On 7/8/2012 2:52 PM, Christian Heimes wrote:
You are contradicting yourself. Either the OS is providing a fully
atomic rename or it doesn't. All POSIX compatible OS provide an atomic
rename functionality that renames the file atomically or fails without
loosing the target side. On POSIX OS it doesn't matter if the target exists.
Rename on some file system types (particularly NFS) may not be atomic.
You don't need locks or any other fancy stuff. You just need to make
sure that you flush the data and metadata correctly to the disk and
force a re-write of the directory inode, too. It's a standard pattern on
POSIX platforms and well documented in e.g. the maildir RFC.
You can use the same pattern on Windows but it doesn't work as good.
That's because you're using the wrong approach. See how to use
ReplaceFile under Win32:
http://msdn.microsoft.com/en-us/library/aa365512%28VS.85%29.aspx
Renaming files is the wrong way to synchronize a
crawler. Use a database that has ACID properties, such as
SQLite. Far fewer I/O operations are required for small updates.
It's not the 1980s any more.
I use a MySQL database to synchronize multiple processes
which crawl web sites. The tables of past activity are InnoDB
tables, which support transactions. The table of what's going
on right now is a MEMORY table. If the database crashes, the
past activity is recovered cleanly, the MEMORY table comes back
empty, and all the crawler processes lose their database
connections, abort, and are restarted. This allows multiple
servers to coordinate through one database.
John Nagle
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