/ 
FileHandle-Rollback-1.05
/ FileHandle::Rollback

NAME

FileHandle::Rollback - FileHandle with commit, rollback, and journaled crash recovery

SYNOPSIS

use FileHandle::Rollback;
my ($fh);

# open file handle
$fh = FileHandle::Rollback->new("+< $path")
  or die "cannot open filehandle: $!";

# put some data at a specific address
$fh->seek(80, 0);
print $fh '1500';

# read some data, partially including data 
# that was written in this rollback segment
$fh->seek(70, 0);
$fh->read($data, 100);

# if you want to cancel the changes:
$fh->rollback;

# or, if you want save the changes:
$fh->commit;

INSTALLATION

FileHandle::Rollback can be installed with the usual routine:

perl Makefile.PL
make
make test
make install

You can also just copy Rollback.pm into the FileHandle/ directory of one of your library trees.

DESCRIPTION

FileHandle::Rollback allows you to open a filehandle, write data to that handle, read the data back exactly as if it were already in the file, then cancel the whole transaction if you choose. FileHandle::Rollback works like FileHandle, with a few important differences, most notably the addition of rollback() and commit(). Those additions and differences are noted below.

$fh->rollback()

Cancels all changes since the last rollback, commit, or since you opened the file handle.

$fh->commit()

Writes changes to the file.

$fh->flock($mode)

The flock method locks the file like the built-in flock command. Use the same mode arguments: LOCK_SH, LOCK_EX, and LOCK_UN.

use Fcntl ':flock';

unless ($fh->flock(LOCK_EX))
  {die "cannot lock filehandle: $!"}

binmode

FileHandle::Rollback only works in binmode, so it will automatically put itself into binmode.

read/write

FileHandle::Rollback only works in read/write mode. Regardless of what you begin the file path with (+<, +>, >, >>, etc) FileHandle::Rollback opens the file with +< . However, if > is anywhere in the path then FileHandle::Rollback will create the file if it doesn't already exist.

autmatic crash recovery

This feature journals the data being written to your file so that if there is a server crash while the data is being written, FileHandle::Rollback automatically finishes the data write. In short, crash recovery protects you against invalid data formats: either all the data is written or none of it is.

To implement crash recovery, simply add the journal option to the new command. journal consists of an anonymous array containing two elements:

$fh = FileHandle::Rollback->new('members.db', journal=>['members.journal', 'members.sem'])
   or die $!;

The first element is the file name of a "journal" file, a file where data is temporarily stored before being written to the real data file. The second element is the file name of a "semaphore" file whose existence indicates that data is being written to the data file.

When a FileHandle::Rollback object is created with journaling, the first thing it does is check if the semaphore file exists. If that file does exist, then the object knows that there was a crash the last time the data was being written. The object pulls the stored data from the journal file and tries again to write the data to the data file. Once the data is fully written, it deletes the semaphore and journal files.

This form of crash recovery is dependent on the atomicity of file creation and deletion on your computer, so on some systems (particularly NFS) there is a small chance that crash recovery will not work properly. Caveat programmer.

TERMS AND CONDITIONS

Copyright (C) 2002, 2003 Miko O'Sullivan

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

AUTHOR

Miko O'Sullivan miko@idocs.com

A lot of the code in this module was copied from MemHandle.pm by Sheridan C. Rawlins. In fact, I started with Sheridan's module and just changed code until it worked the way I wanted, so Sheridan gets a lot of credit for FileHandle::Rollback.

VERSION

Version 1.00, June 29, 2002
First public release

Version 1.01, June 30, 2002
Minor tweaks to 1.00

Version 1.02, June 30, 2002
Small but important correction to documentation

Version 1.03, July 1, 2002
Another small but important correction to documentation.

Version 1.04, July 10, 2002
Yet another small but important correction to documentation.  Sheesh.

and then a long time went by...

Version 1.05, June 12, 2003
Added journaled automatic crash recovery