What are the best practices for avoiding getting blocked while using WebMagic?

WebMagic is a scalable web crawling framework for Java that provides a simple way to extract information from the web. It's important to use it responsibly to avoid getting blocked by websites. Here are some best practices you should follow to avoid getting blocked while using WebMagic:

1. Respect robots.txt: Always check the site's robots.txt file and follow the rules set by the website. If the site disallows crawling on certain pages or with certain user-agents, you should respect these rules.

2. Use a User-Agent String: Set a legitimate user-agent to identify yourself as a browser or a well-known crawler instead of using the default one or none at all. Some websites block requests with non-standard user-agents.

   Spider.create(new MyPageProcessor())
       .setUserAgent("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3")
       // ...
   

3. Limit Request Rate: Do not send too many requests in a short period. Implement delays between the requests. You can do this by setting a download delay in WebMagic.

   Spider.create(new MyPageProcessor())
       .addUrl("http://www.example.com")
       .thread(5)
       .setDownloader(new HttpClientDownloader())
       .setSleepTime(1000) // in milliseconds
       .run();
   

4. Rotate IP Addresses/Proxy: If you need to make many requests, consider using proxies to distribute the load across different IP addresses.

   HttpClientDownloader httpClientDownloader = new HttpClientDownloader();
   httpClientDownloader.setProxyProvider(SimpleProxyProvider.from(new Proxy("host1", port1), new Proxy("host2", port2)));
   Spider.create(new MyPageProcessor())
       .setDownloader(httpClientDownloader)
       // ...
   

5. Rotate User-Agents: Along with rotating IP addresses, sometimes rotating user-agent strings can help in avoiding detection as a bot.

   // You would have to implement a custom Downloader or a custom HttpUriRequestConverter to rotate user-agents per request.
   

6. Handle CAPTCHAs: Some websites will present CAPTCHAs to block scrapers. You can use CAPTCHA solving services to handle this, but be aware that this can be a controversial practice and may violate the website's terms of service.

7. Be Prepared for JavaScript: If the website uses JavaScript to load content dynamically, consider using a headless browser or tools like Selenium with WebMagic to render the JavaScript.

8. Manage Cookies: Some websites track your session with cookies. Make sure you're handling cookies correctly, potentially by using session management in WebMagic.

9. Be Ethical: Do not scrape data at the expense of the website's performance. Avoid scraping personal data without consent and adhere to legal and ethical standards.

10. Handle Errors Gracefully: Implement proper error handling to manage HTTP errors or connection timeouts. This helps prevent repeated requests to the server that might get you blocked.

Here's a simple example of how you might set up a Spider in WebMagic with some of these best practices in mind:

Spider.create(new MyPageProcessor())
    // Set user-agent and manage cookies
    .setUserAgent("MyWebMagicBot/1.0 (+http://example.com/bot)")
    // Add initial URLs to start crawling from
    .addUrl("http://www.example.com")
    // Set the number of threads
    .thread(5)
    // Set sleep time between requests
    .setSleepTime(1000)
    // Optionally set a proxy
    //.setDownloader(httpClientDownloader)
    // Start the spider
    .run();

Remember, best practices are not just about avoiding getting blocked; they are also about maintaining a good relationship with the website providers and respecting the content and services they offer.