Hi Prakash,
Thanks for your reply!
On Thu, 2006-11-16 at 12:15 -0800, Prakash Sangappa wrote:
> As it has been previously discussed, watching for file events on an entire
> filesystem or a directory tree can be a scalability issue. So the File
> events API may not be suitable for an application like the desktop search.
I beg to differ, as we're already using inotify pretty effectively on
Linux for just this purpose.
It's important to maintain a sense of scope. I fear that we as software
developers try to find a perfect general solution at the expense of
real, more limited use cases.
In the previous thread I recall someone saying that this won't scale on
a 10 TB filesystem, and that person is probably right. However, I use
Beagle on my 120 GB home directory and there definitely isn't a torrent
of activity most of the time.
Beagle is a bit of a pathological case, though; a more realistic case
would be GNOME VFS, which monitors directories based on folders that are
open in the Nautilus file manager.
> For a desktop search system like Beagle or spotlight, it appears that a
> better and an useful method would be for the filesystem to provide an
> interface using which we could efficiently collect all the changes
> that have occurred since some given time.
This would be an incredibly useful feature -- the initial crawl of the
filesystem in Beagle is expensive and painful -- but it's only truly
useful when used in tandem with a file notification system. Otherwise
we're still stuck in the world of polling, just with a nicer API.
Beagle will be forced to run that files_changed_since_time() function
once a second in a loop to pick up changes. Is that really more
efficient?
> The approach we are taking for the file events notification API, is to
> address the needs where applications have to repeatedly stat
> files/directories for changes. Now using the file events API,
> applications have to just wait for file events that are sent when a
> file or directory status changes.
This is essentially what Beagle wants too, just for all the files under
your home directory. :)
> File events interface:
>
> Event types:
> * FILE_ACCESS /* Monitored file/directory was accessed */
> * FILE_MODIFIED /* Monitored file/directory was modified */
> * FILE_ATTRIB /* Monitored file/directory's ATTRIB was
> changed */
>
>
> Exception events:
> * FILE_DELETE /* Monitored file/directory was deleted */
> * FILE_RENAME_TO /* Monitored file/directory was renamed */
> * FILE_RENAME_FROM /* Monitored file/directory was renamed */
> * UNMOUNTED /* Monitored file system got unmounted */
How is file creation handled? It seems like you would need an event
analogous to FILE_DELETE here to notice any newly added files.
> The application can only watch the following events. The exception events
> are reported as they occur. They don't have to be watched for.
>
> FILE_ACCESS,
> FILE_MODIFIED,
> FILE_ATTRIB.
>
> typedef struct file_obj {
> timestruc_t atime; /* Access time got from stat(2) */
> timestruc_t mtime; /* Modification time from stat(2) */
> timestruc_t ctime; /* Change time from stat(2) */
> char *name; /* Null terminated file name */
> } file_obj_t;
Does watching a directory imply that all the contained files are
watched? That is, if I watch /home/joe, will touching /home/joe/foo
cause a FILE_MODIFIED event to be thrown for /home/joe/foo? The two use
cases I can think of (Beagle and gnome-vfs) are more interested in
watching all the files in a directory rather than individual files.
(Although there are certainly use cases for individual files as well.)
> To activate monitoring(watching) a file, it needs to be registered
> Upon delivering an event, the file monitor is disabled. It needs to be
> re-registered again to reactivate the monitor and receive further events.
Why is this? Usually apps that monitor a file want to do it on their
own terms, because they have the state they need to determine when to
watch a file or not. This constraint seems like it will just be a
burden on programmers.
Joe
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