Abstract
Causal request traces are valuable to developers of large concurrent and distributed applications, yet difficult to obtain. Traces show how a request is processed, and can be analyzed by tools to detect performance or correctness errors and anomalous behavior. We present BorderPatrol, which obtains precise request traces through suystems built from a litany of unmodified modules. Traced components include Apache, thttpd, PostgreSQL, TurboGears, BIND and notably Zeus, a closed-source event-driven web server. BorderPatrol obtains traces using active observation which carefully modifies the event stream observed by modules, simplifying precise observation. Protocol processors leverage knowledge about standard protocols, avoiding application-specific instrumentation. BorderPatrol obtains precise traces for black-box systems that cannot be traced by any other technique. We confirm the accuracy of BorderPatrol's traces by comparing to manual instrumentation, and compare the developer effort required for each kind of trace. BorderPatrol imposes limited overhead on real systems (approximately 10-15%) and it may be enabled or disabled in at run-time, making it a viable option for deployment in production environments.
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