Wrong data storage in GAE NDB

[Ankur Jatt]

Here is the situation

I have a model like

class Content(ndb.Model):

    likeCount=ndb.IntegerProperty(default=0)

    likeUser  = ndb.KeyProperty(kind=User, repeated=True)

When a new content generate than a new "Content" object is generated like

content_obj = Content(parent=objContentData.key, #Where ContentData is another ndb.Model subclass

                                 likeUser=[],

                                 likeCount=0

                               )

And when any user like the same content than below function gets called

def modify_like(contentData_key, user_key):

    like_obj = Content.query(parent=contetData_key).get()

    if like_obj:

        like_obj.likeUser.append(user_key)

        like_obj.likeCount += 1

        like_obj.put()

###################Problem#############

Now the problem is that when at the same time more than 4 user like the same content than this object write wrong data.

So how can I solve this problem

[Ryan (Cloud Platform Support)]

Salutations Ankur,

You should look into sharded counters. When you have too many updates to the same entity you can run into situations where you have multiple updates happening at once, which leads to to incorrect results.

[Christian F. Howes]

depending on how many writes you expect per second, transactions might be sufficient to solve the problem.  they are write rate limited, so you may still need sharded counters or some other way to batch writes as suggested by Ryan.