How do you handle redirects when using Mechanize?

HTTP redirects are a fundamental part of web navigation, and handling them properly is crucial for effective web scraping with Ruby's Mechanize library. Mechanize provides robust built-in support for following redirects automatically, while also giving you fine-grained control when needed.

Understanding Redirects in Mechanize

By default, Mechanize automatically follows HTTP redirects (301, 302, 303, and 307 status codes) up to a maximum of 20 redirects. This behavior makes it easy to navigate websites that use redirects for URL shortening, domain changes, or routing logic.

Basic Redirect Handling

The simplest approach is to let Mechanize handle redirects automatically:

require 'mechanize'

agent = Mechanize.new
page = agent.get('http://example.com/redirect-url')

# Mechanize automatically follows redirects
puts page.uri  # Shows the final URL after redirects
puts page.title

Configuring Redirect Behavior

Setting Maximum Redirect Limit

You can customize the maximum number of redirects Mechanize will follow:

agent = Mechanize.new
agent.max_history = 50  # Increase redirect limit to 50

# Or set it to 0 to disable automatic redirects
agent.max_history = 0

Accessing Redirect History

Mechanize maintains a history of all pages visited, including redirects:

agent = Mechanize.new
page = agent.get('http://example.com/redirect-url')

# Access the redirect chain
agent.history.each_with_index do |visited_page, index|
  puts "#{index}: #{visited_page.uri}"
end

# Get the previous page (useful after redirects)
previous_page = agent.back

Manual Redirect Handling

For more control over redirect behavior, you can disable automatic redirects and handle them manually:

require 'mechanize'

agent = Mechanize.new
agent.redirect_ok = false  # Disable automatic redirects

begin
  page = agent.get('http://example.com/redirect-url')
rescue Mechanize::RedirectLimitReachedError => e
  puts "Redirect detected: #{e.message}"

  # Get the redirect location from response headers
  redirect_url = e.page.response['location']
  puts "Redirecting to: #{redirect_url}"

  # Manually follow the redirect
  page = agent.get(redirect_url)
end

Handling Specific Redirect Types

Different redirect status codes have specific meanings. Here's how to handle them:

agent = Mechanize.new
agent.redirect_ok = false

response = agent.get('http://example.com/some-url')

case response.code.to_i
when 301
  puts "Permanent redirect"
  new_url = response.response['location']
  # Update bookmarks/cache since this is permanent

when 302, 303
  puts "Temporary redirect"
  new_url = response.response['location']
  # Don't update permanent references

when 307
  puts "Temporary redirect (preserve method)"
  new_url = response.response['location']
  # Must preserve the original HTTP method

else
  puts "No redirect needed"
end

Advanced Redirect Scenarios

Handling Relative Redirects

Sometimes redirect locations are relative URLs. Mechanize handles this automatically, but you can also resolve them manually:

require 'uri'

agent = Mechanize.new
page = agent.get('http://example.com/page')

# If you need to manually resolve relative redirects
if page.response['location']
  redirect_uri = URI.join(page.uri, page.response['location'])
  puts "Full redirect URL: #{redirect_uri}"
end

Detecting Redirect Loops

Prevent infinite redirect loops by tracking visited URLs:

agent = Mechanize.new
visited_urls = Set.new
max_redirects = 10
current_url = 'http://example.com/start'

max_redirects.times do |i|
  if visited_urls.include?(current_url)
    puts "Redirect loop detected at: #{current_url}"
    break
  end

  visited_urls.add(current_url)

  begin
    page = agent.get(current_url)
    break  # Successfully loaded page
  rescue Mechanize::RedirectLimitReachedError => e
    current_url = e.page.response['location']
    puts "Redirect #{i + 1}: #{current_url}"
  end
end

Redirect Handling with Sessions and Cookies

When dealing with authentication or session-based redirects, ensure cookies are properly maintained:

agent = Mechanize.new
agent.cookie_jar = Mechanize::CookieJar.new

# Login and handle redirects
login_page = agent.get('http://example.com/login')
form = login_page.form_with(name: 'login')
form.username = 'your_username'
form.password = 'your_password'

# Submit form and follow authentication redirects
result_page = agent.submit(form)

puts "Final URL after login redirects: #{result_page.uri}"

Error Handling and Debugging

Comprehensive error handling for redirect scenarios:

require 'mechanize'

agent = Mechanize.new
agent.log = Logger.new(STDOUT)  # Enable logging for debugging

begin
  page = agent.get('http://example.com/complex-redirect')

rescue Mechanize::RedirectLimitReachedError => e
  puts "Too many redirects: #{e.message}"
  puts "Last attempted URL: #{e.page.uri}"

rescue Mechanize::ResponseCodeError => e
  puts "HTTP Error during redirect: #{e.response_code}"

rescue Net::HTTPBadResponse => e
  puts "Malformed redirect response: #{e.message}"

rescue StandardError => e
  puts "Unexpected error: #{e.message}"
end

Best Practices for Redirect Handling

1. Monitor redirect chains: Long redirect chains can indicate configuration issues or malicious behavior.

2. Respect redirect limits: Don't set arbitrarily high redirect limits as they can lead to infinite loops.

3. Handle relative URLs: Always be prepared for relative redirect locations.

4. Preserve important headers: When manually following redirects, ensure important headers like authentication tokens are maintained.

5. Log redirect activity: Enable logging to debug complex redirect scenarios.

Similar to how to handle page redirections in Puppeteer, Mechanize provides multiple approaches for handling redirects. For authentication-related redirects, you might also want to learn about how to handle authentication in Puppeteer for browser-based scenarios.

Testing Redirect Handling

Here's a simple test to verify your redirect handling:

require 'mechanize'
require 'webrick'

# Create a simple test server with redirects
server = WEBrick::HTTPServer.new(Port: 8080)
server.mount_proc('/redirect') do |req, res|
  res.status = 302
  res['Location'] = '/final'
end

server.mount_proc('/final') do |req, res|
  res.body = 'Final destination reached!'
end

# Start server in background
Thread.new { server.start }

# Test redirect handling
agent = Mechanize.new
page = agent.get('http://localhost:8080/redirect')
puts page.body  # Should show "Final destination reached!"

server.shutdown

Conclusion

Mechanize's redirect handling capabilities make it an excellent choice for web scraping scenarios involving complex navigation patterns. Whether you need automatic redirect following or fine-grained control over each redirect, Mechanize provides the tools necessary for robust web scraping applications.

The key is understanding when to use automatic redirects versus manual control, and implementing proper error handling to deal with edge cases like redirect loops or malformed responses. With these techniques, you can confidently handle any redirect scenario in your web scraping projects.