Hybrid cooperative schemes for scalable and stable performance of Web content delivery

Systems consisting of multiple edge servers are a popular solution to deal with performance and network resource utilization problems related to the growth of the Web. After a first period of prevalent enthusiasm towards cooperating edge servers, the research community is exploring in a more systematic way the real benefits and limitations of cooperative caching. Hierarchical cooperation has clearly shown its limits. We show that the pure protocols (e.g., directory-based, query-based) applied to a flat cooperation topology do not scale as well. For increasing numbers of cooperating edge servers, the amount of exchanged data necessary for cooperation augments exponentially, or the cache hit rates fall down, or both events occur. We propose and evaluate two hybrid cooperation schemes for document discovery and delivery. They are based on a semi-flat architecture that organizes the edge servers in groups and combines directory-based and query-based cooperation protocols. A large set of experimental results confirms that the combination of directory-based and query-based schemes increases the scalability of flat architectures based on pure protocols, guarantees more stable performance and tends to reduce pathologically long response times.