The Effect of Consistency on Cache Response Time John Dilley and HP Laboratories IEEE Network, May-June 2000 Chun-Fu Kung System Laboratory Dept. of Computer Engineering and Science Yuan-Ze University 2000/9/6

Outline Introduction Cache consistency HTTP consistency validation Some limitations Simulation and results Conclusion

Introduction Proxy cache can improve response time, reduce network demand, and lighten the load on the origin Web servers. Cache servers are typically placed close to a group of end users and handle all HTTP requests from those users. A cached copy of an object may differ from the current copy of that object at the origin server.

Introduction(cont.) Currently origin servers do not communicate changes to caches; a cache must ask about them. The cache must decide whether to serve the object immediately or to validate it with the origin server. The cache will serve locally any object it considers to be fresh, but will attempt to check an object’s consistency before serving the request if the object is stale.

Cache Consistency Fast hits – the cache return objects data to the requester without remote communication. Fast validations – the cache return only a validation of freshness to the requester. Slow hits – the cache return object data to the requester after validating it with origin server. Slow validations – the cache return the validation of freshness after contacting the origin server.

Cache Consistency(cont.) Consistency miss – the cache return a copy of the object after contacting the origin server and getting a fresh copy of the object. Regular miss – the cache return an object that was not in cache after contacting the origin server to retrieve a copy of that object (cold miss and capacity miss). Direct – the request is send to the origin server, and the response data is relay to the client (the cache does not keep the copy of that object).

HTTP Consistency Validation Object age – the time between modification and current time (the value is Age1). TTL – the cache computes a percentage of the object’s age (the value is %Age1) During the TTL period the object is considered fresh. After the TTL expires the object in cache is stale.

Some Limitations Cache log files usually includes the object URI, the object size, the service time, whether object data was returned to the client, and whether the cache made a consistency check with remote server. There are some limitations with such a log-based analysis scheme. Cache log don’t indicate when the client received and displayed the response in the browser application.

Some Limitations(cont.) Service time don’t include processing time of the operating system kernel or network card on the cache system. The logs cannot identify which objects are served from browser’s cache. The log don’t indicate whether external requests are served by other parent proxies.

Server Studied The analysis is based on the study of log files from three Web cache servers: – granite.hpl.hp.com : 3.3 million/month(16) – proxy.hpl.hp.com : 11.4 million/month(1) – Cable modem site : 8.2 million/week(1)

ResponseMean sizeResponse time (s) type%(kbytes)mean50th80th Fast val Slow val Fast hit Slow hit Cons. miss Miss Result-1

ResponseMean sizeResponse time (s) type%(kbytes)mean50th80th Fast val Slow val Fast hit Slow hit Cons. miss Miss Direct n/a Result-2

ResponseMean sizeResponse time (s) type%(kbytes)mean50th80th Fast val Slow val Fast hit Slow hit Miss Result-3

Conclusion When an object is in cache it takes approximately half the time to validation and return the object to client quickly. We have proposed such a mechanism and shown via simulation that it provides better object consistency, is faster for end users, consumes slightly less network bandwidth, and reduce origin server load.