As more and more multi-tier services are developed from commercial off-the-shelf components or heterogeneous middleware without source code available, both developers and administrators need a request tracing tool to (1) exactly know how a user request of interest travels through services of black boxes; (2) obtain macro-level user request behaviors of services without manually analyzing massive logs. This need is further exacerbated by IT system "agility", which mandates tracing tools to provide on-line performance data since off-line approaches cannot reflect system changes in real time. Moreover, considering the large scales of the deployed services, a pragmatic tracing approach should be scalable in terms of the cost in collecting and analyzing logs. In this paper, we introduce a precise, scalable, and online request tracing tool for multi-tier services of black boxes. Our contributions are three-fold. First, we propose a precise request tracing algorithm for multi-tier s...