This is a robust and modular simple web crawler designed to efficiently traverse and fetch web pages. Built with a focus on extensibility, configurability, and best coding practices.

• Modular Design: Clear separation of concerns with distinct modules for fetching, link extraction, retry logic, and queue management.
• Concurrency Control: Efficiently manages multiple concurrent requests without overwhelming target servers.
• URL Deduplication: Ensures each URL is processed only once for optimal performance.
• Retry Mechanism: Handles transient failures by re-enqueuing URLs with a delay.
• Configurable: Easily swap out core components to suit different needs, thanks to the use of interfaces and the Strategy Pattern.
• Robustness: Designed to handle edge cases and the unpredictable nature of the web.

• Single Responsibility Principle: Each module has a distinct responsibility, making the code maintainable and testable.
• Strategy Pattern: Use of interfaces allows for runtime selection of algorithms, making the crawler highly configurable.
• Observer Pattern: The crawler observes all the URLs fetches and delegates retry logic, ensuring resilience.

1. Fetcher: Handles HTTP requests.
2. Link Extractor: Parses fetched HTML content to extract links.
3. Retry Manager: Implements retry logic for failed requests.
4. Queue: Manages the list of URLs to be processed, ensuring concurrency control and deduplication.

crawler := NewCrawler(
    startURL,
    fetcherInstance,
    linkExtractorInstance,
    retryManagerInstance,
)
crawler.Crawl()

Replace startURL, fetcherInstance, linkExtractorInstance, and retryManagerInstance with appropriate values.

Because of its modular design, extending the crawler is straightforward:

• New Fetch Mechanisms: Implement the FetcherInterface to add new fetch mechanisms.
• Link Extraction Techniques: Implement the LinkExtractorInterface for different link extraction strategies.
• Custom Retry Logic: Implement the RetryInterface to define custom retry behaviors.

This web crawler is simple but yet versatile tool designed with a balance of efficiency, extensibility, and robustness.