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High Performance Web Sites

Cover of High Performance Web Sites by Steve Souders Published by O'Reilly Media, Inc.
  1. High Performance Web Sites
    1. SPECIAL OFFER: Upgrade this ebook with O’Reilly
    2. Praise for High Performance Web Sites
    3. Foreword
    4. Preface
      1. How This Book Is Organized
      2. Conventions Used in This Book
      3. Code Examples
      4. Comments and Questions
      5. Safari® Books Online
      6. Acknowledgments
    5. 1. The Importance of Frontend Performance
      1. Tracking Web Page Performance
      2. Where Does the Time Go?
      3. The Performance Golden Rule
    6. 2. HTTP Overview
      1. Compression
      2. Conditional GET Requests
      3. Expires
      4. Keep-Alive
      5. There's More
    7. 3. Rule 1: Make Fewer HTTP Requests
      1. Image Maps
      2. CSS Sprites
      3. Inline Images
      4. Combined Scripts and Stylesheets
      5. Conclusion
    8. 4. Rule 2: Use a Content Delivery Network
      1. Content Delivery Networks
      2. The Savings
    9. 5. Rule 3: Add an Expires Header
      1. Expires Header
      2. Max-Age and mod_expires
      3. Empty Cache vs. Primed Cache
      4. More Than Just Images
      5. Revving Filenames
      6. Examples
    10. 6. Rule 4: Gzip Components
      1. How Compression Works
      2. What to Compress
      3. The Savings
      4. Configuration
      5. Proxy Caching
      6. Edge Cases
      7. Gzip in Action
    11. 7. Rule 5: Put Stylesheets at the Top
      1. Progressive Rendering
      2. sleep.cgi
      3. Blank White Screen
      4. Flash of Unstyled Content
      5. What's a Frontend Engineer to Do?
    12. 8. Rule 6: Put Scripts at the Bottom
      1. Problems with Scripts
      2. Parallel Downloads
      3. Scripts Block Downloads
      4. Worst Case: Scripts at the Top
      5. Best Case: Scripts at the Bottom
      6. Putting It in Perspective
    13. 9. Rule 7: Avoid CSS Expressions
      1. Updating Expressions
      2. Working Around the Problem
      3. Conclusion
    14. 10. Rule 8: Make JavaScript and CSS External
      1. Inline vs. External
      2. Typical Results in the Field
      3. Home Pages
      4. The Best of Both Worlds
    15. 11. Rule 9: Reduce DNS Lookups
      1. DNS Caching and TTLs
      2. The Browser's Perspective
      3. Reducing DNS Lookups
    16. 12. Rule 10: Minify JavaScript
      1. Minification
      2. Obfuscation
      3. The Savings
      4. Examples
      5. Icing on the Cake
    17. 13. Rule 11: Avoid Redirects
      1. Types of Redirects
      2. How Redirects Hurt Performance
      3. Alternatives to Redirects
    18. 14. Rule 12: Remove Duplicate Scripts
      1. Duplicate Scripts—They Happen
      2. Duplicate Scripts Hurt Performance
      3. Avoiding Duplicate Scripts
    19. 15. Rule 13: Configure ETags
      1. What's an ETag?
      2. The Problem with ETags
      3. ETags: Use 'Em or Lose 'Em
      4. ETags in the Real World
    20. 16. Rule 14: Make Ajax Cacheable
      1. Web 2.0, DHTML, and Ajax
      2. Asynchronous = Instantaneous?
      3. Optimizing Ajax Requests
      4. Caching Ajax in the Real World
    21. 17. Deconstructing 10 Top Sites
      1. Page Weight, Response Time, YSlow Grade
      2. How the Tests Were Done
      3. Amazon
      4. AOL
      5. CNN
      6. eBay
      7. Google
      8. MSN
      9. MySpace
      10. Wikipedia
      11. Yahoo!
      12. YouTube
    22. Index
    23. About the Author
    24. Colophon
    25. SPECIAL OFFER: Upgrade this ebook with O’Reilly
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Chapter 11. Rule 9: Reduce DNS Lookups

The Internet is based on finding servers through IP addresses. Because IP addresses are hard to remember, URLs typically contain hostnames instead, but the IP address is still necessary for the browser to make its request. That's the role of the Domain Name System (DNS). DNS maps hostnames to IP addresses, just as phonebooks map people's names to their phone numbers. When you type www.yahoo.com into your browser, a DNS resolver is contacted by the browser and returns that server's IP address.

This explanation highlights another benefit of DNS—a layer of indirection between URLs and the actual servers that host them. If a server is replaced with one that has a different IP address, DNS allows users to use the same hostname to connect to the new server. Or, as is the case with www.yahoo.com, multiple IP addresses can be associated with a hostname, providing a high degree of redundancy for a web site.

However, DNS has a cost. It typically takes 20–120 milliseconds for the browser to look up the IP address for a given hostname. The browser can't download anything from this hostname until the DNS lookup is completed. The response time depends on the DNS resolver (typically provided by your ISP), the load of requests on it, your proximity to it, and your bandwidth speed. After reviewing how DNS works from the browser's perspective, I'll describe what you can do to reduce the amount of time your pages spend doing DNS lookups.

DNS Caching and TTLs

DNS ...

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