redis-list-index.py

import redis
from contextlib import contextmanager

@contextmanager
def redisLock(r, key):
    '''
    Acquire a lock on a particular key (makes a new mutex key).
    This is not meant for very high concurrency, just to avoid
    data corruption in simple cases.
    '''

    tries = 15

    lockKey = '%s-mutex' % (key)
    locked = 0
    for i in range(tries):
        locked = r.setnx(lockKey, 1)
        if not locked:
            time.sleep(0.1)
        else: break
        
    if not locked: # Assume a crash or deadlock
        r.set(lockKey, 1)
        
    yield
    r.delete(lockKey)

def setListItemIndex(r, key, oldIndex, newIndex):
    ''' Set the index of any redis list item efficiently. '''

    with redisLock(r, key):
        listSize = r.llen(key)
        pipe = r.pipeline()
        maxIndex = listSize - 1
        queueKey = '%s-queue' % (key)
        targetKey = '%s-target' % (key)
        
        # First half: shuffle everything out of the list
        for i in range(oldIndex, maxIndex):
            pipe.rpoplpush(key, queueKey)
            
        pipe.rpoplpush(key, targetKey)
        
        for i in range(oldIndex):
            pipe.rpoplpush(key, queueKey)
            
        # Second half: put back in the list, in the new order
        for i in range(newIndex, maxIndex):
            pipe.rpoplpush(queueKey, key)
            
        pipe.rpoplpush(targetKey, key)
        
        for i in range(newIndex):
            pipe.rpoplpush(queueKey, key)
            
        pipe.execute()