October 25, 2001Stanford Networking Seminar Aging Through Cascaded Caches: Performance Issues in the Distribution of Web Content. Edith Cohen AT&T Labs-research Haim Kaplan Tel-Aviv University

October 25, 2001Stanford Networking Seminar HTTP Freshness Control Cached copies have: –Freshness lifetime –Age (elapsed time since fetched from origin) TTL (Time to Live) = freshness lifetime – age Expired copies must be validated before they can be used (request constitutes a ”cache miss”). Body (content) header Cache-directives

October 25, 2001Stanford Networking Seminar Aging of Copies Origin server Freshness Lifetime = 10 hours Age = 0 TTL = 10 8:00am

October 25, 2001Stanford Networking Seminar Aging of Copies Origin server Freshness Lifetime = 10 hours Age = 1 TTL = 9 9:00am 12:00pm Age = 4 TTL = 6 3:00pm Age = 7 TTL = 3

October 25, 2001Stanford Networking Seminar Aging of Copies Origin server Freshness Lifetime = 10 hours 6:00pm Age = 10 TTL = 0

October 25, 2001Stanford Networking Seminar Aging thru Cascaded Caches reverse-proxy cache origin server 8:00am proxy caches Age = 0 TTL = 10

October 25, 2001Stanford Networking Seminar 5:00pm Age = 9 TTL = 1 Aging thru Cascaded Caches reverse-proxy cache origin server proxy caches

October 25, 2001Stanford Networking Seminar 6:00pm Aging thru Cascaded Caches reverse-proxy cache origin server proxy caches Age = 10 TTL = 0

October 25, 2001Stanford Networking Seminar 6:00pm Aging thru Cascaded Caches reverse-proxy cache origin server proxy caches Age = 0 TTL = 10

October 25, 2001Stanford Networking Seminar TTL of a Cached Copy Freshness -lifetime t TTL Requests at client cache: From Origin MM From CacheMMM

October 25, 2001Stanford Networking Seminar Age-Induced Performance Issues for Cascaded Caches Caches are often cascaded (path between web server and end-user includes 2 or more caches.). Copies obtained thru a cache are less effective than copies obtained thru an origin server.Copies obtained thru a cache are less effective than copies obtained thru an origin server. Reverse proxies increase validation traffic !! Reverse proxies increase validation traffic !! More misses at downstream caches mean: – Increased traffic between cascaded caches. – Increased user-perceived latency.

October 25, 2001Stanford Networking Seminar Research Questions How does miss-rate depend on the configuration of upstream cache(s) and on request patterns ? Can upstream caches improve performance by proactively reducing content age ? how? Can downstream caches improve performance by better selection or use of a source? Request sequences: Arbitrary, Poisson, Pareto, fixed-frequency, Traces. Models for Cache/Source/Object relation: Authoritative, Independent, Exclusive. Our analysis:

October 25, 2001Stanford Networking Seminar Basic Relationship Models cache/source/object Authoritative: “Origin server:” 0 age copies. Exclusive: all misses directed to the same cache. Independent: each miss is directed to a different independent upstream cache. Cache-3Cache-2 Cache-1 Cache-BCache-ACache-CCache-D

October 25, 2001Stanford Networking Seminar Basic Models… Theorem: On all sequences, the number of misses obeys: Authoritative < Exclusive < Independent Authoritative age(t) = 0 Exclusive age(t) = T - (t+ a ) mod T Independent age(t) e U[0,T] Object has fixed freshness-lifetime of T. Miss at time t results in a copy with age: Theorem: Exclusive < 2*Authoritative Independent < e*Authoritative

October 25, 2001Stanford Networking Seminar TTL of “Supplied” Copy Freshness -lifetime t TTL Requests Received at source: Exclusive Authoritative Independent Source:

October 25, 2001Stanford Networking Seminar How Much More Traffic? Log\Model AuthoritativeExclusiveIndependent NLANR UC47%55%57% NLANR SD52%60%62% Miss-rate for different configurations

October 25, 2001Stanford Networking Seminar Rejuvenation at Source Caches Rejuvenation: refresh your copy pre-term once its TTL drops below a certain fraction v of the Lifetime duration. t TTL Requests at client: 24h 12h v=0.5 no rejuv. source client

October 25, 2001Stanford Networking Seminar Rejuvenation’s Basic Tradeoff: Is increase/decrease monotone in V (?) Increases traffic between upstream cache and origin (fixed cost) origin Upstream cache Downstream Client caches Decreases traffic to client caches (larger gain with more clients)

October 25, 2001Stanford Networking Seminar Interesting Dependence on V… Independent(v) <> Exclusive(v) Independent(v) is monotone: if v1 > v2, Independent(v1) > Independent(v2) Exclusive(v) is not monotone (miss-rate can increase !!) Integral 1/v (synchronized rejuvenation): Exclusive(v) < Independent(v) and is monotone (Pareto, Poisson, not with fixed- frequency).

October 25, 2001Stanford Networking Seminar

October 25, 2001Stanford Networking Seminar

October 25, 2001Stanford Networking Seminar How Can Non-integral 1/v Increase Client Misses? Freshness -lifetime t TTL Upstream Cache Downstream Client Cache Copy at client is not synchronized with source. When it expires, the rejuv source has an aged copy. Requests at Client cache: Pre-term refreshes

October 25, 2001Stanford Networking Seminar Why Integral 1/v Works Well? Freshness -lifetime t TTL Upstream Cache Cached copies remain synchronized Requests at Upstream cache: Downstream Client Cache Pre-term refreshes v=0.5

October 25, 2001Stanford Networking Seminar Some Conclusions Configuration: Origin (“Authoritative”) is best. Otherwise, use a consistent upstream cache per object (“Exclusive”). “No-cache” request headers: resulting sporadic refreshes may increase misses at other client caches. (But it is possible to compensate…). Rejuvenation: potentially very effective, but a good parameter setting (synchronized refreshes) is crucial. Behavior patterns: Similar for Poisson, Pareto, traces, (temporal locality). Different for fixed-frequency. For more go to Full versions of: Cohen, Kaplan SIGCOMM 2001 Cohen, Halperin, Kaplan, ICALP 2001