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A scalable e-commerce server should be able to provide different levels of quality of service (QoS) to different types of requests according to clients' navigation patterns and the server capacity. In this paper, we propose a two-dimensional (2D) service differentiation (DiffServ) model for online transactions: inter-session and intra-session. The inter-session model aims to provide different levels of QoS to sessions from different customer classes, and the intra-session model aims to provide different levels of QoS to requests in different states of a session. We introduce service slowdown as a QoS metric of e-commerce servers. It is defined as the weighted sum of request slowdown in different sessions and in different session states. We formulate the problem of 2D DiffServ provisioning as an optimization of processing rate allocation with the objective of minimizing service slowdown. We derive the optimal allocations for an M/G/1 server under various server load conditions and prove that the optimal allocations guarantees requests' slowdown to be square-root proportional to their pre-specified differentiation weights in both dimensions. We evaluate the optimal allocation scheme via extensive simulations and compare it with a tailored proportional DiffServ scheme. Simulation results validate that both allocation schemes can achieve predictable, controllable, and fair 2D slowdown differentiation on e-commerce servers. The optimal allocation scheme guarantees 2D DiffServ at a minimum cost of service slowdown.