Web caching architectures: hierarchical and distributed caching

In this paper we analyze the performance of hierarchical and distributed caching architectures. With hierarchical caching, caches are placed at multiple levels of the network. With distributed caching, caches are only placed at the bottom levels of the network and there are no intermediate caches. Our main performance measurement is the expected latency to retrieve a Web document. We find that hierarchical caching has shorter connection times than distributed caching, thus, placing additional copies at intermediate network levels reduces the retrieval latency for small documents. We also find that distributed caching has shorter transmission times than hierarchical caching. Distributed caching has higher bandwidth usage than hierarchical caching. However, the network traffic generated by a distributed scheme is better distributed, using more bandwidth in the lower network levels, which are less congested. We also discuss administrative issues concerning the large scale deployment of distributed caching. Additionally, we study a hybrid scheme where a certain number of caches cooperate at every level of a caching hierarchy using distributed caching. We find that a well configured" hybrid scheme can combine the advantages of both hierarchical and distributed caching, reducing the connection time as well as the transmission time. Depending on the hybrid caching architecture, the current parent caches load, and the document size, there is a certain number of caches that should cooperate at each network level to minimize the overall retrieval latency. We propose small modifications of the existing cache-sharing protocols to dynamically determine the degree of cooperation between caches at every level of a hybrid caching scheme.