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High-concurrency Crawler Implementation

Build a high-performance web crawler that can efficiently crawl and index web pages from multiple domains. This project combines network programming, concurrency, and data processing techniques.

Project Structure​

The crawler is organized as follows:

phase4/crawler/
├── CMakeLists.txt
├── include/
│   └── crawler.h  # Crawler interface and classes
└── src/
    └── crawler.cpp  # Implementation of crawling functionality

Learning Objectives​

During this project, you will:

  • Design and implement a concurrent web crawling system
  • Handle HTTP requests and responses efficiently
  • Manage large-scale URL discovery and scheduling
  • Implement politeness policies (robots.txt, crawl delays)
  • Build URL filtering and duplicate detection systems
  • Create efficient HTML parsing and link extraction
  • Apply rate limiting and resource management techniques
  • Design persistent storage for crawled data

Core Concepts​

Crawler Architecture​

A typical web crawler consists of several components:

  1. URL Frontier: Queue of URLs to be crawled
  2. Scheduler: Determines which URL to crawl next
  3. Fetcher: Downloads web pages via HTTP
  4. Extractor: Parses HTML to find new links
  5. Storage: Saves crawled content and metadata
  6. Politeness Policy: Respects website crawl rules

Concurrency Model​

The crawler should use a producer-consumer pattern:

  • Multiple fetcher threads download pages concurrently
  • Extractor threads process downloaded pages
  • Scheduler manages the URL queue
  • Storage component handles data persistence

Implementation Architecture​

Crawler Interface​

class Crawler {
public:
    struct CrawlConfig {
        std::vector<std::string> seed_urls;
        int max_depth = 3;
        int max_pages = 1000;
        std::chrono::milliseconds crawl_delay = std::chrono::milliseconds(1000);
        int concurrent_threads = 4;
        std::string robots_policy = "obey";  // obey, ignore, custom
        std::string output_dir = "./crawl_data";
    };
    
    explicit Crawler(const CrawlConfig& config);
    void start();
    void stop();
    void pause();
    void resume();
    
    // Statistics
    CrawlStats get_stats() const;
    
private:
    CrawlConfig config_;
    std::atomic<bool> running_;
    std::atomic<bool> paused_;
    
    // Core components
    URLFrontier url_frontier_;
    std::vector<std::thread> fetcher_threads_;
    ContentProcessor content_processor_;
    std::mutex stats_mutex_;
    CrawlStats stats_;
    
    void fetcher_worker(int thread_id);
    void process_page(const CrawledPage& page);
    bool should_crawl(const std::string& url) const;
};

Supporting Structures​

struct CrawledPage {
    std::string url;
    std::string content;
    std::vector<std::string> links;
    std::string content_type;
    int status_code;
    std::chrono::system_clock::time_point timestamp;
};

struct CrawlStats {
    size_t pages_crawled;
    size_t pages_failed;
    size_t urls_discovered;
    size_t urls_skipped;
    std::chrono::system_clock::time_point start_time;
};

URL Frontier​

class URLFrontier {
public:
    void add_url(const std::string& url, int depth = 0);
    std::optional<std::pair<std::string, int>> get_next_url();
    bool is_duplicate(const std::string& url) const;
    size_t size() const;
    
private:
    std::queue<std::pair<std::string, int>> url_queue_;  // URL + depth
    std::unordered_set<std::string> seen_urls_;
    mutable std::mutex mutex_;
};

Implementation Requirements​

Core Features​

  1. Concurrent Fetching: Download multiple pages simultaneously
  2. URL Filtering: Skip certain URLs based on patterns or rules
  3. Duplicate Detection: Prevent crawling the same URL multiple times
  4. Politeness: Respect robots.txt and implement crawl delays
  5. Error Handling: Deal with network errors, timeouts, and invalid content
  6. Storage: Save crawled content to persistent storage
  7. Statistics: Track crawl progress and performance metrics

