configured using environment variables. You can set these in your shell or use a .env file loader (not included by default, but easy to add).

General Settings

Variable	Default	Description
URLS	https://www.google.com https://www.amazon.com	Space-separated list of URLs to monitor.
TIMEOUT	1000	Request timeout in milliseconds.
SLEEP	15	Sleep time (seconds) between checking all URLs.
INTERVAL	300	Minimum interval (seconds) between repeat alerts for the same URL.

Email Alerter Settings

Variable	Description
EMAIL_USER	SMTP username (e.g., your gmail address).
EMAIL_PASS	SMTP password (or App Password).
EMAIL_TO	Recipient email address.
EMAIL_SUBJ	(Optional) Subject line for alerts.

AWS SNS Alerter Settings

Variable	Description
AWS_ACCESS_KEY_ID	AWS Access Key.
AWS_SECRET_ACCESS_KEY	AWS Secret Key.
AWS_REGION	AWS Region (e.g., us-east-1).
AWS_TOPIC_ARN	The ARN of the SNS topic to publish to.

Usage

Start the application:

npm start

Running Tests

The project uses Jest for automated unit testing.

npm test

This will run tests for:

• Network utility
• Checker logic (state transitions, alerts)
• Alerter implementations (Email, AWS)

Project Structure

• src/index.ts: Entry point.
• src/lib/: Core logic (Checker, Network, Logger).
• src/alerters/: Alerting implementations (Email, AWS) and Manager.
• src/config.ts: Configuration loading.
• src/types.ts: TypeScript interfaces.