System Software Lab. A Scalable Web Cache Consistency Architecture Kim Sangyup SSLAB. EE. KAIST SIGCOMM ’ 99 Haobo Yu, Lee Breslau

System Software Lab. 2 Contents  Introduction  Previous approaches Time-To-Live Invalidation Lease  New Approach  Analytic Performance Evaluation  Conclusion & Critique

System Software Lab. 3 Introduction  Web caching Lowers the load on servers Reduces the overall network bandwidth required Lowers the latency of responses  But, disadvantage in cache consistency  Cache consistency Stale page in the cache is problem Perishable Mark uncachable, expecting “ reload ” button

System Software Lab. 4 Previous Approaches  Time-To-Live Server allocate TTL on each page To request on expired page send IMS message to server Set TTL to small : provide tight consistency  But decrease benefits of web caching TTL = 0 : poll away Adaptive TTL  Not give an upper bound on the staleness of a page

System Software Lab. 5 Previous Approaches(cont ’ d)  Invalidation When page is modified, server send invalidation signal Server keeps state on every client of each page  Scaling problems when there are many readers Multicast to transmit the invalidation  Assigning a multicast group to each page  Other unscalable overhead on the routing infrastructure

System Software Lab. 6 Previous Approaches(cont ’ d)  Lease Combines TTL and invalidation Whenever stores a page, requests a lease from server  Page change -> server notifies all caches(has valid page)  Request for expired lease of page -> send IMS message Short lease -> same to TTL Two volume lease algorithm  Reduce validation traffic of short lease  Page lease : long  Volume lease : short

System Software Lab. 7 New Approach  Multicast-based invalidation on application level  Using a cache hierarchy to avoid the scalability problem

System Software Lab. 8 New Approach(cont ’ d)  Hierarchy Multicast group are associated with caches, not pages Each parent cache owns a unique multicast group  Allocating group address  Unique sender in the group  Heartbeats Keep alive hierarchy Group owner sends out a periodic heartbeat(=t) Same to volume lease of length T(T/t = 5) (current time – highest timestamp) > T : lease expiring

System Software Lab. 9 New Approach(cont ’ d)  Attaching servers JOIN/LEAVE message Update the server routing table in each cache  Top level cache knows about all servers attached in hierarchy  Handling request

System Software Lab. 10 New Approach(cont ’ d)  Invalidations On top of heartbeats, piggyback explicit invalidations Only invalidate pages that have been requested(read page) Each heartbeat contains a list of all read pages that have been rendered invalid the last time period T In child cache  Have valid read page->mark invalid and propagate the invalidation  Have no valid read page->ignores the invalidation

System Software Lab. 11 New Approach(cont ’ d)  Adding Push to the Architecture Reduce first access latency Selective push  Each cache and server make their own independent decision about whether or not to push a page  Receives an invalidate of page P :  Request for page P :  Push page P In case

System Software Lab. 12 Analytic Performance Evaluation  Reading & writing : Poisson processed of rate r, w  Four event : RR, RW, WR, WW  Frequency of event

System Software Lab. 13 Conclusion & Critique  Conclusion Multicast invalidation with volume leases A Caching hierarchy to make the design more scalable  Critique Stable and well-managed cache hierarchy Higher level cache have more overload Make response time longer