Bill Freeman writes:
Scenario,
Apache or nginx is serving a file, in response to an AJAX request, because
serving static files is fast. The file contains data to be displayed on a
web page (via jQueryUI bar graphs, among, if that's of interest).
While I might come up with a scheme for updating file contents in place
(open for writing,don't truncate, seek, write), I would feel less as hazard
if I wrote a new file and swapped it in under the the same directory entry.
(Hopefully, if the server tends to cache static files in memory, it at
least does a stat on the file name at each request, to see if the
modification time has changed.)
The plan is to create a new file under a different name, then hard link the
critical name to the new file.
I know that I can use 'ln -f' on the command line, but I don't know whether
that turns into an unlink followed by a link, leaving a tiny window during
which the critical name doesn't exist, or whether (other than updating link
counts) this just does the equivalent of writing a new inode number into
the directory entry.
This plan isn't going to work, because the call to link() is going to
fail with EEXIST.
Instead of using link(), you should use rename(), which has all of the
atomic semantics that you seem to be looking for. Think of this as
being like double-buffering, on your fs.
Regards,
--kevin
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