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Architectural Issues of Web−Enabled Electronic Business phần 4

Hình 7: Nội dung giao hàng: (a) chuyển hướng DNS và (b) chuyển hướng đối tượng nhúng vào máy chủ nhân bản thích hợp nhất cho một yêu cầu người sử dụng có thể được xác định bằng cách sử dụng một điều phối viên tập trung (một máy chủ chuyển hướng chuyên dụng) hoặc cho phép ra quyết định phân phối | Redirection Protocol servers. Therefore this approach does not lend itself to intelligent load balancing. Since dynamic content delivery is very sensitive to the load on the servers however this approach can not be preferred in e-commerce systems. Note that it is also possible to use various hybrid approaches. Akamai Technologies www.akimi.com for instance is using an hybrid of the two approaches depicted in Figures 7 a and b . But whichever implementation approach is chosen the main task of the redirection is given a user request to identify the most suitable server for the current server and network status. fa fb Figure 7 Content delivery a DNS redirection and b embedded object redirection The most appropriate mirror server for a given user request can be identified by either using a centralized coordinator a dedicated redirection server or allowing distributed decision making each server performs redirection independently . In Figure 8 a there are several mirror servers coordinated by a main server. When a particular server experiences a request rate higher than its capability threshold it requests the central redirection server to allocate one or more mirror servers to handle its traffic. In Figure 8 b each mirror server software is installed to each server. When a particular server experiences a request rate higher than its capability threshold it checks the availability at the participating servers and determines one or more servers to serve its contents. 111 Redirection Protocol Figure 8 a Content delivery through central coordination and b through distributed decision making Note however that even when we use the centralized approach there can be more than one central server distributing the redirection load. In fact the central server s can broadcast the redirection information to all mirrors in a sense converging to a distributed architecture shown in Figure 8 b . In addition a central redirection server can act either as a passive directory server Figure