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36 lines
2.3 KiB
TeX
36 lines
2.3 KiB
TeX
\subsection{Transaction Model}
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\inlinedefinition Some terms for Transaction model:
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\begin{itemize}
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\item \bi{Begin of transaction (BOT)}: Often implicit
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\item \bi{Commit}: Transaction has finished, database confirms to client when all changes of the transaction have been made persistent
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\item \bi{Abort/rollback}: transaction is cancelled, all changes made already are rolled back
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\item $a <_T b$: $a$ happens before $b$, is a \textit{partial order}, which implies that $a$ will be done before $b$.
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\end{itemize}
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In real systems, transactions are often associated with sessions and all SQL statements after an update statement are considered to be part of the transaction.
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This means they are explicitly managed, using a \texttt{BEGIN TRANSACTION} or \texttt{BEGIN} statement and ended with either a \texttt{COMMIT} or \texttt{ROLLBACK} statement.
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So, we have the following statements:
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\begin{itemize}
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\item \texttt{COMMIT}: Transaction will not make any order
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\item \texttt{ROLLBACK}: Roll back all changes
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\item \texttt{SAVEPOINT $<$name$>$}: Create a save point / history point, used to roll back some changes and try some others in case of failure.
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\item \texttt{ROLLBACK TO SAVEPOINT $<$name$>$}: Roll back to a specific save point.
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\item \texttt{RELEASE SAVEPOINT $<$name$>$}: Delete a save point (doesn't delete the changes made before, just the log entry)
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\end{itemize}
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\subsubsection{Operations}
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Below a list of very important syntax for the exam (where the subscript $i$ denotes that this operation is part of transaction $T_i$):
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\begin{itemize}
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\item Read operations: $r_i[x]$ or $r_i(x)$ is an access to tuple $x$ without modifying it.
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\item Write operations: $w_i[x]$ or $r_i(x)$ represents a write operation on the tuple $x$.
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\item Abort: $a_j$ is an abort of transaction $T_j$
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\item Commit $c_j$ is the commit of transaction $T_j$
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\end{itemize}
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Conflicting operations are two operations on the same item with at least one of them being a write operation, such as $r_1[x] w_2[x]$ or $w_1[y] w_2[y]$.
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\subsubsection{History}
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A history $H$ is a partially ordered $<_H$ sequence of operations from a set of transactions.
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If two operations are ordered within a transaction, they are equally ordered in the history.
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If two operations $p$ and $q$ conflict, then they are ordered with respect to each other ($p <_H q$ or $q <_H p$)
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