Re: notify all processes of database modification

On Sat, 8 Feb 2020 at 04:02, Jens Alfke <[email protected]> wrote:

> > On Feb 7, 2020, at 6:23 AM, Kees Nuyt <[email protected]> wrote:
> >
> > Anyway, SQLite doesn't have such a mechanism by itself.
> > Maybe inotify is useful to you :
> >
> > https://en.wikipedia.org/wiki/Inotify <
> https://en.wikipedia.org/wiki/Inotify>
> > http://man7.org/linux/man-pages/man7/inotify.7.html <
> http://man7.org/linux/man-pages/man7/inotify.7.html>
>
> Or on Apple platforms, FSEvents.
>
> On any platform, you'd need to monitor both the main database and the .wal
> file.
>
> And the notification would trigger soon after a transaction began making
> changes, although the changes wouldn't be visible to you until the commit,
> so you'd probably need to start polling until you see the changes, with
> some heuristic about timing out if nothing happens for a while (e.g. if the
> transaction is aborted.)
>

See also PRAGMA data_version when it comes to polling the DB, the return
value of which changes when another process modifies the DB. IIRC the
implementation of this depends on a value in the DB header page, so it may
be sufficient to only monitor the main DB file for changes.

https://www.sqlite.org/pragma.html#pragma_data_version
-Rowan
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