Web Scraping Techniques at Scale

While large, curated datasets like Common Crawl provide a significant starting point (as discussed in "Utilizing Common Crawl Data"), there are scenarios where you need more targeted or fresher data than readily available archives offer. Web scraping, the process of automatically extracting data from websites, becomes a necessary tool. However, scaling this process to collect terabytes of text suitable for LLM pre-training introduces substantial engineering challenges beyond simple script execution.

This section focuses on the techniques and considerations required for building and operating web crawlers capable of fetching vast amounts of data efficiently and responsibly.

Asynchronous Operations for I/O Efficiency

Web scraping is fundamentally an I/O-bound task. Your crawler spends most of its time waiting for network responses. Using standard synchronous code (like the popular requests library) means each request blocks execution until it completes. This severely limits throughput.

To overcome this, asynchronous programming is essential. Python's asyncio library, combined with HTTP clients like aiohttp, allows your crawler to initiate many network requests concurrently. While one request is waiting for a response, the program can work on initiating others or processing completed ones, drastically improving overall speed.

Here's an example using aiohttp to fetch multiple URLs concurrently:

import asyncio
import aiohttp
import time

async def fetch(session, url):
    """Asynchronously fetches a single URL."""
    print(f"Fetching: {url}")
    try:
        # Set a timeout to avoid hanging indefinitely
        async with session.get(url, timeout=10) as response:
            # Ensure we only process successful responses
            if response.status == 200:
                # In a real scraper, you'd process the content here
                # content = await response.text()
                # print(f"Got content from {url}, length: {len(content)}")
                print(
                    f"Successfully fetched: {url} "
                    f"(Status: {response.status})"
                )
                return await response.text() # Return content or status
            else:
                print(
                    f"Failed: {url} (Status: {response.status})"
                )
                return None
    except asyncio.TimeoutError:
        print(f"Timeout fetching: {url}")
        return None
    except aiohttp.ClientError as e:
        print(f"Client error fetching {url}: {e}")
        return None

async def main(urls):
    """Manages the concurrent fetching of multiple URLs."""
    # Create a single session for connection pooling
    async with aiohttp.ClientSession() as session:
        tasks = [fetch(session, url) for url in urls]
        # Gather runs tasks concurrently and waits for them all to finish
        results = await asyncio.gather(*tasks)
        # Process results (optional, here just filtering out None)
        successful_fetches = [r for r in results if r is not None]
        print(
            f"\nFinished fetching. "
            f"Successful: {len(successful_fetches)}/{len(urls)}"
        )

# Example Usage
target_urls = [
    "https://example.com",
    "https://httpbin.org/get",
    "https://httpbin.org/delay/1", # Simulate a slow response
    "https://nonexistent-domain-xyz123.org", # Simulate connection error
    "https://httpbin.org/status/404" # Simulate HTTP error
]

start_time = time.time()
# In a Jupyter Notebook or script, run the asyncio event loop
# asyncio.run(main(target_urls)) # Use this in a standard Python script

# If running in an environment where an event loop is already running
# (like Jupyter):
# await main(target_urls) # Use this in Jupyter/IPython with await support

# For demonstration purposes, we'll run it here directly
if __name__ == "__main__":
    asyncio.run(main(target_urls))
    end_time = time.time()
    print(f"Total time: {end_time - start_time:.2f} seconds")

Running this code typically takes slightly longer than the longest individual request (the 1-second delay in this example), rather than the sum of all request times, demonstrating the benefit of concurrency.

