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Cooperative Caching of Dynamic Content on a Distributed Web Server

Cooperative Caching of Dynamic Content on a Distributed Web Server. Vegard Holmedahl, Ben Smith, Tao Yang Speaker: SeungLak Choi, DB Lab., CS Dept. Contents. Introduction Access log analysis Design of the Swala Experiments Conclusions Critiques. Introduction (1/2).

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Cooperative Caching of Dynamic Content on a Distributed Web Server

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  1. Cooperative Caching of Dynamic Content on a Distributed Web Server Vegard Holmedahl, Ben Smith, Tao Yang Speaker: SeungLak Choi, DB Lab., CS Dept.

  2. Contents • Introduction • Access log analysis • Design of the Swala • Experiments • Conclusions • Critiques

  3. Introduction (1/2) • Web caching reduces mainly the network delay • Processor bottleneck is also important in sites serving extensive requests to dynamic contents

  4. Introduction (2/2) • Swala • Distributed Web server • Cooperatively caches the results of CGI requests • Alexandria Digital Library (ADL) system at UCSB • Accessing multi-resolution images • Spatial database queries

  5. Access Log Analysis (1/2) • There is significant potential for reducing response time by optimizing CGI

  6. Access Log Analysis (2/2) • By caching a few number of CGI results, achieve significant response time reduction

  7. Design of the Swala (1/3) • Module design • HTTP Module • Handling HTTP requests • Cache module • Updates local directory • Send contents to other node • Delete expired cache entries

  8. Design of the Swala (2/3) • Cache table consistency • Intra-node consistency • Avoiding corruption from simultaneous updates • Locking granularity – directory, table, entry • Inter-node consistency • Weak consistency • Broadcast update information to the other nodes • False cache miss, false cache hit

  9. Design of the Swala (3/3) • Content consistency • Weak consistency – TTL • Reasonable for dynamic contents with infrequent updates • Store the cache directory in main memory • Store CGI results in disk • System administrator must specify uncacheable CGI

  10. Experiments (1/6) • Environment • Sun Ultra 1 143-MHz • Sun Ultra 2 167-MHz • 64 or 128 MB • Connected by 100Mbps Ethernet • WebStone

  11. Experiments (2/6) • Single-node performance and overhead of cache fetch • 24 clients request nullcgi that does no work and produces <1000 bytes output

  12. Experiments (3/6) • Multi-node performance

  13. Experiments (4/6) • Cache insertion overhead • Each unique request generates a cache miss

  14. Experiments (5/6) • Cache directory maintenance overhead • Generate update messages to Swala

  15. Experiments (6/6) • A comparison between cooperative and stand-alone caching Cache size 2000 Cache size 20

  16. Conclusions • Caching CGI results on a cluster of workstations • Significant potential for time saving through caching CGI results • Overhead of cooperative cache management is small • Cooperative caching of CGI results substantially improves the response time

  17. Critiques • Strengths • Attack the important problem occurred in common web sites • CGI programs don’t have to be rewritten • Weaknesses • Weak content consistency • Remote cache fetch can incur network congestion in heavy network traffic • Contents hashing of requests?

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