Question

我想用蒙戈德字更新一些数据我的逻辑如下:

#find the specific document with "md5,time,size",
if collection.find({"src_md5":file_md5,"src_time":file_time,"src_size":file_size}).count() == 0:
    #if not found
    #   find the idx,if idx is not yet exist,set idx equa 1

    if collection.find({},{"idx":1}).count() == 0:
        idx = 1

    #if idx is alread there, sort idx and get the biggest idx
    else:
        idx = collection.find({},{"idx":1}).sort( idx ,-1).limit(5)[0][ idx ]
        idx = idx + 1

        #insert the info with idx
        if not self.insertFileInfo(collection,file_obj,file_md5,file_time,file_size,long(idx)):
            return None
#if the specific document with "md5,time,size" is found
else:
#just get the idx with the specific md5
    idx = collection.find({"src_md5":file_md5,"src_time":file_time,"src_size":file_size},{"idx":1})[0][ idx ]
    return None

i will run the above code in 4 machines,which means 4 process would update mongodb almost simultaneously,how can i ensure the atomic of the operations? my recored schema is

{"src_md5":"djapijfdakfiwqjfkasdj","src_size":2376498,"src_time":1338179291,"idx":1}
{"src_md5":"jdfipajkoijjipjefjidwpj","src_size":234876323,"src_time":1338123873,"idx":2}
{"src_md5":"djapojfkdasxkjipkjkf","src_size":3829874,"src_time":1338127634,"idx":3}

it s not a simple auto increment key,it should be increased when md5,size,time changed,and shuld be insert with them,as a record. i create a compound unique index on {"src_md5","src_time","src_size"},and create a unique index on {"idx"},but before i insert new info, i should get the idx alread exist,then increase it. there are two situation: 1,idx with the specific md5,size,time,if is already exist,just return the idx 2,if not exist, increase idx with 1

Answer 1

这个中也讨论了类似的问题。

您想要做的是类似于拥有一个独特的单调增长键, 它将保存在您自己的收藏和递增中, 使用 $inc, 正如相关问题中所描述的。

这将确保您永远不会尝试使用相同的 idx 两次。现在仍然存在着两种线索尝试插入两个不同的 idx 键的新组合( md5, size, time) 的可能性, 但第二个则会失败, 因为您有独特的索引( md5, size, time) 。

现在这里存在的唯一种族条件是,如果第二线由于独特的索引而不能插入,你最终会有一个未使用的 IDx 值(即每次发生时,增加 IDx 值都会跳过一个 ) 。问题对你有多大? 如果大, 您需要强制锁定您的应用程序代码, 或者您可以改变您的系统结构来处理这个案子。

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