The paper formalizes the Causal Observability Problem and proves that an external monitor of shared-memory operations cannot be both sound and complete, with external instrumentation giving soundness and internal giving completeness. Of three timestamping implementations, FAInc and Striped are linearizable while Collect fails all standard consistency models, yet all give identical soundness under external placement.

— via arXiv Distributed Computing, Gilde Valeria Rodr'iguez, Armando Casta~neda, Miguel Pi~na