Review of the thumbnailer spec

[Jannis Pohlmann]

On Fri, 15 May 2009 12:34:17 +0200
Alexander Larsson <alexl at redhat.com> wrote:

>  On Fri, 2009-05-15 at 04:00 +0200, Jannis Pohlmann wrote:
> 
> > I think we should improve the description of Unqueue a little bit.
> > Unqueue doesn't really prevent service-side implementations of
> > unqueing batch tasks (more than one URI queued as one handle) in
> > the middle of the task (e.g. after having generated thumbnails for
> > 2 out of 10 URIs).
> >
> >   This is exactly what e.g. file managers need though. Just imagine
> > a file manager enters a directory, queues all its files for
> > thumbnailing and then switches the directory before all files are
> > processed by the thumbnailer. So IMHO it should be pointed out more
> > clearly that this is not a violation of the spec. Right now it just
> > says "You can't unqueue requests that are currently running." This
> > should be something like "You cannot cancel the processing of
> > individual URIs but thumbnailers are free to support cancelling
> > running tasks between the processing two URIs when a task unqueued."
> 
> I don't think using this spec from a file manager is really a good
> idea, its quite a lot of overhead doing all this dbus i/o when
> loading a large directory. I certainly won't be using it in nautilus.

I don't know. The D-Bus communication between file manager and generic
thumbnailer can be reduced by checking against the supported MIME types
and by requesting thumbnails for groups of files instead of individual
files (which is still ok if you need a thumbnail for a newly created
file for instance).

I think the main overhead is caused by the communication between the
generic thumbnailer and specialized thumbnailers to which it delegates
requests file by file. Implementing the generic interface in the file
manager doesn't really help here. 

An implementation of the spec that wants to be useful for a file
manager could ship a few built-in specialized thumbnailers for often
required MIME types which it doesn't expose over D-Bus, and which are
used unless they are overriden by other specialized thumbnailers.

Thumbnailing is expensive and I'm curious whether the D-Bus overhead
will actually be noticable (the annoying kind of way). I really want
thumbnailing moved out of *our* file manager at least. I might be wrong
and later find myself at a dead end, but I'll give it a shot.

  - Jannis
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