Popular Internet applications deploy a multi-tier architecture, with each tier provisioning a certain functionality to its preceding tier. In this paper, we address the challenging session-based admission control issue for multi-tier Internet applications. The session-based admission control approach (SBAC) designed for a single Web server is not effective for a multi-tier architecture. This is due to the fact that the bottleneck in a multi-tier website dynamically shifts among tiers as client access patterns change. Admission control based on only the bottleneck tier is not efficient as different sessions impose different resource consumptions at the different tiers. First, we propose a multi-tier measurement based admission control (MBAC), which pro-actively accepts different session mixes based on the utilization state of all tiers. More importantly, we design a coordinated session-based admission control approach (CoSAC) based on a machine learning technique. It uses a Bayesi...