if a change occurs we need to be careful about backward compatibility / mixing of versions with replication, which is common.

Jason Toffaletti to mongodb-user
show details 4:42 AM (4 hours ago)
I started up a mongod v1.4.4 with --master and ran some multi-field
updates like this from mongo shell:

> db.test2.update(

{thing:'stuff'}

, {$set:

{stuff:14}

, $inc:{version:1}})

While doing this I have a pymongo 1.5.1 script running a tailable
cursor on local.oplog.$main, printing out results:

update {u'o2':

{u'_id': ObjectId('4c610b301a632ffb5161c3e8')}

, u'ns':
u'test.test2', u'ts': Timestamp(1281428434, 1), u'o': {u'$set':
{u'version': 2.0}}, u'op': u'u'}
update {u'o2':

{u'_id': ObjectId('4c610b301a632ffb5161c3e8')}

, u'ns':
u'test.test2', u'ts': Timestamp(1281428450, 1), u'o': {u'$set':
{u'version': 3.0}}, u'op': u'u'}
update {u'o2':

{u'_id': ObjectId('4c610b301a632ffb5161c3e8')}

, u'ns':
u'test.test2', u'ts': Timestamp(1281428586, 1), u'o': {u'$set':
{u'version': 4.0}}, u'op': u'u'}

This is strange, the "o" field only shows the update to version. What
does mongo shell show?

> db['oplog.$main'].find(

{op:"u"}

)
{ "ts" :

{ "t" : 1281428434000, "i" : 1 }

, "op" : "u", "ns" :
"test.test2", "o2" :

{ "_id" : ObjectId("4c610b301a632ffb5161c3e8") }

,
"o" : { "$set" :

{ "stuff" : 1 }

, "$set" :

{ "stuff" : 1 }

} }
{ "ts" :

{ "t" : 1281428450000, "i" : 1 }

, "op" : "u", "ns" :
"test.test2", "o2" :

{ "_id" : ObjectId("4c610b301a632ffb5161c3e8") }

,
"o" : { "$set" :

{ "stuff" : 1346 }

, "$set" :

{ "stuff" : 1346 }

} }
{ "ts" :

{ "t" : 1281428586000, "i" : 1 }

, "op" : "u", "ns" :
"test.test2", "o2" :

{ "_id" : ObjectId("4c610b301a632ffb5161c3e8") }

,
"o" : { "$set" :

{ "stuff" : 14 }

, "$set" :

{ "stuff" : 14 }

} }

Wait, what? This doesn't have the version update, but shows the $set
twice. I know mongod replication works, so I must be doing something
wrong?