您如何在Mongo中诊断write_concern问题？

Question

I have a Mongo database (v2.4.10) setup with a secondary and I'm saving data to it. Occassionally after saving a new document and trying to retrieve it I get an exception for the document not being found, even though on manual inspection it's there.

Here's a minmal example but unfortunately the problem does not reproduce consistently. I suspect the problem due to trying to read from the database before the secondary has a copy of the data?

from mongoengine import connect, Document

class User(Document):
    date_creation = DateTimeField(default=datetime.datetime.utcnow)
    username = StringField(required=True, unique=True)

user = User()
user.username = 'nick'
user.save()

user = User.objects.get(username='nick') # exception thrown here matching User not found
user.username = 'changed'
user.save()

Is there a way to diagnose whether this is true? Is so what is the recommended solution should I just be calling user.save(write_concern={'w':0})

Answer 1

使用save（...，write_concern = {w：2，fsync：True}，...），因为mongoengine的默认写入关注值为None 。