This document demonstrates how the oplog in MongoDB replica sets works by replicating operations from the primary to secondaries. It shows insert, update, remove operations on a collection on the primary and then verifies that the same operations have been replicated to the oplog and applied on a secondary by finding the same document changes. It also shows getMore commands from secondaries tailing the oplog on the primary to stay up to date as new operations are applied.

18. http://www.mongodb.org/display/DOCS/Replica+Sets+-+Oplog

19. myset:PRIMARY> db.mycoll.insert({x:1})
myset:PRIMARY> db.mycoll.update({x:1}, {$set : {y:1}})
myset:PRIMARY> db.mycoll.update({x:2}, {$set : {y:1}}, true)
myset:PRIMARY> db.mycoll.remove({x:1})
myset:PRIMARY> db.mycoll.find()
{ "_id" : ObjectId("4def8e5fb07da951204c60e0"), "x" : 1, "y" : 1 }
{ "_id" : ObjectId("4def8e5fd60f868d9ea77776"), "x" : 2, "y" : 1 }

20. myset:PRIMARY> use local
myset:PRIMARY> db.oplog.rs.find().sort({$natural:-1})
{ "ts" : { "t" : 1307545183000, "i" : 4 }, "h" : NumberLong("-1214445074120755058"), "op" : "d",
"ns" : "test.mycoll", "b" : true, "o" : { "_id" : ObjectId("4def8e5fb07da951204c60e0") } }
{ "ts" : { "t" : 1307545183000, "i" : 3 }, "h" : NumberLong("296197708926251535"), "op" : "i",
"ns" : "test.mycoll", "o" : { "_id" : ObjectId("4def8e5fd60f868d9ea77776"), "x" : 2, "y" : 1 } }
{ "ts" : { "t" : 1307545183000, "i" : 2 }, "h" : NumberLong("6923456793495821492"), "op" : "u",
"ns" : "test.mycoll", "o2" : { "_id" : ObjectId("4def8e5fb07da951204c60e0") }, "o" : { "$set" :
{ "y" : 1 } } }
{ "ts" : { "t" : 1307545183000, "i" : 1 }, "h" : NumberLong("-4148240362977057242"), "op" : "i",
"ns" : "test.mycoll", "o" : { "_id" : ObjectId("4def8e5fb07da951204c60e0"), "x" : 1 } }
...

21. Wed Jun 8 23:53:38 [conn6] getmore local.oplog.rs cid:3687673685314762755 getMore:
{ ts: { $gte: new Date(5615858237044687812) } } bytes:20 nreturned:0 3022ms
Wed Jun 8 23:53:38 [conn8] getmore local.oplog.rs cid:9178181051262829411 getMore:
{ ts: { $gte: new Date(5615858237044687812) } } bytes:20 nreturned:0 3046ms
Wed Jun 8 23:53:41 [conn8] getmore local.oplog.rs cid:9178181051262829411 getMore:
{ ts: { $gte: new Date(5615858237044687812) } } bytes:20 nreturned:0 3041ms
Wed Jun 8 23:53:41 [conn6] getmore local.oplog.rs cid:3687673685314762755 getMore:
{ ts: { $gte: new Date(5615858237044687812) } } bytes:20 nreturned:0 3043ms
...

22. myset:SECONDARY> db.oplog.rs.find().sort({$natural:-1})
{ "ts" : { "t" : 1307545183000, "i" : 4 }, "h" : NumberLong("-1214445074120755058"), "op" : "d",
"ns" : "test.mycoll", "b" : true, "o" : { "_id" : ObjectId("4def8e5fb07da951204c60e0") } }
{ "ts" : { "t" : 1307545183000, "i" : 3 }, "h" : NumberLong("296197708926251535"), "op" : "i",
"ns" : "test.mycoll", "o" : { "_id" : ObjectId("4def8e5fd60f868d9ea77776"), "x" : 2, "y" : 1 } }
{ "ts" : { "t" : 1307545183000, "i" : 2 }, "h" : NumberLong("6923456793495821492"), "op" : "u",
"ns" : "test.mycoll", "o2" : { "_id" : ObjectId("4def8e5fb07da951204c60e0") }, "o" : { "$set" :
{ "y" : 1 } } }
{ "ts" : { "t" : 1307545183000, "i" : 1 }, "h" : NumberLong("-4148240362977057242"), "op" : "i",
"ns" : "test.mycoll", "o" : { "_id" : ObjectId("4def8e5fb07da951204c60e0"), "x" : 1 } }
...