Simple(3) User Contributed Perl Documentation Simple(3)NAMELockFile::Simple - simple file locking scheme
SYNOPSIS
use LockFile::Simple qw(lock trylock unlock);
# Simple locking using default settings
lock("/some/file") || die "can't lock /some/file\n";
warn "already locked\n" unless trylock("/some/file");
unlock("/some/file");
# Build customized locking manager object
$lockmgr = LockFile::Simple->make(-format => '%f.lck',
-max => 20, -delay => 1, -nfs => 1);
$lockmgr->lock("/some/file") || die "can't lock /some/file\n";
$lockmgr->trylock("/some/file");
$lockmgr->unlock("/some/file");
$lockmgr->configure(-nfs => 0);
# Using lock handles
my $lock = $lockmgr->lock("/some/file");
$lock->release;
DESCRIPTION
This simple locking scheme is not based on any file locking system
calls such as "flock()" or "lockf()" but rather relies on basic file
system primitives and properties, such as the atomicity of the
"write()" system call. It is not meant to be exempt from all race
conditions, especially over NFS. The algorithm used is described below
in the ALGORITHM section.
It is possible to customize the locking operations to attempt locking
once every 5 seconds for 30 times, or delete stale locks (files that
are deemed too ancient) before attempting the locking.
ALGORITHM
The locking alogrithm attempts to create a lockfile using a temporarily
redefined umask (leaving only read rights to prevent further create
operations). It then writes the process ID (PID) of the process and
closes the file. That file is then re-opened and read. If we are able
to read the same PID we wrote, and only that, we assume the locking is
successful.
When locking over NFS, i.e. when the one of the potentially locking
processes could access the lockfile via NFS, then writing the PID is
not enough. We also write the hostname where locking is attempted to
ensure the data are unique.
CUSTOMIZING
Customization is only possible by using the object-oriented interface,
since the configuration parameters are stored within the object. The
object creation routine "make" can be given configuration parmeters in
the form a "hash table list", i.e. a list of key/value pairs. Those
parameters can later be changed via "configure" by specifying a similar
list of key/value pairs.
To benefit from the bareword quoting Perl offers, all the parameters
must be prefixed with the "-" (minus) sign, as in "-format" for the
format parameter.. However, when querying the object, the minus must
be omitted, as in "$obj->format".
Here are the available configuration parmeters along with their
meaning, listed in alphabetical order:
autoclean
When true, all locks are remembered and pending ones are
automatically released when the process exits normally (i.e.
whenever Perl calls the END routines).
delay
The amount of seconds to wait between locking attempts when the
file appears to be already locked. Default is 2 seconds.
efunc
A function pointer to dereference when an error is to be reported.
By default, it redirects to the logerr() routine if you have
Log::Agent installed, to Perl's warn() function otherwise.
You may set it explicitely to "\&LockFile::Simple::core_warn" to
force the use of Perl's warn() function, or to "undef" to suppress
logging.
ext The locking extension that must be added to the file path to be
locked to compute the lockfile path. Default is ".lock" (note that
"." is part of the extension and can therefore be changed). Ignored
when format is also used.
format
Using this parmeter supersedes the ext parmeter. The formatting
string specified is run through a rudimentary macro expansion to
derive the lockfile path from the file to be locked. The following
macros are available:
%% A real % sign
%f The full file path name
%D The directory where the file resides
%F The base name of the file
%p The process ID (PID)
The default is to use the locking extension, which itself is
".lock", so it is as if the format used was "%f.lock", but one
could imagine things like "/var/run/%F.%p", i.e. the lockfile does
not necessarily lie besides the locked file (which could even be
missing).
When locking, the locking format can be specified to supersede the
object configuration itself.
hold
Maximum amount of seconds we may hold a lock. Past that amount of
time, an existing lockfile is removed, being taken for a stale
lock. Default is 3600 seconds. Specifying 0 prevents any forced
unlocking.
max Amount of times we retry locking when the file is busy, sleeping
delay seconds between attempts. Defaults to 30.
nfs A boolean flag, false by default. Setting it to true means we could
lock over NFS and therefore the hostname must be included along
with the process ID in the stamp written to the lockfile.
stale
A boolean flag, false by default. When set to true, we attempt to
detect stale locks and break them if necessary.
wafter
Stands for warn after. It is the number of seconds past the first
warning during locking time after which a new warning should be
emitted. See warn and wmin below. Default is 20.
warn
A boolean flag, true by default. To suppress any warning, set it to
false.
wfunc
A function pointer to dereference when a warning is to be issued.
