1File::NFSLock(3) User Contributed Perl Documentation File::NFSLock(3)
2
6 File::NFSLock - perl module to do NFS (or not) locking
7
9 use File::NFSLock qw(uncache);
10 use Fcntl qw(LOCK_EX LOCK_NB);
11 my $file = "somefile";
13 ### set up a lock - lasts until object looses scope
15 if (my $lock = new File::NFSLock {
16 file => $file,
17 lock_type => LOCK_EX|LOCK_NB,
18 blocking_timeout => 10, # 10 sec
19 stale_lock_timeout => 30 * 60, # 30 min
20 }) {
21 ### OR
23 ### my $lock = File::NFSLock->new($file,LOCK_EX|LOCK_NB,10,30*60);
24 ### do write protected stuff on $file
26 ### at this point $file is uncached from NFS (most recent)
27 open(FILE, "+<$file") || die $!;
28 ### or open it any way you like
30 ### my $fh = IO::File->open( $file, 'w' ) || die $!
31 ### update (uncache across NFS) other files
33 uncache("someotherfile1");
34 uncache("someotherfile2");
35 # open(FILE2,"someotherfile1");
36 ### unlock it
38 $lock->unlock();
39 ### OR
40 ### undef $lock;
41 ### OR let $lock go out of scope
42 }else{
43 die "I couldn't lock the file [$File::NFSLock::errstr]";
44 }
45
47 Program based of concept of hard linking of files being atomic across
48 NFS. This concept was mentioned in Mail::Box::Locker (which was
49 originally presented in Mail::Folder::Maildir). Some routine flow is
50 taken from there -- particularly the idea of creating a random local
51 file, hard linking a common file to the local file, and then checking
52 the nlink status. Some ideologies were not complete (uncache
53 mechanism, shared locking) and some coding was even incorrect (wrong
54 stat index). File::NFSLock was written to be light, generic, and fast.
55
57 Locking occurs by creating a File::NFSLock object. If the object is
58 created successfully, a lock is currently in place and remains in place
59 until the lock object goes out of scope (or calls the unlock method).
60 A lock object is created by calling the new method and passing two to
62 four parameters in the following manner:
63 my $lock = File::NFSLock->new($file,
65 $lock_type,
66 $blocking_timeout,
67 $stale_lock_timeout,
68 );
69 Additionally, parameters may be passed as a hashref:
71 my $lock = File::NFSLock->new({
73 file => $file,
74 lock_type => $lock_type,
75 blocking_timeout => $blocking_timeout,
76 stale_lock_timeout => $stale_lock_timeout,
77 });
78
80 Parameter 1: file
81 Filename of the file upon which it is anticipated that a write will
82 happen to. Locking will provide the most recent version (uncached)
83 of this file upon a successful file lock. It is not necessary for
84 this file to exist.
85 Parameter 2: lock_type
87 Lock type must be one of the following:
88 BLOCKING
90 BL
91 EXCLUSIVE (BLOCKING)
92 EX
93 NONBLOCKING
94 NB
95 SHARED
96 SH
97 Or else one or more of the following joined with '|':
99 Fcntl::LOCK_EX() (BLOCKING)
101 Fcntl::LOCK_NB() (NONBLOCKING)
102 Fcntl::LOCK_SH() (SHARED)
103 Lock type determines whether the lock will be blocking, non
105 blocking, or shared. Blocking locks will wait until other locks
106 are removed before the process continues. Non blocking locks will
107 return undef if another process currently has the lock. Shared
108 will allow other process to do a shared lock at the same time as
109 long as there is not already an exclusive lock obtained.
110 Parameter 3: blocking_timeout (optional)
112 Timeout is used in conjunction with a blocking timeout. If
113 specified, File::NFSLock will block up to the number of seconds
114 specified in timeout before returning undef (could not get a lock).
115 Parameter 4: stale_lock_timeout (optional)
117 Timeout is used to see if an existing lock file is older than the
118 stale lock timeout. If do_lock fails to get a lock, the modified
119 time is checked and do_lock is attempted again. If the
120 stale_lock_timeout is set to low, a recursion load could exist so
121 do_lock will only recurse 10 times (this is only a problem if the
122 stale_lock_timeout is set too low -- on the order of one or two
123 seconds).
