\section{Implementation of the Backends}
Opie provides backend implementations for XML, VCF and SQL. XML and VCF are file based
implementations, while the SQL backend use a real database.

\section{Locking, Journal and Visibility}
Opie PIM encourages that all changes are made visible 
immediately and are propagated via Qt Signals by the frontend.
Due to performance issues, we recognize that not every backend
implementation - especially the file based - is capable of making every 
change physically available instantly, called ``Live Update''. It would be too complex to 
implement this efficiently, thus we declare this as ``nice to have'' feature, 
which may be implemented by the backend.\\
By default changes should not be lost by concurrent access to the files. 
The requested feature to avaoid this, is called ``Auto-Update''.  
If one backend need to save the data, it has to lock the access (it can use API \comment{Was?}), check whether external changes
occurred since last save and reload and merge the data, if necessary. This merging guarantees to include all external changes 
to the local dataset. Afterwards it saves all data and removes the lock.
\comment{ Was willst Du hier genau sagen? The user may call reload() at any time and the access needs to be locked
for that.} The backends has to use the appropriate technique to provide
fast and secure operations.\\
\comment{Ich würde im Folgenden eher das vorschlagen:
The reload feature may be turned off by the frontend to disable automatic update. In this case, the
backend should work in ``read-only'' mode to prevent possible overwrite of changes}
The reload feature may be turned off by the frontend so that a save()
might possible override changes that occurred.

\subsubsection{Journal and Out Of Memory}
The backend need to make sure that it is capable of surviving crashes
and storage out of memory situation without losing data and changes.\\
The file based backends is encouraged to use a journal-file on a per user basis 
to keep track of changes. The journal can be applied in case of a crash.\\
In out of disk storage situation the user needs to be informed, so he should get the chance to
free space before the operation is stopped.
Database based backends should use mechanisms which are provided or suggested for the database system, used.

\subsubsection{Features of the backends}

\begin{center}
\begin{tabular}{|l|l|l|l|l|}
\hline
backend & Live Update & Auto Update & Journal  & Available\\ \hline
XML & no  & yes & no but SQL & after save \\ \hline
SQL & yes & yes & yes        & immediately \\ \hline
VCF & no  & no  & no         & never      \\ \hline
\end{tabular}
\label{Feature of backends}
\end{center}

\subsection{Generic Implementation}
\subsubsection{Recurrence}
FIXME
\subsubsection{OPimState}
FIXME
\subsubsection{Reminders and Alarms}

\subsection{Todo}
\subsubsection{XML}

% supported attributes
\begin{tabular}{|l|l|}
\hline
Key            & Possible Values \\ \hline
Categories     & Comma separated list of Integers \\ \hline
Uid            & Unique negative Integer generated by timet \\ \hline
HasDate        & 0 for false and 1 for true \\ \hline
Completed      & 1 for completed and 0 for not completed \\ \hline
Description    & Escaped multi line Text \\ \hline
Summary        & One line summary \\ \hline
Priority       & Between 0 and 5, 5 high priority \\ \hline
DateDay        & The Day for the DueDate \\ \hline
DateMonth      & The Month for the DueDate \\ \hline
DateYear       & The year for the DueDate \\ \hline
Progress       & 0,10,20,30,40,50,60,70,80,90,100 \\ \hline
CompleteDate   & YYYYMMDD formated Date \\ \hline
StartDate      & YYYYMMDD formated Date \\ \hline
CrossReference & TODO!!  \\ \hline
State          & Relates to OPimState 0-4 arre current values \\ \hline
Alarms         & YYYYMMDD:durationOfTheAlarm:sound:future 
                 multiple alarams are joined with ; \\ \hline
Reminders      & Notused TODO!!! \\ \hline
rtype          & Daily, Weekly, MonthlyDay, MonthlyDate, Yearly, None\\ \hline
rweekdays      & Char OPimReccruence::Days ored together \\ \hline
rposition      & The day in the week TODO\\ \hline
rfreq          & Every n (as int) \\ \hline
start          & The creation of the recurrence in UTC time\\ \hline
rhasenddate    & 1 if the recurrence has an end \\ \hline
enddt          & The end date of the recurrence from UTC time\\ \hline
\end{tabular}

\subsubsection{VCF}


\subsubsection{SQL}


\subsection{Contact}
\subsubsection{XML}
\subsubsection{VCF}
\subsubsection{SQL}


\subsection{Datebook}
\subsubsection{XML}
\subsubsection{VCF}
\subsubsection{SQL}