By default, it redirects to the logwarn() routine if you have
Log::Agent installed, to Perl's warn() function otherwise.
You may set it explicitely to "\&LockFile::Simple::core_warn" to
force the use of Perl's warn() function, or to "undef" to suppress
logging.
wmin
The minimal amount of time when waiting for a lock after which a
first warning must be emitted, if warn is true. After that, a
warning will be emitted every wafter seconds. Defaults to 15.
Each of those configuration attributes can be queried on the object
directly:
$obj = LockFile::Simple->make(-nfs => 1);
$on_nfs = $obj->nfs;
Those are pure query routines, i.e. you cannot say:
$obj->nfs(0); # WRONG
$obj->configure(-nfs => 0); # Right
to turn of the NFS attribute. That is because my OO background chokes
at having querying functions with side effects.
INTERFACE
The OO interface documented below specifies the signature and the
semantics of the operations. Only the "lock", "trylock" and "unlock"
operation can be imported and used via a non-OO interface, with the
exact same signature nonetheless.
The interface contains all the attribute querying routines, one for
each configuration parmeter documented in the CUSTOMIZING section
above, plus, in alphabetical order:
configure(-key => value, -key2 => value2, ...)
Change the specified configuration parameters and silently ignore
the invalid ones.
lock(file, format)
Attempt to lock the file, using the optional locking format if
specified, otherwise using the default format scheme configured in
the object, or by simply appending the ext extension to the file.
If the file is already locked, sleep delay seconds before retrying,
repeating try/sleep at most max times. If warning is configured, a
first warning is emitted after waiting for wmin seconds, and then
once every wafter seconds, via the wfunc routine.
Before the first attempt, and if hold is non-zero, any existing
lockfile is checked for being too old, and it is removed if found
to be stale. A warning is emitted via the wfunc routine in that
case, if allowed.
Likewise, if stale is non-zero, a check is made to see whether any
locking process is still around (only if the lock holder is on the
same machine when NFS locking is configured). Should the locking
process be dead, the lockfile is declared stale and removed.
Returns a lock handle if the file has been successfully locked,
which does not necessarily needs to be kept around. For instance:
$obj->lock('ppp', '/var/run/ppp.%p');
<do some work>
$obj->unlock('ppp');
or, using OO programming:
my $lock = $obj->lock('ppp', '/var/run/ppp.%p') ||;
die "Can't lock for ppp\n";
<do some work>
$lock->relase; # The only method defined for a lock handle
i.e. you don't even have to know which file was locked to release
it, since there is a lock handle right there that knows enough
about the lock parameters.
lockfile(file, format)
Simply compute the path of the lockfile that would be used by the
lock procedure if it were passed the same parameters.
make(-key => value, -key2 => value2, ...)
The creation routine for the simple lock object. Returns a blessed
hash reference.
trylock(file, format)
Same as lock except that it immediately returns false and does not
sleep if the to-be-locked file is busy, i.e. already locked. Any
stale locking file is removed, as lock would do anyway.
Returns a lock hande if the file has been successfully locked.
unlock(file)
Unlock the file.
BUGS
The algorithm is not bullet proof. It's only reasonably safe. Don't
bet the integrity of a mission-critical database on it though.
The sysopen() call should probably be used with the "O_EXCL|O_CREAT"
flags to be on the safer side. Still, over NFS, this is not an atomic
operation anyway.
BEWARE: there is a race condition between the time we decide a lock is
stale or too old and the time we unlink it. Don't use "-stale" and set
"-hold" to 0 if you can't bear with that idea, but recall that this
race only happens when something is already wrong. That does not make
it right, nonetheless. ;-)
AUTHOR
Raphael Manfredi <Raphael_Manfredi@pobox.com>
SEE ALSOFile::Flock(3).
perl v5.14.0 2008-07-12 Simple(3)