HTTP Client Implementation​

Your crawler needs to handle:

  • HTTP/1.1 requests with appropriate headers
  • Redirect following (with cycle detection)
  • Content type detection and filtering
  • Connection pooling for efficiency
  • Timeouts and retry logic

Sample Implementation Approach​

class Crawler {
private:
    struct Fetcher {
        static std::optional<CrawledPage> fetch(const std::string& url, 
                                              const CrawlConfig& config);
    };
    
    struct LinkExtractor {
        static std::vector<std::string> extract_links(const std::string& html, 
                                                    const std::string& base_url);
    };
    
    CrawlConfig config_;
    URLFrontier frontier_;
    std::vector<std::thread> workers_;
    std::unordered_set<std::string> processed_urls_;
    
    void worker_thread(int id);
    void save_page(const CrawledPage& page);
    
public:
    void start();
    void stop();
};

Politeness Implementation​

robots.txt Handling​

  • Download and parse robots.txt for each domain
  • Respect crawl delays specified in robots.txt
  • Obey allow/disallow rules
  • Implement user-agent specific rules

Rate Limiting​

  • Implement per-domain request throttling
  • Use exponential backoff for failed requests
  • Respect HTTP response headers (Retry-After, etc.)

Performance Considerations​

  • Connection Pooling: Reuse HTTP connections to reduce overhead
  • Bandwidth Throttling: Limit total bandwidth usage
  • Memory Management: Efficiently manage large numbers of URLs in memory
  • Storage Efficiency: Use appropriate formats for storing crawled data
  • Duplicate Detection: Efficiently detect and prevent duplicate crawling
  • Thread Safety: Proper synchronization for shared data structures

Storage Strategies​

Consider implementing different storage options:

  1. File System: Store each page in a separate file
  2. Database: Use SQLite or similar for structured storage
  3. Key-Value Store: Use a format suitable for large-scale crawling
  4. Compressed Storage: Compress content to save space

Testing Strategy​

Test your crawler with:

  • Small, controlled website crawls
  • Robots.txt compliance verification
  • Concurrency and performance under load
  • Error handling for various HTTP status codes
  • Duplicate detection accuracy
  • Memory usage with large seed lists
  • Politeness behavior verification

Usage Example​

#include "crawler.h"
#include <iostream>

int main() {
    Crawler::CrawlConfig config;
    config.seed_urls = {"https://example.com"};
    config.max_depth = 2;
    config.max_pages = 100;
    config.concurrent_threads = 4;
    config.crawl_delay = std::chrono::milliseconds(1000);
    
    Crawler crawler(config);
    
    std::cout << "Starting crawl..." << std::endl;
    crawler.start();
    
    // Wait for completion or user interruption
    // In a real application, you might want to implement proper signal handling
    
    auto stats = crawler.get_stats();
    std::cout << "Crawl completed:" << std::endl;
    std::cout << "Pages crawled: " << stats.pages_crawled << std::endl;
    std::cout << "Pages failed: " << stats.pages_failed << std::endl;
    std::cout << "URLs discovered: " << stats.urls_discovered << std::endl;
    
    return 0;
}

Advanced Features (Optional)​

Consider implementing additional capabilities:

  1. Sitemap Support: Parse and follow XML sitemaps
  2. Content Analysis: Extract and index text content for search
  3. Media Crawling: Download images, documents, and other media
  4. Real-time Processing: Process pages as they're crawled
  5. Distributed Crawling: Split crawling across multiple machines
  6. JavaScript Rendering: Use headless browser for JavaScript-heavy sites

Building and Testing​

# Build the project
cd build
cmake ..
make -j4

# Test crawling a local test site or controlled external site
./phase4/crawler/web_crawler

Next Steps​

After completing the crawler project, review the Development Standards documentation to ensure your code follows the project's guidelines. Then proceed to implement all the projects you've learned about throughout this comprehensive playground!