Abstract. Conformance checking, a method of process mining, is commonly used to assess how well a set of historic log traces fits a given process model, or vice versa. Here we explore online conformance checking, i.e., to check conformance on logs while they are written. This can be useful for near-realtime detection of errors and deviations from the desired path. While the online aspect leads to some challenges, we also study efficient detection of timing anomalies and violations of numerical invariants. The approach is implemented in CCaaS, a RESTful service that detects unfitting events and other errors in split seconds.