"Instantaneous Notification" on OS X
anant at kix.in
Mon Mar 17 22:22:49 CDT 2008
I'm interested in applying for the "Instantaneous Notification" GSoC
project idea as listed on your Wiki . I hope this is the right
place to discuss SoC project ideas!
While I'm not familiar with how notifications work in Windows, I have
some idea of how things are laid out in the Mac. In OS X 10.4, there
is no public API for filesystem changes; although /dev/fsevents is
available (which is what Spotlight uses). However, using the fsevents
file has several drawbacks (the API is private for a reason) - one
badly behaved client is sufficient to spoil it for everyone. The
kernel holds all events written to the file in a buffer until all
interested applications have been notified. When heavy filesystem
activity occurs (eg. when a huge tarball is unpacked or a large
application is installed), it is possible that the buffer fills up and
events are dropped. This method also requires us to run a daemon that
continuously monitors the file, lest we miss certain changes, although
this requirement can be overcome with 3rd party wrappers over the
fsevents file, like fslogger .
In OS X 10.5 (Leopard) however, a public API for filesystem changes
called FSEvents  has appeared. The kernel does the job of
continuously monitoring the fsevents file and writing them to a log,
so our application can start at any time and say "What's changed since
the last time I asked?". Also, since the API is public (i.e. meant to
be used), this seems like the the ideal way to go. There is no chance
of us missing events.
Using the FSEvents API has one disadvantage when compared to probing /
dev/fsvents, it notifies us of directory changes only (much like the
older dnotify system in Linux). We still have to scan the directories
FSEvents returns and compare them to our cached versions to tell the
difference. Hence, the speedup may not be as significant as that
gained by inotify on Linux.
To sum up, is an FSEvents based (hence 10.5 only) solution appear to
something that would interest the Mercurial project?
Thanks in advance for your comments :)
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