Crawler

A crawler (also known as a spider or bot) is an automated program that systematically browses and indexes web pages. Search engines use crawlers to discover, analyze, and index web content, making it findable in search results.

How Crawlers Work

1. Basic Crawling Process

# Basic Crawler Implementation
class WebCrawler:
    def __init__(self):
        self.visited_urls = set()
        self.url_queue = []
        self.robots_cache = {}

    async def crawl(self, start_url):
        if not self.is_allowed(start_url):
            return
        
        self.url_queue.append(start_url)
        
        while self.url_queue:
            url = self.url_queue.pop(0)
            if url in self.visited_urls:
                continue
                
            try:
                content = await self.fetch_page(url)
                self.process_page(content)
                self.extract_links(content)
                self.visited_urls.add(url)
            except Exception as e:
                self.log_error(url, e)

    def is_allowed(self, url):
        # Check robots.txt rules
        return self.check_robots_txt(url)

2. Robots.txt Handling

# Robots.txt Parser
class RobotsParser:
    def parse_robots_txt(self, content):
        rules = {
            'allow': [],
            'disallow': [],
            'crawl_delay': None,
            'sitemap': []
        }
        
        for line in content.split('\n'):
            if line.startswith('Allow:'):
                rules['allow'].append(line.split(':', 1)[1].strip())
            elif line.startswith('Disallow:'):
                rules['disallow'].append(line.split(':', 1)[1].strip())
            elif line.startswith('Crawl-delay:'):
                rules['crawl_delay'] = float(line.split(':', 1)[1].strip())
            elif line.startswith('Sitemap:'):
                rules['sitemap'].append(line.split(':', 1)[1].strip())
                
        return rules

Crawler Directives

1. Meta Robots

<!-- Basic Meta Robots Tags -->
<meta name="robots" content="index, follow">
<meta name="robots" content="noindex, nofollow">
<meta name="robots" content="noarchive">

<!-- Specific Crawler Directives -->
<meta name="googlebot" content="index, follow">
<meta name="bingbot" content="noindex">
<meta name="googlebot-news" content="noindex">

2. HTTP Headers

# Nginx X-Robots-Tag Configuration
location /private/ {
    add_header X-Robots-Tag "noindex, nofollow";
}

location /temporary/ {
    add_header X-Robots-Tag "noarchive";
}

location /beta/ {
    add_header X-Robots-Tag "noindex, nofollow, noarchive";
}

Crawl Optimization

1. Sitemap Implementation

<!-- XML Sitemap Example -->
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <url>
    <loc>https://example.com/</loc>
    <lastmod>2024-03-15</lastmod>
    <changefreq>daily</changefreq>
    <priority>1.0</priority>
  </url>
  <url>
    <loc>https://example.com/products</loc>
    <lastmod>2024-03-14</lastmod>
    <changefreq>weekly</changefreq>
    <priority>0.8</priority>
  </url>
</urlset>

2. Crawl Budget Management

// Crawl Rate Controller
class CrawlRateManager {
  constructor(maxRequestsPerSecond) {
    this.maxRequestsPerSecond = maxRequestsPerSecond;
    this.requestQueue = [];
    this.lastRequestTime = Date.now();
  }

  async scheduleRequest(url) {
    const now = Date.now();
    const timeSinceLastRequest = now - this.lastRequestTime;
    const minimumInterval = 1000 / this.maxRequestsPerSecond;

    if (timeSinceLastRequest < minimumInterval) {
      await new Promise(resolve => 
        setTimeout(resolve, minimumInterval - timeSinceLastRequest)
      );
    }

    this.lastRequestTime = Date.now();
    return this.makeRequest(url);
  }
}

Performance Optimization

1. Resource Hints

<!-- Resource Hints for Crawlers -->
<link rel="preload" href="/assets/critical.css" as="style">
<link rel="preload" href="/assets/main.js" as="script">
<link rel="dns-prefetch" href="//cdn.example.com">
<link rel="preconnect" href="https://api.example.com">

2. Server Configuration

# Nginx Crawler Optimization
location / {
    # Gzip compression
    gzip on;
    gzip_types text/plain text/html text/css application/javascript;
    
    # Cache control
    expires 1h;
    add_header Cache-Control "public, no-transform";
    
    # Crawler specific settings
    if ($http_user_agent ~* (googlebot|bingbot)) {
        set $crawl_rate "slow";
    }
}

Monitoring and Analysis

1. Log Analysis

# Crawler Log Analyzer
class CrawlerLogAnalyzer:
    def analyze_logs(self, log_file):
        crawler_stats = {
            'googlebot': {'requests': 0, 'bytes': 0},
            'bingbot': {'requests': 0, 'bytes': 0},
            'errors': []
        }
        
        for line in log_file:
            if 'Googlebot' in line:
                self.process_googlebot_entry(line, crawler_stats)
            elif 'bingbot' in line:
                self.process_bingbot_entry(line, crawler_stats)
            
            if '"status": "4' in line or '"status": "5' in line:
                crawler_stats['errors'].append(self.parse_error(line))
                
        return crawler_stats

2. Performance Tracking

// Crawler Performance Monitor
class CrawlerPerformance {
  constructor() {
    this.metrics = {
      crawlRate: 0,
      responseTime: [],
      errors: [],
      crawlDepth: new Map()
    };
  }

  trackRequest(url, response) {
    this.metrics.crawlRate++;
    this.metrics.responseTime.push(response.timing);
    this.updateCrawlDepth(url);
    
    if (response.status >= 400) {
      this.metrics.errors.push({
        url,
        status: response.status,
        timestamp: new Date()
      });
    }
  }

  generateReport() {
    return {
      averageResponseTime: this.calculateAverageResponse(),
      errorRate: this.calculateErrorRate(),
      crawlDistribution: this.analyzeCrawlPattern()
    };
  }
}

Common Issues and Solutions

1. Crawl Errors

// Crawl Error Handler
class CrawlErrorHandler {
  async handleError(error) {
    const solutions = {
      '404': async (url) => {
        await this.checkRedirects(url);
        await this.updateSitemap(url);
        await this.notifyTeam(url, '404');
      },
      '500': async (url) => {
        await this.checkServerHealth();
        await this.notifyTeam(url, '500');
      },
      'robots_blocked': async (url) => {
        await this.checkRobotsRules();
        await this.updateRobotsTxt();
      }
    };

    return solutions[error.type]?.(error.url);
  }
}

2. Rate Limiting

// Rate Limiting Implementation
class CrawlerRateLimit {
    private $redis;
    private $window = 60; // seconds
    private $limit = 10;  // requests per window

    public function isAllowed($userAgent) {
        $key = "rate_limit:" . $userAgent;
        $current = $this->redis->get($key) ?? 0;

        if ($current >= $this->limit) {
            return false;
        }

        $this->redis->incr($key);
        $this->redis->expire($key, $this->window);
        
        return true;
    }
}

Remember that proper crawler management is essential for effective SEO. Optimizing your site for crawlers while maintaining performance for users requires careful balance and regular monitoring.