Distributed Crawling Architectures

Even with asynchronous operations, a single machine has limits on bandwidth, CPU, and memory. To scrape at the scale needed for LLMs (potentially billions of pages), a distributed architecture is required. Common components include:

1. URL Frontier/Queue: A system to manage the list of URLs to be crawled. This needs to handle potentially billions of entries, prioritize URLs, and prevent re-crawling recently visited pages. Message queues like RabbitMQ, Kafka, or even Redis lists are often used. Workers fetch batches of URLs from the queue.
2. Crawler Workers: Multiple processes or machines, each running an asynchronous crawler instance (like the aiohttp example, often enhanced using frameworks like Scrapy). They fetch URLs from the queue, download pages, potentially extract links for new URLs (adding them back to the queue), and process/store the content.
3. Content Storage: A scalable storage system for raw HTML or extracted text. Cloud object storage (like AWS S3, Google Cloud Storage) or distributed file systems (HDFS) are suitable choices (covered further in Chapter 8).
4. Metadata Store: Often needed to track visited URLs, crawl timestamps, robots.txt rules, and potentially page checksums for deduplication. Databases (SQL or NoSQL) or specialized key-value stores can serve this purpose.

A simplified view of a distributed crawling system. Workers fetch URLs from a central queue, process pages, store content, update metadata, and potentially discover new URLs to add back to the queue.

Implementing Respectful Crawling

Aggressive scraping can overload websites, impacting their availability for legitimate users and potentially leading to your IP addresses being blocked. Large-scale crawling must be done responsibly.

robots.txt

Most websites provide a /robots.txt file outlining rules for automated agents (bots/crawlers). These rules specify which parts of the site should not be accessed (Disallow) and sometimes suggest a preferred crawl delay. Respecting robots.txt is a fundamental aspect of ethical scraping.

Python's urllib.robotparser can help:

from urllib.robotparser import RobotFileParser
from urllib.parse import urljoin

# Define your crawler's User-Agent string
# Be specific and provide contact info if possible
USER_AGENT = "MyLLMDataCrawler/1.0 (+http://mycrawlerinfo.example.com)"

def can_fetch_url(robot_parser, url):
    """Checks if the URL is allowed by robots.txt for our user agent."""
    try:
        return robot_parser.can_fetch(USER_AGENT, url)
    except Exception as e:
        # Handle potential parsing errors gracefully
        print(f"Error checking robots.txt permission for {url}: {e}")
        return False # Default to not fetching if unsure

# --- Usage within a crawler ---
robots_url_cache = {}
# Cache parser objects per domain to avoid re-fetching

async def check_and_fetch(session, url):
    # Simplified: assumes URL is well-formed http/https
    base_url = urljoin(url, '/')
    robots_url = urljoin(base_url, 'robots.txt')

    parser = robots_url_cache.get(base_url)
    if not parser:
        print(f"Fetching robots.txt for {base_url}")
        parser = RobotFileParser()
        parser.set_url(robots_url)
        try:
             # Use an async request here in a real async crawler
            # For simplicity, using read() which might block in async code
            # In aiohttp, you'd fetch robots_url and then call parser.parse()
            # await parser.read()
            # # Ideal async way (requires async adaptation of parser)
            # --- Synchronous simulation for example clarity ---
            import requests
            response = requests.get(
                robots_url,
                timeout=5,
                headers={'User-Agent': USER_AGENT}
            )
            if response.status_code == 200:
                 parser.parse(response.text.splitlines())
            else:
                 print(
                     f"No valid robots.txt found for {base_url} "
                     f"(Status: {response.status_code})"
                 )
            # --- End Synchronous simulation ---
            robots_url_cache[base_url] = parser
            # Cache even if fetch failed or no rules
        except Exception as e:
            print(f"Failed to fetch or parse robots.txt for {base_url}: {e}")
            robots_url_cache[base_url] = None # Mark as failed
            return None # Cannot proceed without checking robots.txt

    if parser and can_fetch_url(parser, url):
        print(f"Allowed to fetch: {url}")
        # Proceed with actual fetching using aiohttp (fetch function from earlier)
        return await fetch(session, url)
    elif parser:
        print(f"Disallowed by robots.txt: {url}")
        return None
    else:
        # Handle case where robots.txt fetching failed earlier
        print(f"Could not verify robots.txt permission for {url}, skipping.")
        return None

# --- Example call within async context ---
# async with aiohttp.ClientSession(
#         headers={'User-Agent': USER_AGENT}) as session:
#     await check_and_fetch(
#         session, "https://example.com/some/allowed/path")
#     await check_and_fetch(
#         session, "https://example.com/some/disallowed/path")
#     # Assuming disallowed in robots.txt

Rate Limiting

Even if allowed by robots.txt, hitting a single server with hundreds or thousands of requests per second can overwhelm it. Implement rate limiting per domain.

