Reader Writer Lock

A python implementation of a solution for the three Reader-Writer problems.

Not only does it implement the reader-writer problems, it is also compliant with the python lock interface which among others include support for timeout.

For reading about the theory behind the reader-writer problems refer to Wikipedia.

Installation

Install the python package readerwriterlock

python3 -m pip install -U readerwriterlock

Usage

1. Choose a rwlock class base on your access priority need and feature need which is going to be use by the threads:

Priority	+Speed	+Downgradable*
Reader priority (aka First readers-writers problem)	RWLockRead	RWLockReadD
Writer priority (aka Second readers-writers problem)	RWLockWrite	RWLockWriteD
Fair priority (aka Third readers-writers problem)	RWLockFair	RWLockFairD

* Downgradable feature allows the locks to be atomically downgraded from being locked in write-mode to locked in read-mode

ⓘ Downgradable classes come with a theoretical ~20% negative effect on performance for acquiring and releasing locks.

2. Instantiate an instance of the chosen RWLock class:

from readerwriterlock import rwlock
a = rwlock.RWLockFairD()

3. Generate read locks and write locks using the following methods:

      a_reader_lock = a.gen_rlock()

      a_writer_lock = a.gen_wlock()

4. Use the generated read/write locks to protect section in your code:

Pythonic usage example

with a.gen_rlock():
  #Read stuff

with a.gen_wlock():
  #Write stuff

Use case (Timeout) example

b = a.gen_wlock()
if b.acquire(blocking=True, timeout=5):
  try:
    #Do stuff
  finally:
    b.release()

Use case (Downgrade) example

b = a.gen_wlock()
if b.acquire():
    try:
        #Read/Write stuff
        b = b.downgrade()
        #Read stuff
    finally:
        b.release()

Live example

Refer to the file test_rwlock.py which has above 90% line coverage of rwlock.py.

The tests can be initiated by doing

make check.test.coverage

Contact

• Project: GitHub
• Éric Larivière ericlariviere@hotmail.com