1. #MongoDBTokyo
Replication and Replica Sets
Steve Francia
Chief Evangelist, 10gen

2. Agenda
• Replica Sets Lifecycle
• Developing with Replica Sets
• Operational Considerations
• Behind the Curtain

3. Why Replication?
• How many have faced node failures?
• How many have been woken up from sleep to do
a fail-over(s)?
• How many have experienced issues due to
network latency?
• Different uses for data
– Normal processing
– Simple analytics

4. ReplicaSet Lifecycle

5. Node 1 Node 2
Node 3
Replica Set – Creation

6. Node 1 Node 2
Secondary Secondary
Heartbeat
Re
n
tio
pli
ica
cat
pl
on i
Re
Node 3
Primary
Replica Set – Initialize

7. Primary Election
Node 1 Node 2
Secondary Heartbeat Secondary
Node 3
Replica Set – Failure

8. Replication
Node 1 Node 2
Secondary Primary
Heartbeat
Node 3
Replica Set – Failover

9. Replication
Node 1 Node 2
Secondary Primary
Heartbeat
n
tio
ica
pl
Re
Node 3
Recovery
Replica Set – Recovery

10. Replication
Node 1 Node 2
Secondary Primary
Heartbeat
n
tio
ica
pl
Re
Node 3
Secondary
Replica Set – Recovered

11. ReplicaSet Roles &
Configuration

12. Node 1 Node 2
Secondary Arbiter
Heartbeat
Re
pli
cat
on i
Node 3
Primary
Replica Set Roles

13. Configuration Options
> conf = {
_id : "mySet",
members : [
{_id : 0, host : "A”, priority : 3},
{_id : 1, host : "B", priority : 2},
{_id : 2, host : "C”},
{_id : 3, host : "D", hidden : true},
{_id : 4, host : "E", hidden : true, slaveDelay :
3600}
]
}
> rs.initiate(conf)

14. Configuration Options
> conf = {
_id : "mySet”,
members : [ Primary DC
{_id : 0, host : "A”, priority : 3},
{_id : 1, host : "B", priority : 2},
{_id : 2, host : "C”},
{_id : 3, host : "D", hidden : true},
{_id : 4, host : "E", hidden : true, slaveDelay :
3600}
]
}
> rs.initiate(conf)

15. Configuration Options
> conf = {
Secondary DC
_id : "mySet”, Default Priority = 1
members : [
{_id : 0, host : "A”, priority : 3},
{_id : 1, host : "B", priority : 2},
{_id : 2, host : "C”},
{_id : 3, host : "D", hidden : true},
{_id : 4, host : "E", hidden : true, slaveDelay :
3600}
]
}
> rs.initiate(conf)

16. Configuration Options
> conf = {
_id : "mySet”,
members : [
{_id : 0, host : "A”, priority : 3}, Analytics
{_id : 1, host : "B", priority : 2}, node
{_id : 2, host : "C”},
{_id : 3, host : "D", hidden : true},
{_id : 4, host : "E", hidden : true, slaveDelay :
3600}
]
}
> rs.initiate(conf)

17. Configuration Options
> conf = {
_id : "mySet”,
members : [
{_id : 0, host : "A”, priority : 3},
{_id : 1, host : "B", priority : 2},
{_id : 2, host : "C”},
{_id : 3, host : "D", hidden : true},
{_id : 4, host : "E", hidden : true, slaveDelay :
3600}
]
}
Backup node
> rs.initiate(conf)

18. Developing with Replica Sets

19. Client Application
Driver
Write
Read
Primary
Secondary Secondary
Strong Consistency

20. Client Application
Driver
Write
ad
Re
Re
ad
Primary
Secondary Secondary
Delayed Consistency

21. Write Concern
• Network acknowledgement
• Wait for error
• Wait for journal sync
• Wait for replication

22. Driver
write
Primary
apply in
memory
Unacknowledged

23. Driver
getLastError
Primary
apply in
memory
MongoDB Acknowledged (wait for error)

24. Driver
getLastError
j:true
write
Primary
apply in write to
memory journal
Wait for Journal Sync

25. Driver
getLastError
write
w:2
Primary
replicate
apply in
memory
Secondary
Wait for Replication

26. Tagging
• New in 2.0.0
• Control where data is written to, and read from
• Each member can have one or more tags
– tags: {dc: "ny"}
– tags: {dc: "ny",
subnet: "192.168",
rack: "row3rk7"}
• Replica set defines rules for write concerns
• Rules can change without changing app code

27. Tagging Example
{
_id : "mySet",
members : [
{_id : 0, host : "A", tags : {"dc": "ny"}},
{_id : 1, host : "B", tags : {"dc": "ny"}},
{_id : 2, host : "C", tags : {"dc": "sf"}},
{_id : 3, host : "D", tags : {"dc": "sf"}},
{_id : 4, host : "E", tags : {"dc": "cloud"}}],
settings : {
getLastErrorModes : {
allDCs : {"dc" : 3},
someDCs : {"dc" : 2}} }
}
> db.blogs.insert({...})
> db.runCommand({getLastError : 1, w : "someDCs"})

28. Driver
getLastError
W:allDCs
write
Primary (SF)
replicate
apply in
memory
Secondary (NY)
replicate
Secondary (Cloud)
Wait for Replication (Tagging)

29. Read Preference Modes
• 5 modes (new in 2.2)
– primary (only) - Default
– primaryPreferred
– secondary
– secondaryPreferred
– Nearest
When more than one node is possible, closest node is used for
reads (all modes but primary)

30. Tagged Read Preference
• Custom read preferences
• Control where you read from by (node) tags
– E.g. { "disk": "ssd", "use": "reporting" }
• Use in conjunction with standard read preferences
– Except primary

31. Operational Considerations

32. Maintenance and Upgrade
• No downtime
• Rolling upgrade/maintenance
– Start with Secondary
– Primary last

33. Replica Set – 1 Data Center
• Single datacenter
Datacenter
• Single switch & power
Member 1 • Points of failure:
– Power
Member 2 – Network
– Data center
Datacenter 2
Member 3 – Two node failure
• Automatic recovery of
single node crash

34. Replica Set – 2 Data Centers
• Multi data center
Datacenter 1
• DR node for safety
Member 1
• Can’t do multi data
Member 2 center durable write
safely since only 1 node
in distant DC
Datacenter 2
Member 3

35. Replica Set – 3 Data Centers
Datacenter 1 • Three data centers
Member 1
Member 2
• Can survive full data
center loss
Datacenter 2
Member 3 • Can do w= { dc : 2 } to
Member 4 guarantee write in 2
data centers (with tags)
Datacenter 3
Member 5

36. Just Use It
• Use replica sets
• Easy to setup
– Try on a single machine
• Check doc page for RS tutorials
– http://docs.mongodb.org/manual/replication/#tutorials

37. #MongoDBSeoul
Thank You
Steve Francia
Chief Evangelist, 10gen