\begin{frame}
  \frametitle{Deadlock Handling via Wait-for-Graphs}
  
  \begin{goal}{Deadlock Detection via Wait-for-Graphs}
    The system maintains a \emph{waits-for-graph}:
      \begin{itemize}
        \item Nodes of the graph are transactions.
        \item Edge $T_1 \to T_2$ means $T_1$ is blocked by a lock held by $T_2$.\\
          \remark{Hence $T_1$ waits for $T_2$ to release the lock.}
      \end{itemize}
    \pause\smallskip
    
    The system checks periodically for \emph{cycles} in the graph:
    \begin{itemize}
      \item If a cycle is detected, then the deadlock is \emph{resolved} by \emph{aborting}
        one or more transactions.
    \end{itemize}
  \end{goal}
  \pause\smallskip
  
  \begin{alertblock}{Selecting the \emph{victim} is a challenge}
  \begin{itemize}
    \item Aborting \emph{young} transactions might lead to starvation.
      The same transaction may be cancelled again and again.
    \item Aborting \emph{old} transactions may cause a lot of computational investment to be thrown away.
  \end{itemize}
  \end{alertblock}
\end{frame}