High-Throughput Web Crawlers: Politeness Queues, Robots.txt Parsing & DNS Caching
High-throughput web crawling requires balancing aggressive bandwidth utilization with strict server etiquette and distributed fault tolerance.
1. The Mercator Frontier Architecture
A production web crawler manages millions of URLs using a two-tier queueing system: Front Queues enforce priority based on PageRank and domain trust; Back Queues enforce host politeness by guaranteeing only one worker thread interacts with a single domain at any given timestamp.
2. Robots.txt Compliance & AST Caching
Before initiating an HTTP request, the crawler fetches and parses the target domain's /robots.txt file. The parser constructs an Abstract Syntax Tree (AST) evaluating User-agent, Disallow, Allow, and Crawl-delay directives. Parsed rules are cached in Redis with a 24-hour TTL to eliminate redundant network roundtrips.
3. DNS Resolution Bottlenecks
DNS resolution is frequently the primary bottleneck in web crawler throughput. Enterprise crawlers maintain dedicated asynchronous DNS resolver pools coupled with localized in-memory LRU caches to resolve thousands of hostnames per second without hitting public recursive nameservers.
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