How the statement based replication works internal on secondary node of mongodb?
Mongodb do support only row based replication for the data durability among the master and slaves, when you hit a update command like below

db.collname.update({"empid":43123},{$set:{"age":{$inc:1}}})

the above statement search for the employee with employee id empid 43123 and increaments his age to 1, if his age is 26 it will make it as 27, this operation happens only in master, if you go and check the oplog for this statement the oplog entry will be like below

db.collname.update({"empid":43123},{$set:{"age":27}})

so mongodb replication always work on the basis of row which is updated.