Fair Ordering

A growing class of applications demands \emph{fair ordering/sequencing} of events which ensures that events generated earlier by one client are processed before later events from other clients. However, achieving such sequencing is fundamentally challenging due to the inherent limitations of clock synchronization. We advocate for an approach that embraces, rather than eliminates, clock variability. Instead of attempting to remove error from a timestamp, Tommy, our proposed system, leverages a statistical model to compare two noisy timestamps probabilistically by learning per-clock offset distributions. Our preliminary statistical model computes the probability that one event precedes another w.r.t. the wall-clock time without access to the wall-clock. This serves as a foundation for a new relation: \emph{likely-happened-before} denoted by $\xrightarrow{p}$ where $p$ represents the probability of an event to have happened before another. The $\xrightarrow{p}$ relation provides a basis for ordering multiple events which are otherwise considered \emph{concurrent} by the typical \emph{happened-before} ($\rightarrow$) relation. We highlight various related challenges including intransitivity of $\xrightarrow{p}$ relation as opposed to the transitive $\rightarrow$ relation. We also outline several research directions: online fair sequencing, stochastically fair total ordering, host-level support for fairness and more.