papers/lp-210-block-stm/sections/03-zapdb-integration.tex

\section{Read/Write-Set Tracking via ZapDB MVCC}

LP-210's correctness reduces to ZapDB's MVCC version vectors. Each
cell in ZapDB has a version pointer; reads observe the version
visible from the snapshot; writes create a new version. The
commit-phase conflict check is a version-vector intersection test:

\begin{table}[h]
\centering
\small
\begin{tabular}{@{}p{6cm}p{7cm}@{}}
\toprule
\textbf{Concept} & \textbf{ZapDB primitive} \\
\midrule
Read $\textrm{tx}_i$ at key $K$, observing version $V$ &
  \texttt{read\_set[i] $\ni$ (K, V)} \\
Write $\textrm{tx}_i$ at key $K$, replacing version $V_{\textrm{prior}}$ with $V_{\textrm{new}}$ &
  \texttt{write\_set[i] $\ni$ (K, $V_{\textrm{new}}$, $V_{\textrm{prior}}$)} \\
Conflict: $\textrm{tx}_i$ read $V_{\textrm{prior}}$ of $K$ but $\textrm{tx}_j$ ($j<i$, committed) wrote $V_{\textrm{new}}$ of $K$ &
  $\textrm{tx}_i.\textrm{read\_set}$ contains $(K, V_{\textrm{prior}})$ and \texttt{staged\_overlay} has key $K$ with a different version \\
Resolution: roll back $\textrm{tx}_i$, re-execute &
  LP-202 \texttt{txn.Discard()} on $\textrm{tx}_i$'s per-tx overlay; re-execute \\
\bottomrule
\end{tabular}
\caption{Conflict detection reduces to ZapDB MVCC version-vector
intersection.}
\label{tab:mvcc}
\end{table}

No global lock. No serialization across the wave. The pattern is
identical to a Software Transactional Memory implementation -- hence
the name.