Web Crawler Architecture
Build scalable web crawlers: distributed architecture, rate limiting, politeness policies, and data processing.
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Problem Statement
Design a web crawler that can systematically discover and download billions of web pages for a search engine. The system must be scalable, respectful of websites, and handle various content types while avoiding traps and duplicate content.
Key Requirements
- • Crawl 1 billion pages per month
- • Respect robots.txt and rate limiting
- • Handle different content types (HTML, PDF, images)
- • Avoid crawler traps and infinite loops
- • Prioritize important pages and fresh content
Step 1: Clarify Requirements
Functional Requirements
- •Web Crawling: Download web pages from given seed URLs
- •Content Types: HTML, PDF, images, videos (configurable)
- •Politeness: Respect robots.txt and rate limits
- •Freshness: Re-crawl pages based on update frequency
Non-Functional Requirements
- •Scale: 1 billion pages/month (~400 pages/second
- •Robustness: Handle failures, malicious websites
- •Efficiency: Avoid duplicate crawling, optimize bandwidth
- •Extensibility: Easy to add new content processors
Step 2: Back-of-Envelope Estimation
Crawl Rate
1B pages/month
≈ 400 pages/second average
Peak: 800 pages/second
Need distributed crawling
Storage
Average page: 500KB
1B × 500KB = 500TB
Metadata: ~50TB
URLs to crawl: ~10TB
Bandwidth
400 pages/sec × 500KB
= 200 MB/second
= 17TB per day
Need high bandwidth
Step 3: High-Level Design
Crawler Architecture
Seed URLs → URL Frontier → Crawler Workers → Content Processor → Storage
Seed URLs
URL Frontier
Crawlers
Processor
Storage
URL Frontier
- • Queue of URLs to be crawled
- • Priority-based ordering
- • Politeness per domain
- • Freshness scheduling
Crawler Workers
- • Download pages from URLs
- • Respect robots.txt
- • Handle timeouts/errors
- • Extract new URLs
Content Processor
- • Parse HTML and extract links
- • Detect duplicate content
- • Content validation
- • Malware scanning
Storage Layer
- • Store crawled content
- • URL database and metadata
- • Duplicate detection cache
- • Robots.txt cache
Step 4: Deep Dive - URL Frontier
Politeness & Priority Management
Politeness (Per Domain)
- • Separate queue per domain/host
- • Configurable delay between requests
- • Respect robots.txt crawl-delay
- • Monitor server response times
Priority Queues
- • High: News sites, popular domains
- • Medium: Business sites, blogs
- • Low: Personal pages, forums
- • Dynamic priority based on PageRank
Duplicate Detection Strategies
URL Level
- • Normalize URLs (remove fragments)
- • Hash table for seen URLs
- • Bloom filter for memory efficiency
- • Handle URL redirects
Content Level
- • Hash content (MD5, SHA-256)
- • Simhash for near-duplicates
- • Compare text similarity
- • Handle dynamic content
Performance
- • Distributed hash tables
- • LRU cache for hot URLs
- • Periodic cleanup
- • Probabilistic data structures
Robustness & Error Handling
Crawler Traps
- • Infinite calendar/pagination
- • URL length limits (2KB max)
- • Depth limits per domain
- • Detect cycles with heuristics
Failure Handling
- • Retry with exponential backoff
- • Circuit breaker for bad hosts
- • Timeout configuration
- • Dead letter queue
Scalability & Performance
Horizontal Scaling
- • Partition URLs by domain hash
- • Independent crawler workers
- • Shared distributed queue (Kafka)
- • Auto-scaling based on queue depth
Caching Strategy
- • DNS caching (reduce lookups)
- • Robots.txt cache (per domain)
- • URL seen cache (bloom filter)
- • Content hash cache
Storage Optimization
- • Compress content before storage
- • Deduplicate at storage layer
- • Hot/cold data separation
- • Archive old crawl data
Network Optimization
- • HTTP/2 for multiplexing
- • Connection pooling per domain
- • Geographic distribution
- • Bandwidth rate limiting
Content Processing Pipeline
Processing Stages
1. Download
- • HTTP client
- • Handle redirects
- • Timeout management
2. Parse
- • Extract links
- • Parse HTML/XML
- • Handle malformed content
3. Filter
- • Content validation
- • Spam detection
- • Quality scoring
4. Store
- • Index content
- • Update metadata
- • Queue new URLs
Real-World Web Crawlers
Googlebot
- • Crawls trillions of pages
- • Mobile-first indexing
- • JavaScript rendering (Chromium)
- • Respects crawl budget per site
Common Crawl
- • Non-profit web crawling
- • Monthly crawls of entire web
- • Open dataset for research
- • Processes 25+ billion pages
Internet Archive
- • Preserves web history
- • Crawls for long-term storage
- • Heritrix crawler framework
- • 70+ billion web pages archived
Scrapy (Open Source)
- • Python web crawling framework
- • Built-in duplicate filtering
- • Async networking with Twisted
- • Extensible middleware system
Crawler Ethics & Best Practices
- • Always respect robots.txt directives
- • Implement rate limiting to avoid overwhelming servers
- • Use appropriate User-Agent headers
- • Handle error responses gracefully
- • Consider legal and copyright implications
- • Monitor and respond to website owner requests