Web Crawler
Overview
A web crawler systematically traverses web pages by discovering URLs, fetching content, parsing HTML, and storing results. Uses BFS or priority-based frontier management. Performance is I/O-bound, typically limited by politeness constraints rather than compute.
When to Use
Trigger conditions:
- Building a site audit tool to discover all pages and their link structure
- Collecting structured data from websites at scale
- Mapping site architecture for SEO analysis
When NOT to use:
- When you need data from a single API endpoint (use HTTP client directly)
- When a sitemap.xml provides all needed URLs (parse sitemap instead)
Algorithm
IRON LAW: Respect robots.txt and Rate Limits
A crawler MUST:
1. Parse and obey robots.txt before crawling any path
2. Enforce crawl-delay (default 1s if unspecified)
3. Identify itself with a descriptive User-Agent
Ignoring these is unethical and will get your IP blocked.Phase 1: Input Validation
Parse seed URLs, fetch and parse robots.txt for each domain, set crawl scope (same-domain, subdomain, or cross-domain). Gate: Valid seed URLs, robots.txt rules loaded, scope defined.
Phase 2: Core Algorithm
- Initialize URL frontier with seed URLs (priority queue or FIFO)
- Dequeue URL, check: not visited, allowed by robots.txt, within scope
- Fetch page with timeout and retry logic, respect crawl-delay
- Parse HTML: extract links (normalize, deduplicate), extract content/metadata
- Enqueue discovered URLs, store parsed data
- Repeat until frontier empty or limit reached
Phase 3: Verification
Check: no robots.txt violations in crawl log, no duplicate pages stored, all discovered URLs accounted for. Gate: Crawl completed within scope, politeness maintained.
Phase 4: Output
Return site map with pages, link graph, and extracted metadata.
Output Format
{
"pages": [{"url": "...", "status": 200, "title": "...", "links_out": 15, "depth": 2}],
"metadata": {"pages_crawled": 500, "errors": 12, "duration_seconds": 300, "domain": "example.com"}
}Examples
Sample I/O
Input: Seed: "https://example.com", max_depth: 2, max_pages: 100 Expected: Crawl tree with homepage at depth 0, linked pages at depth 1-2, respecting robots.txt
Edge Cases
| Input | Expected | Why |
|---|---|---|
| robots.txt disallows / | Zero pages crawled | Must respect full disallow |
| Redirect loop | Stop after 5 redirects | Prevent infinite loop |
| Soft 404 (200 with error page) | Flag as soft 404 | Status code alone is insufficient |
Gotchas
- URL normalization:
http://Example.COM/path/andhttp://example.com/pathare the same URL. Normalize: lowercase host, remove default port, remove trailing slash, sort query params. - JavaScript-rendered content: A basic HTTP fetch misses JS-rendered content. Use headless browser (Playwright/Puppeteer) for SPAs.
- Trap detection: Calendar pages, session IDs in URLs, and infinite pagination create crawler traps. Set max depth and URL pattern limits.
- Rate limiting yourself: Parallel fetching without per-domain rate limiting will overwhelm small servers. Use per-domain semaphores.
- Character encoding: Not all pages are UTF-8. Detect encoding from HTTP headers and meta tags; fall back to charset detection libraries.
References
- For URL normalization rules (RFC 3986), see
references/url-normalization.md - For distributed crawling architecture, see
references/distributed-crawl.md