Question

我正在尝试创建某种队列，许多进程可以访问这些队列并以原子方式处理项目：

while True:

  s= db.queue.find_and_modify({'service':'lock'},{'$set':{'locked':True}},upsert=True,new=False) # serves as a lock over the whole collection

  if s and s['locked']: # already locked by another process
    time.sleep(random.randint(2,10)) # sleep and try again
    continue
  else: # the lock was put by this process (atomic find_and_modify)
    db.queue.update({'service':'lock'},{'$set':{'pid':process_id}}, upsert=True) # register the process who put the lock

  res = db.queue.find({'when':{'$lte':last},'locked':False}).limit(1).sort('when',1) #select an available item

  to_return = None #initialize with None

  if res.count():  # 1 or more
     to_return = res[0]  #get the first item 
     to_return['locked'] = True      # lock it
     to_return['pid'] = process_id   # register the process locking it
     db.queue.save(to_return)

  db.queue.update({'service':'lock'},{'$set':{'locked':False}}) # remove the global lock

  return to_return

有时候我会看到一个进程DEFUNCT 在日志中我看到了例外：

Traceback (most recent call last):
  File "driver.py", line 22, in getUserFromQueue
    to_return = res[0]

IndexError("no such item for Cursor instance")

这意味着光标res为空，但是由于存在全局锁定，它怎么可能是空的：只有一个进程查询项目，它已经通过了条件if res.count()，即。光标中至少有一个项目

我必须创建全局锁的原因是我不能使用限制并使用find_and_modify进行排序

pymongo空光标与count（）＆gt; 0

0 个答案: