\begin{frame}
  \frametitle{Pessimistic: Lock-based Concurrency Control}
  
  \begin{goal}{Lock-based concurrency control}
    Transactions must \emph{lock} objects before using them.
  \end{goal}
  \pause\medskip
  
  \begin{block}{Types of \emph{locks}}
    \begin{itemize}
      \item \emph{Shared lock (S-lock)} is acquired on Y before reading Y.\\[1ex]
        Many transactions can hold a shared lock on Y.
    \medskip
      \item \emph{Exclusive lock (X-lock)} is acquired on Y before writing Y.\\[1ex]
        A transaction can hold an exclusive lock on Y only if
        no other transaction holds any lock on Y.
    \end{itemize}
  \end{block}
  \smallskip
  
   If a transaction has an X-lock on Y it can also read Y.
\end{frame}