eth-summaries/semester4/dmdb/parts/05_concurrency-control-recovery/08_implementation-of-recovery.tex

\subsection{Implementation of Recovery}
The most common implementation of these ideas is Write Ahead Logging (WAL).
There, we separate the storage used for the log and data. The log contains enough information for whatever policy is chosen and log records corresponding to a change in the DB
must be written to the log before changes to the data in the buffer cache are flushed to permanent storage.
The COMMIT record in the log is us ed to mark the end of a transaction.

This is typically used to implement UNDO/REDO (on 2PL-based systems) or no-UNDO/REDO (on Snapshot Isolation (SI)-based systems)


\subsubsection{Checkpoints}
A log would grow forever if we don't do anything. Storage would then become an issue and recovery time would increase.
Instead, Checkpoints are used. At a checkpoint, we do the following:
\begin{itemize}
    \item Push all dirty blocks to the disk
    \item Push all the logs in the log buffer to disk
    \item We now have an active translation table and dirty page table
    \item We mark the log with a checkpoint label and flush it to the log
\end{itemize}
Then, recovery happens from a checkpoint instead from the beginning. However note that a checkpoint is not necessarily a consistent copy of the DB.


\subsubsection{Recovery with WAL and checkpoints}
Also called ARIES style recovery, if we use WAL, the recovery with a checkpoint works as follows:

\begin{algorithm}
    \caption{WAL checkpoint recovery}
    \begin{algorithmic}[1]
        \Procedure{WALCheckpointRecovery}{}
            \State Find latest completed checkpoint in the log
            \While{Traversing the log to the end}
                \State identify active transactions at time of crash and dirty pages that may not have made it to disk at time of crash for each element
            \EndWhile
            \State Apply all updates (REDO) starting from the log entry matching the lowest SCN in the dirty pages
            \State UNDO all transactions that were active at the time of the crash
        \EndProcedure
    \end{algorithmic}
\end{algorithm}