Re: [gnome-db] trigger support or simulation with libgda ?

[Vivien Malerba <vmalerba gmail com>]

On 4/15/05, Daniel Espinosa <esodan yahoo com mx> wrote:
> 
> --- "reinecke fox gmx net" <reinecke fox gmx net>
> wrote:
> >
> >
> > hello together,
> > I'm new to libgda, so I took some time to review
> > your library the last days:
> > its great, thanks for your work !
> >
> > I'm looking for a mechanism to watch the changes
> > made in a table ...
> > Is it possible to define a listener (and a callback)
> > that monitors a table and throws the
> > callback or a signal if one of the typical
> > DML-commands (insert, update, delete) ran ???
> >
> > The program I'm writing is a service, what has to
> > make some actions, if the data in
> > the monitored table was changed from outside (maybe
> > by a webfrontend or anything else)...
> >
> > PostgreSQL has TRIGGERS implemented to support such
> > requirements. But I
> > really don't want to use libpq ;) Can I use libgda
> > to use such postgres triggers ?
> >
> > Maybe I found a usable mechanism within the
> > GdaDataModel - e.g. the gda_data_model_row_inserted
> > function and others. Within the
> > 'gda_data_model_changed()' function documentation,
> > listeners
> > are spoken to - but where can I define one ? I
> > couldn't find any listener defining function in the
> > lib. I think I was blind ;)
> >
> > I hope there's any possibility to bypass the
> > inefficient table polling, I had to use before.
> >
> > Thanks in advance for some help.
> > darius
> 
> In LibGda 1.3.1 GdaDataModel is a GObject, then you
> can associate a callback to any event. The events know
> sopported are:
> 
> changed
> row_inserted
> row_updated
> row_removed
> column_inserted
> column_updated
> column_removed
> begin_update
> cancel_update
> commit_update

This would not work to be notified of a table's contents change as the
data model notifies of changes made to its own contents (defined in
any way you want).

Postgres has the notification system which allows to be notified of a
named condition which has to be "emitted" by the NOTIFY command. The
condition then needs to be emitted by a trigger. This is not really
portable...

Libgda has support for notifiction propagation through the
"event-notification" signal of the GdaClient object. However for now
the only events to be notified do not include table content change.
More work would be required to add such support, but it is doable.

Vivien