Analysis of Web Caching Architectures: Hierarchical and Distributed CachingPablo Rodriguez, Christian Spanner, and Ernst W. BiersackIEEE/ACM TRANSACTIONS ON NETWORKINGVOL. 9, NO. 4, Auguest 2001

AbstractCaching architecturesHierarchicalDistributedHybridAnalytical modelsPerformanceConnection timeTransmission timeTotal latencyBandwidthCache load

Caching architecturesHierarchical cachingInstitutional cacheIntermediate cacheNational cacheDistributed cachingInstitutional cache

Network topology

The modelNetwork modelFull O-ary treeDocument modelRequest Poisson distributionPopularity - Zipf distributionHierarchical cachingCaches are placed at the access points between two different networks.Distributed cachingCaches are placed at the institutional network.

Network model

Document model

Properties and limitations of the modelO-ary trees are good models.Modifying the height or the number of tiers of the tree can easily model other networks.The model assumes homogeneous client communities.Heterogeneous client communities can be easily modeled.Simulations results in this paper should be considered as relative results.

Connection timeDepend on the number of network links from the client to the cache.

Connection time (contd)Distance of transmissionA request first travels up then downTCP three-way handshakeServer

Transmission timeCaches operate in a cut-through mode.Requestrate

ComparisonO = 4H = 3z = 10N = 250 million

Connection time

Network traffic at every tree level

Expected transmission time(a) Non-congested national network(b) Congested national network

Total latency

Heterogeneous client communities(a) Expected connection time(b) Expected transmission time

Bandwidth usageThe expected number of links traversed to distribute one packet to the clients.(a) Regional network(b) National network

Cache loadThe filtered request rate

Disk spaceThe average Web document size S times the average number of copies present in the caching infrastructure.The average number of copies present in the caching infrastructure can be calculated using the probability that a new document copy is created at every cache level.

Disk space (contd)

A hybrid caching schemeA certain number of caches k cooperate at every network level.When a document cannot be found in a cacheThe cache checks if the document resides in any of the cooperating caches.If multiple caches have a document copy, the neighbor cache with the lowest latency is selected.Otherwise, the request is then forwarded to the immediate parent cache or to the server.

Connection time

Connection time (contd)

Transmission time

Transmission time (contd)

Total latency

Bandwidth usage(a) National network(b) Regional network

Cache load

ConclusionsHierarchical caching architectureReduce the expected distance to hit a documentDecrease the bandwidth usageReduce the administrative concernsNeed powerful intermediate caches or load-balancing algorithmsDistributed caching architectureLarge network distancesHigh bandwidth usagesAdministrative issuesHybrid scheme is the best