| Summary: | Delivering fast response times for user transactions is a critical requirement for Web services. Often, a Web service has Service Level Agreements (SLA) with its users that quantify how quickly the service has to respond to a user transaction. Typically, SLAs stipulate requirements for Web service response time percentiles, e.g., a specified target for the 95<sup>th</sup> percentile of response time. Violating SLAs can have adverse consequences for a Web service operator. Consequently, operators require systematic techniques to predict Web service response time percentiles. Existing prediction techniques are very time consuming since they often involve manual construction of queuing or machine learning models. To address this problem, we propose Prospective, a data-driven approach for predicting Web service response time percentiles. Given a specification for workload expected at the Web service over a planning horizon, Prospective uses historical data to offer predictions for response time percentiles of interest. At the core of Prospective is a lightweight simulator that uses collaborative filtering to estimate response time behaviour of the service based on behaviour observed historically. Results show that Prospective significantly outperforms other baseline techniques for a wide variety of workloads. In particular, the technique provides accurate estimates even for workload scenarios not directly observed in the historical data. We also show that Prospective can provide a Web service operator with accurate estimates of the types and numbers of Web service instances needed to avoid SLA violations.
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