/// <summary>
/// 取出队头，并放在队尾
/// 若已取空，从DB中移入
/// because user buffer class does not have this operation, lstWaitingIDInDB is treated as long type here
/// on 2011-5-22, it is modified that only lstWaitingID.Count>0, MaxLengthInMem items are removed from DB into memory, so that the performance is enhanced
/// </summary>
public long RollQueue()
{
//从数据库队列缓存中移入元素
while (lstWaitingID.Count == 0)
{
lstWaitingID = lstWaitingIDInDB.GetFirstValues(iMaxLengthInMem);
}
if (lstWaitingID.Count == 0)
{
return(0);
}
//记录队头
long lFirstValue = lstWaitingID.First.Value;
lock (oLock)
{
//移入队尾，并从队头移除
if (lstWaitingID.Count < iMaxLengthInMem && lstWaitingIDInDB.Count == 0)
{
lstWaitingID.AddLast(lFirstValue);
}
else
{
lstWaitingIDInDB.Enqueue(lFirstValue);
}
lstWaitingID.RemoveFirst();
//从数据库队列缓存中移入元素
while (lstWaitingID.Count == 0)
{
lstWaitingID = lstWaitingIDInDB.GetFirstValues(iMaxLengthInMem);
}
}
return(lFirstValue);
}