Agar: What-If Analysis for Microservice Applications
Microservice operators must reason about how applications would behave under changes that have not yet occurred: different workloads, resource allocations, or SLO targets. Evaluating every alternative experimentally is expensive, but predicting such counterfactuals is difficult because microservices contain many interacting components. End-to-end models must learn behavior over a combinatorial space of conditions, but composing per-component predictors causes prediction errors to accumulate. We address this challenge by changing what each component model must predict. Rather than predicting exact values, we learn simple, interpretable bounds that contain every observed behavior while remaining tight enough to support useful end-to-end conclusions. We build AGAR, a counterfactual performance analysis tool that composes these models to answer end-to-end SLO-feasibility questions. This allows us to formulate autoscaling as a counterfactual query: what is the smallest resource allocation under which an SLO is met? Our evaluation demonstrates that across three microservice applications, AGAR's bounds are actionable: its derived autoscaler reduces CPU allocation by up to 36.8% over state-of-the-art horizontal autoscalers while satisfying application SLOs. Further, AGAR is able to predict operational cost given workload traces, provide guidance on which service's startup time to optimize and localize performance bugs within each application.