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MongoDB
Replication
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Philipp Krenn
@xeraa
ecosio & ViennaDB
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Motivation
Availability & data safety
Read scalability
Helping backups
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Data migration
Delayed members
Oplog Tailing (Meteor.js)
https://meteorhacks.com/mongodb-oplog-and-meteor.html
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Basics
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Terminology
Primary + Secondaries
Master + Slaves problematic — renamed
Arbiter
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http://docs.mongodb.org
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http://docs.mongodb.org
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http://docs.mongodb.org
> rs.addArb("arbiter.example.com:3000")
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http://docs.mongodb.org
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Limits
50 replica set members
12 before 2.7.8
7 voting members
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Example
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No content
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Single instance
$ mkdir 1
$ mongod --dbpath 1 --port 27001 --logpath log1
$ mongo --port 27001
> db.test.insert({ name: "Philipp", city: "Wien" })
> db.test.find()
Stop instance
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Add replication
$ mkdir 2
$ mkdir 3
$ mongod --replSet javantura --dbpath 1 --port 27001
--logpath log1 --oplogSize 20
$ mongod --replSet javantura --dbpath 2 --port 27002
--logpath log2 --oplogSize 20
$ mongod --replSet javantura --dbpath 3 --port 27003
--logpath log3 --oplogSize 20
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Connect
$ hostname
$ mongo --port 27001
> db.test.find()
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Configure replication
Start on the old instance, otherwise data lost
rs.initiate()
rs.status()
rs.add("PK-MBP:27002")
rs.add("PK-MBP:27003")
rs.status()
db.isMaster()
db.test.find()
db.test.insert({ name: "Peter", city: "Steyr" })
db.test.find()
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Read from secondaries
$ mongo --port 27002
> db.test.find()
> rs.slaveOk()
> db.test.find()
> db.test.insert({ name: "Dieter", city: "Graz" })
slaveOk only valid for the current connection
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Failover
Kill primary with [Ctrl]+[C]
Write to new primary
> rs.status()
> db.test.insert({ name: "Dieter", city: "Graz" })
> db.test.find()
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Restart old primary
$ mongod --replSet name --dbpath 1 --port 27001
--logpath log1 --oplogSize 20
$ mongo --port 27001
> rs.status()
> rs.slaveOk()
> db.test.find()
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Election
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Heartbeat
2s interval
10s until election
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Election rules
1. Priority
2. Optime
3. Connections
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Priority
cfg = rs.conf()
cfg.members[0].priority = 0
cfg.members[1].priority = 1
cfg.members[2].priority = 2
rs.reconfig(cfg)
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Optime
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Connections
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Election
Candidate node asks for a vote
Others can veto
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Election
One yes for one node within 30s
Majority yes elects a new primary
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No content
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Issues
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CAP
Select Availability or Consistency
Partition-tolerance is a prerequisite
for distributed systems
"The network is reliable":
http://aphyr.com/posts/288-the-network-is-reliable
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Rollback
Old primary rolls back unreplicated
changes once it rejoins the replica set
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Rollback file
rollback/ in data folder
File name:
..
.bson
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Election time
At times 5 to 7 minutes
http://www.tokutek.com/2014/07/explaining-ark-
part-2-how-elections-and-failover-currently-work/
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Missing synchronization
during election
Old primary sends last changes to a
single node
If not new primary: rollback
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Remember
Replication is
asynchronous
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Multiple primaries
Unlikely but possible
Bugs: https://jira.mongodb.org/browse/SERVER-9765
Test script with no replies: https://groups.google.com/
forum/#!topic/mongodb-dev/-mH6BOYyzeI
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Kyle Kingsbury @aphyr:
Call Me Maybe
http://aphyr.com/tags/jepsen
PostgreSQL, Redis, MongoDB, Riak,
Zookeeper, RabbitMQ, etcd + Consul,
ElasticSearch
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http://aphyr.com/posts/284-call-me-
maybe-mongodb
05/2013 version 2.4
Up to 42% data lost
Data written to old primary: rollback
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No content
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WriteConcern
Configure durability vs performance
https://github.com/mongodb/mongo-java-driver/blob/
master/src/main/com/mongodb/WriteConcern.java
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WriteConcern.
UNACKNOWLEDGED
w=0, j=0
Fire and forget
Default until 11/2012
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No content
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WriteConcern.
ACKNOWLEDGED
w=1, j=0
Current default
Operation successful in memory
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WriteConcern.
JOURNALED
w=1, j=1
Operation written to the journal file
Since 1.8, single server durability
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WriteConcern.FSYNCED
w=1, fsync=true
Operation written to disk
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WriteConcern.
REPLICA_ACKNOWLEDGED
w=2, j=0
Acknowledged by primary and at least
one secondary
w is the server number
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WriteConcern.
MAJORITY
w=majority, j=0
Acknowledgement by the majority of
nodes
wtimeout recommended
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WriteConcern.
MAJORITY
Nearly no data lost, but high overhead
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Write concern performance
https://blog.serverdensity.com/mongodb-on-google-
compute-engine-tips-and-benchmarks/
3 x 1,000 inserts on GCE
Local 10GB system disk
Dedicated 200GB disk
Dedicated 200GB for data and journal
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n1-standard-2
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n1-highmem-8
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Thanks!
Questions?
Now, later today, or @xeraa
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Backup Slides
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Oplog
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Replication via logs
MongoDB: Operations log (Oplog)
MySQL: Binary log (Binlog)
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Naiv approach: Transmit original query
Statement Based Replication (SBR)
DELETE FROM test.table WHERE quantity > 20 LIMIT 1
db.collection.remove({ quantity: { $gt: 20 }}, true)
//justOne: true
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Unambiguous
representation
Row-Based Replication (RBR): Oplog
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MongoDB
Asynchronous replication
Secondaries can get the Oplog from:
their primary
a secondary with more recent data
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Oplog size
32bit: 48MB
64bit OS X: 183MB
64bit *nix, Windows: 1GB to 50GB
(5% free disk)
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Inner details
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Capped collection in oplog.rs of the
local database
> use local
> show collections
me 0.000MB / 0.008MB
oplog.rs 0.000MB / 20.000MB
replset.minvalid 0.000MB / 0.008MB
slaves 0.000MB / 0.008MB
startup_log 0.003MB / 10.000MB
system.indexes 0.001MB / 0.008MB
system.replset 0.000MB / 0.008MB
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> db.oplog.rs.find()
{
"h": NumberLong("-265486071808715859"),
"ns": "test.test",
"o": {
"_id": ObjectId("541a8ed285ea5f8ae059d530"),
"name": "Dieter"
"city": "Graz"
},
"op": "i",
"ts": Timestamp(1411026642, 1),
"v": 2
}
...