124
126 After the $lock object is instantiated with new, as outlined above,
127 some methods may be used for additional functionality.
128 unlock
130 $lock->unlock;
131 This method may be used to explicitly release a lock that is aquired.
133 In most cases, it is not necessary to call unlock directly since it
134 will implicitly be called when the object leaves whatever scope it is
135 in.
136 uncache
138 $lock->uncache;
139 $lock->uncache("otherfile1");
140 uncache("otherfile2");
141 This method is used to freshen up the contents of a file across NFS,
143 ignoring what is contained in the NFS client cache. It is always
144 called from within the new constructor on the file that the lock is
145 being attempted. uncache may be used as either an object method or as
146 a stand alone subroutine.
147 newpid
149 my $pid = fork;
150 if (defined $pid) {
151 # Fork Failed
152 } elsif ($pid) {
153 $lock->newpid; # Parent
154 } else {
155 $lock->newpid; # Child
156 }
157 If fork() is called after a lock has been aquired, then when the lock
159 object leaves scope in either the parent or child, it will be released.
160 This behavior may be inappropriate for your application. To delegate
161 ownership of the lock from the parent to the child, both the parent and
162 child process must call the newpid() method after a successful fork()
163 call. This will prevent the parent from releasing the lock when unlock
164 is called or when the lock object leaves scope. This is also useful to
165 allow the parent to fail on subsequent lock attempts if the child lock
166 is still aquired.
167
169 On failure, a global variable, $File::NFSLock::errstr, should be set
170 and should contain the cause for the failure to get a lock. Useful
171 primarily for debugging.
172
174 By default File::NFSLock will use a lock file extenstion of ".NFSLock".
175 This is in a global variable $File::NFSLock::LOCK_EXTENSION that may be
176 changed to suit other purposes (such as compatibility in mail systems).
177
179 Notify paul@seamons.com or bbb@cpan.org if you spot anything.
180 FIFO
182 Locks are not necessarily obtained on a first come first serve basis.
183 Not only does this not seem fair to new processes trying to obtain a
184 lock, but it may cause a process starvation condition on heavily locked
185 files.
186 DIRECTORIES
188 Locks cannot be obtained on directory nodes, nor can a directory node
189 be uncached with the uncache routine because hard links do not work
190 with directory nodes. Some other algorithm might be used to uncache a
191 directory, but I am unaware of the best way to do it. The biggest use
192 I can see would be to avoid NFS cache of directory modified and last
193 accessed timestamps.
194
196 Download and extract tarball before running these commands in its base
197 directory:
198 perl Makefile.PL
200 make
201 make test
202 make install
203 For RPM installation, download tarball before running these commands in
205 your _topdir:
206 rpm -ta SOURCES/File-NFSLock-*.tar.gz
208 rpm -ih RPMS/noarch/perl-File-NFSLock-*.rpm
209
211 Paul T Seamons (paul@seamons.com) - Performed majority of the
212 programming with copious amounts of input from Rob Brown.
213 Rob B Brown (bbb@cpan.org) - In addition to helping in the programming,
215 Rob Brown provided most of the core testing to make sure implementation
216 worked properly. He is now the current maintainer.
217 Also Mark Overmeer (mark@overmeer.net) - Author of Mail::Box::Locker,
219 from which some key concepts for File::NFSLock were taken.
220 Also Kevin Johnson (kjj@pobox.com) - Author of Mail::Folder::Maildir,
222 from which Mark Overmeer based Mail::Box::Locker.
223
225 Copyright (C) 2001
226 Paul T Seamons
227 paul@seamons.com
228 http://seamons.com/
229 Copyright (C) 2002-2003,
231 Rob B Brown
232 bbb@cpan.org
233 This package may be distributed under the terms of either the
235 GNU General Public License
236 or the
237 Perl Artistic License
238 All rights reserved.
240
242 Hey! The above document had some coding errors, which are explained
243 below:
244 Around line 624:
246 You forgot a '=back' before '=head1'
247perl v5.12.0 2003-05-13 File::NFSLock(3)