9 30 2007 Improving HTTP Performance Caching w Content Distribution Networks Integrate forward and reverse caching functionality One overlay network usually administered by one entity e g Akamai CDNs Replication Provide document caching Pull Direct result of clients requests Push Expectation of high access rate Prof Vern Paxson Also do some processing EE122 Fall 2007 Handle dynamic web pages Transcoding TAs Lisa Fowler Daniel Killebrew Jorge Ortiz Improving HTTP Performance Improving HTTP Performance Caching with CDNs cont CDN Example Akamai Akamai Creates new domain names for each client content provider Server e g i a cnn net CNAME custom i cnn net edgesuite net custom i cnn net edgesuite net CNAME a1921 g akamai net CDN Backbone ISP ISP 1 Customer Content Provider Modifies content ISP 2 Forward proxies Embedded URLs reference new domains CNN page s HREF s refer to i a cnn net Clients Pushes content out to Akamai as it changes Or Akamai pulls it on demand w usual caching mech Both CNN Akamai have control over load distribution Improving HTTP Performance Improving HTTP Performance CDN Example Akamai Caching vs Replication cnn com Akamaizes its content Why move content closer to users akamai net DNS servers a Akamai servers store cache secondary content for Akamaized services lookup a1921 g akamai net How b Caching DNS server for cnn com c local DNS server Akamaized response object has inline URLs for secondary content at after resolving CNAMEs a1921 g akamai net and other Akamai managed DNS names Reduce latency for the user Reduce load on the network and the server GET http cnn com 1 DNS Lookup 2 Fetch page w Akamaized content 3 DNS Lookup for Akamai URLs 4 Fetch content Replicate content on demand after a request Store the response message locally for future use Challenges May need to verify if the response has changed and some responses are not cacheable Replication Planned replication of content in multiple locations Update of resources handled outside of HTTP Can replicate scripts that create dynamic responses 1 9 30 2007 Hosting Multiple Machines Per Site Replicate a popular Web site across multiple machines Helps to handle the load Places content closer to clients Helps when content isn t cacheable by proxies CDNs Hosting Multiple Machines Per Site Solution 2 single IP address multiple machines Run multiple machines behind a single IP address Problem Want to direct client to a particular replica Why Balance load across server replicas Pair clients with nearby servers Load Balancer 64 236 16 20 Solution 1 Manual selection by clients Each replica has its own site name A Web page lists the replicas e g by name location and asks clients to click on a hyperlink to pick Hosting Multiple Machines Per Site Solution 3 multiple addresses multiple machines Same name but different addresses for all of the replicas Configure DNS server to return different addresses Ensure all packets from a single TCP connection go to the same replica Hosting Multiple Sites Per Machine Multiple Web sites on a single machine Hosting company runs the Web server on behalf of multiple sites e g www foo com and www bar com Problem GET index html www foo com index html or www bar com index html Solutions Multiple server processes on the same machine 12 1 1 1 64 236 16 20 Internet Have a separate IP address or port for each server Include site name in HTTP request Single Web server process with a single IP address Client includes Host header e g Host www foo com Required header with HTTP 1 1 173 72 54 131 2
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