Web Crawler

Design

The crawler is designed to be fault-tolerant, efficient, and configurable. It consists of the following components:

1. CLI Interface: A simple command-line interface for users to start the crawler with a specified URL and configuration options.
2. Crawler Class: Manages the crawling process, including URL queuing, visiting pages, and ensuring concurrency and rate limiting.
3. Fetcher Class: Handles the fetching of web pages with retry logic.
4. Parser Class: Parses HTML content to extract links.

Design Decisions

• Concurrency: Utilized asyncio for managing concurrent tasks, which allows for crawling multiple pages simultaneously without blocking.
• Error Handling: Implemented retry logic and error handling to ensure the crawler can recover from transient network issues.
• Rate Limiting: Implemented delay between requests inside of each asynchronous task to prevent overwhelming the server.
• HTML Parsing: Used BeautifulSoup for efficient HTML parsing and link extraction.

Trade-offs

• Rate Limiting: A fixed delay is used for simplicity. A more sophisticated approach could be to use a dynamic rate limiter so that the crawler can adapt to the server's response time.
• Exponential Backoff: The retry logic uses a fixed delay between retries instead of exponential backoff. Exponential backoff could be more efficient in some cases.
• Breadth-First vs. Depth-First: The current queue implementation processes URLs in a depth-first manner. Depending on the use case, a breadth-first approach might be more appropriate. This would likely be slower but would ensure that all pages at a given depth are processed before moving to the next depth.

Dependencies

The program uses the following dependencies:

• aiohttp for making asynchronous HTTP requests
• BeautifulSoup for parsing HTML
• asyncio for managing asynchronous tasks
• typer for creating the CLI interface

Installation

This project uses Poetry for dependency management. To install Poetry, follow the instructions on the official website.

Once Poetry is installed, you can set up the project environment and install the dependencies by running:

poetry install

Usage

You can run the crawler from the command line using the provided CLI interface. Here’s how to get started:

poetry run python main.py --url https://en.wikipedia.org --max-concurrent-tasks 5 --rate-limit 1

Command-Line Arguments:

• --url: The starting URL for the crawler (required)
• --max-concurrent-tasks: The maximum number of concurrent tasks (default: 5)
• --rate-limit: The delay between requests in seconds (default: 1)
• --max-retries: The maximum number of retries for failed requests (default: 3)
• --retry-delay: The delay between retries in seconds (default: 1)
• --help: Show the help message

Testing

You can run the test suite using the following command:

poetry run pytest

The tests cover the core functionality of the crawler, including URL extraction, fetching, and parsing.

Future Improvements

Here are some potential improvements that could be made to the crawler:

• User-Agent Rotation: Rotate user agents to prevent being blocked by servers that detect and block crawlers.
• Proxy Support: Add support for rotating proxies to avoid IP-based blocking.
• Persistent Storage: Store visited URLs and links in a database for resuming crawls and analyzing results.
• Depth Limitation: The current implementation does not limit the depth of crawling. Adding a depth limit could prevent excessively deep crawls on large sites.
• Robots.txt Parsing: Implement parsing of robots.txt files to respect crawling rules specified by the website.
• Logging: Add more detailed logging to track the crawling process and errors.