• Fixed Delay: Add a simple await asyncio.sleep(delay_seconds) between requests to the same domain.
• Crawl-delay Directive: Some robots.txt files suggest a delay. Respect this if present.
• Adaptive Delay: Monitor server response times or error rates (like HTTP 429 "Too Many Requests", 503 "Service Unavailable"). Increase the delay automatically if issues arise.
• Concurrent Connection Limits: Limit the number of simultaneous connections open to a single domain/IP address.

A simple per-domain delay mechanism:

import asyncio
import time

# Store the last access time for each domain
last_access_times = {}
# Minimum delay between requests to the same domain (in seconds)
MIN_DELAY_PER_DOMAIN = 1.0

async def rate_limited_fetch(session, url):
    # Requires 'from urllib.parse import urlparse'
    domain = urlparse(url).netloc

    last_access = last_access_times.get(domain, 0)
    now = time.monotonic()
    elapsed = now - last_access

    if elapsed < MIN_DELAY_PER_DOMAIN:
        wait_time = MIN_DELAY_PER_DOMAIN - elapsed
        print(f"Rate limiting {domain}: Waiting {wait_time:.2f}s")
        await asyncio.sleep(wait_time)

    # Update last access time *before* making the request
    last_access_times[domain] = time.monotonic()
    # Now perform the actual fetch
    # (using function like 'fetch' or 'check_and_fetch')
    result = await fetch(session, url) # Assume 'fetch' handles exceptions
    return result

User-Agent Identification

Always set a clear User-Agent string that identifies your crawler and ideally provides a way to contact you (e.g., a URL with information or an email address). This helps site administrators understand the traffic source and contact you if your crawler causes problems. Avoid using generic browser user agents.

Example: User-Agent: LLMBuilderBot/0.1 (+http://www.my-llm-project.org/crawler-info)

Handling Scale Challenges

Beyond politeness, large-scale scraping presents other technical hurdles:

• Dynamic Content (JavaScript): Many modern websites load content using JavaScript after the initial HTML page load. Simple HTTP requests won't capture this data. Tools like Playwright or Selenium, which control a real browser engine, are needed. However, they are significantly more resource-intensive (CPU, memory) than simple HTTP clients, slowing down crawling and increasing costs. Use them selectively only when necessary.
• Crawler Traps: Websites may intentionally or unintentionally create "spider traps" - infinite loops of dynamically generated links (e.g., calendars with endless "next month" links) that can trap a naive crawler. Implement mechanisms like maximum crawl depth per domain, URL pattern filtering, or monitoring the number of URLs discovered from a single page/domain.
• Session Management & Logins: Accessing content behind logins requires handling authentication (cookies, tokens) which adds complexity and may violate terms of service. This is generally avoided for large-scale pre-training data collection unless explicitly permitted.
• Robust Error Handling: Network glitches, server errors (HTTP 5xx), client errors (HTTP 4xx), and timeouts are common at scale. Implement retry logic, often with exponential backoff (waiting progressively longer after each failure), for transient errors like timeouts or 503s. Give up after a few retries for persistent errors (like 404 Not Found).

Building a scalable and robust web scraper is a significant software engineering task. While libraries and frameworks provide building blocks, careful design considering concurrency, distribution, storage, politeness, and error handling is necessary to successfully gather the vast amounts of data needed for training large language models. Remember that data quality is also paramount; the raw output of scraping often requires extensive cleaning and filtering, as discussed in Chapter 7 ("Data Cleaning and Preprocessing Pipelines").