papers/hanzo-webhooks/sections/07-event-ordering.tex

\section{Event Ordering}


\subsection{Causal Ordering}

Events for the same resource are delivered in causal order:

\begin{definition}[Causal Order]
For events $e_1, e_2$ on resource $r$:
\[
e_1 \prec e_2 \iff e_1.timestamp < e_2.timestamp \land e_1.resource = e_2.resource
\]
\end{definition}

\begin{theorem}[Order Preservation]
For causally related events $e_1 \prec e_2$:
\[
delivered(e_1, s) \text{ completes before } delivery(e_2, s) \text{ starts}
\]
\end{theorem}

Implementation: events are partitioned by resource ID, with single-threaded delivery per partition.

\subsection{Handling Out-of-Order Delivery}

Cross-resource events may arrive out of order. We include sequence information:

\begin{lstlisting}[caption=Sequence Metadata]
{
  "id": "evt_abc123",
  "sequence": 12345,
  "previous_sequence": 12344,
  ...
}
\end{lstlisting}

Endpoints can detect and reorder events using sequence numbers.