lockbydir.py = Locking across processes. Using lockdir existence & age.

@summary

An easy to use lock mechanism, with lock timeout, and limited waiting patience.

@usage

L = DLock( "LOCKNAME" ) # step A: instantiate, with name
acquired = L.LoopWhileLocked_ThenLocking() # step B: waitThenTryLocking
if acquired:
    time.sleep(1)       # Step C: Use scarce resource (maximum for TIMEOUT secs!)
    L.unlocking()       # Step D: unlocking = resource free for next user
return acquired         # Optional: Tell the caller. 

@motivation

I want locking of the django DB, across concurrent uwsgi processes, for any underlying DB. See posting: http://stackoverflow.com/q/28030536/3693375 I got great hints, and learnt a lot. But when no one could really help me to the end, I created this class 'DLock'.

@mechanism:

A lock is represented by a lockdir, in the current directory (or a RAMDISK).

• While it exists, and its filedate is recent, the lock is 'locked'.
• If the file does not exist, or is timed out, the lock is 'unlocked'.

Users need not care about the lockdir, but access the lock by two simple functions:

• .LoopWhileLocked_ThenLocking() and
• .unlocking()

@parameters

TIMEOUT, and PATIENCE can be changed in each DLock instance, or by subclassing DLock.

• TIMEOUT: Seconds after which the lock opens automatically.
• PATIENCE: Seconds after which no more hope to acquire the lock.

@examples

• Shortest possible usage is in howToUse().
• The inner workings are well explained in testDLocks().
• Parallel processes are demonstrated in 2 examples in 'lockbydir_concurrent.py'.

@liveplayer

You can see the examples running live(!) in a GITplayer, thanks to PythonAnywhere!

• testDLock() (inner workings)
• lockbydir_concurrent.py example 1 (several processes use same lock)
• lockbydir_concurrent.py example 2 (massive hammering)

Or you download it, and start all 3 examples like this:

git clone https://github.com/drandreaskrueger/lockbydir.git
cd lockbydir
python lockbydir.py 
python lockbydir_concurrent.py 
python -c "import lockbydir_concurrent; lockbydir_concurrent.startMassive()"

@note: Tested on Python 2.7.5. Works on Windows and Linux.

@requires: lockbydir.py, lockbydir_OS.py, lockbydir_concurrent.py

@since: 23 Jan 2015, last change: See commit/file date

@related:

http://stackoverflow.com/a/21444311/3693375 and http://stackoverflow.com/a/28532580/3693375 and probably http://stackoverflow.com/q/1348026/3693375

---

@human

@contact: python (at) AndreasKrueger (dot) de

@license: Never remove my name, nor the examples - and send me job offers.

@todo: I am poor, send bitcoins: 1MT9gazTyodKVU3XFEUgR5aCwG7rXXiuWC Thx!