Data-Intensive infrastructures are increasingly used for on-line processing of live data to guide operations and decision making. VScope is a flexible monitoring and analysis middleware for troubleshooting such large-scale, time-sensitive, multi-tier applications. With VScope, lightweight anomaly detection and interaction tracking methods can be run continuously throughout an application’s execution. The runtime events generated by these methods can then initiate more detailed and heavier weight analyses which are dynamically deployed in the places where they may be most likely fruitful for root cause diagnosis and mitigation. We comprehensively evaluate VScope prototype in a virtualized data center environment with over 1000 virtual machines (VMs), and apply VScope to a representative on-line log processing application. Experimental results show that VScope can deploy and operate a variety of on-line analytics functions and metrics with a few seconds at large scale.Compared to traditional logging approaches, VScope based troubleshooting has substantially lower perturbation and generates much smaller log data volumes. It can also resolve complex cross-tier or cross-software-level issues unsolvable solely by application-level or per-tier mechanisms.