The document discusses the concepts of replication and durability in MongoDB, detailing configuration, features, and scenarios for high availability and fault tolerance. It covers the structure and management of replica sets, data replication methods, and various types of outages. Additionally, it highlights advanced features like priorities, tagging, and durability options within different replication contexts.

1. Replication
& Durability

2. @spf13
AKA
Steve Francia
15+ years
building the
internet
Father, husband,
skateboarder
Chief Solutions Architect @
responsible for drivers,
integrations, web & docs

3. Agenda
• Intro to replication
• How MongoDB does Replication
• Configuring a ReplicaSet
• Advanced Replication
• Durability
• High Availability Scenarios

4. Replication

5. Use cases

6. Use cases
• High Availability (auto-failover)

7. Use cases
• High Availability (auto-failover)
• Read Scaling (extra copies to read from)

8. Use cases
• High Availability (auto-failover)
• Read Scaling (extra copies to read from)
• Backups Delayed Copy (fat finger)
• Online, Time (PiT) backups
• Point in

9. Use cases
• High Availability (auto-failover)
• Read Scaling (extra copies to read from)
• Backups Delayed Copy (fat finger)
• Online, Time (PiT) backups
• Point in
• Use (hidden) replica for secondary
workload
• Analytics
• Data-processingexternal systems
• Integration with

10. Types of outage

11. Types of outage
Planned
• Hardware upgrade
• O/S or file-system tuning
• Relocation of data to new file-system /
storage
• Software upgrade

12. Types of outage
Planned
• Hardware upgrade
• O/S or file-system tuning
• Relocation of data to new file-system /
storage
• Software upgrade
Unplanned
• Hardware failure
• Data center failure
• Region outage
• Human error
• Application corruption

13. Replica Set features

14. Replica Set features
• A cluster of N servers

15. Replica Set features
• A cluster of N servers
• Any (one) node can be primary

16. Replica Set features
• A cluster of N servers
• Any (one) node can be primary
• Consensus election of primary

17. Replica Set features
• A cluster of N servers
• Any (one) node can be primary
• Consensus election of primary
• Automatic failover

18. Replica Set features
• A cluster of N servers
• Any (one) node can be primary
• Consensus election of primary
• Automatic failover
• Automatic recovery

19. Replica Set features
• A cluster of N servers
• Any (one) node can be primary
• Consensus election of primary
• Automatic failover
• Automatic recovery
• All writes to primary

20. Replica Set features
• A cluster of N servers
• Any (one) node can be primary
• Consensus election of primary
• Automatic failover
• Automatic recovery
• All writes to primary
• Reads can be to primary
(default) or a secondary

21. How
MongoDB
Replication

22. How MongoDB
Replication works
Member 1 Member 3
Member 2
• Set is made up of 2 or more nodes

23. How MongoDB
Replication works
Member 1 Member 3
Member 2
Primary
• Election establishes the PRIMARY
• Data replication from PRIMARY to

24. How MongoDB
Replication works
negotiate
new master
Member 1 Member 3
Member 2
DOWN
• PRIMARY may fail
• Automatic election of new PRIMARY if

25. How MongoDB
Replication works
Member 3
Member 1
Primary
Member 2
DOWN
• New PRIMARY elected
• Replica Set re-established

26. How MongoDB
Replication works
Member 3
Member 1
Primary
Member 2
Recovering
• Automatic recovery

27. How MongoDB
Replication works
Member 3
Member 1
Primary
Member
2
• Replica Set re-established

28. How Is Data
Replicated?

29. How Is Data
Replicated? to the
• Change operations are written
oplog
• The oplog is a capped collection (fixed size)
•Must have enough space to allow new secondaries to
catch up (from scratch or from a backup)
•Must have enough space to cope with any applicable
slaveDelay

30. How Is Data
Replicated? to the
• Change operations are written
oplog
• The oplog is a capped collection (fixed size)
•Must have enough space to allow new secondaries to
catch up (from scratch or from a backup)
•Must have enough space to cope with any applicable
slaveDelay
• Secondaries query the primary’s oplog
and apply what they find
• All replicas contain an oplog

31. Configuring
a ReplicaSet

32. Creating a Replica Set
$ ./mongod --replSet <name>
> cfg = {
_id : "<name>",
members : [
{ _id : 0, host : "sf1.acme.com" },
{ _id : 1, host : "sf2.acme.com" },
{ _id : 2, host : "sf3.acme.com" }
]
}
> use admin
> rs.initiate(cfg)

33. Managing a Replica Set

34. Managing a Replica Set
rs.conf()
Shell helper: get current configuration

35. Managing a Replica Set
rs.conf()
Shell helper: get current configuration
rs.initiate(<cfg>);
Shell helper: initiate replica set

36. Managing a Replica Set
rs.conf()
Shell helper: get current configuration
rs.initiate(<cfg>);
Shell helper: initiate replica set
rs.reconfig(<cfg>)
Shell helper: reconfigure a replica set

37. Managing a Replica Set
rs.conf()
Shell helper: get current configuration
rs.initiate(<cfg>);
Shell helper: initiate replica set
rs.reconfig(<cfg>)
Shell helper: reconfigure a replica set
rs.add("hostname:<port>")
Shell helper: add a new member

38. Managing a Replica Set
rs.conf()
Shell helper: get current configuration
rs.initiate(<cfg>);
Shell helper: initiate replica set
rs.reconfig(<cfg>)
Shell helper: reconfigure a replica set
rs.add("hostname:<port>")
Shell helper: add a new member
rs.remove("hostname:<port>")
Shell helper: remove a member

39. Managing a Replica Set

40. Managing a Replica Set
rs.status()
Reports status of the replica set from one
node's point of view

41. Managing a Replica Set
rs.status()
Reports status of the replica set from one
node's point of view
rs.stepDown(<secs>)
Request the primary to step down

42. Managing a Replica Set
rs.status()
Reports status of the replica set from one
node's point of view
rs.stepDown(<secs>)
Request the primary to step down
rs.freeze(<secs>)
Prevents any changes to the current replica
set configuration (primary/secondary status)
Use during backups

43. Advanced
Replication

44. Lots of Features
• Delayed
• Hidden
• Priorities
• Tags

45. Slave Delay

46. Slave Delay
• Lags behind master by configurable
time delay

47. Slave Delay
• Lags behind master by configurable
time delay
• Automatically hidden from clients

48. Slave Delay
• Lags behind master by configurable
time delay
• Automatically hidden from clients
• Protects against operator errors
• Fat fingering
• Application corrupts data

49. Other member
types

50. Other member
types
• Arbiters
• Don’t store a copy of the data
• Vote in elections
• Used as a tie breaker

51. Other member
types
• Arbiters
• Don’t store a copy of the data
• Vote in elections
• Used as a tie breaker
• Hidden
• Not reported in isMaster
• Hidden from slaveOk reads

52. Priorities

53. Priorities
• Priority: a number between 0 and 100
• Used during an election:
• Most up to date
• Highest priority
• Less than 10s behind failed Primary
• Allows weighting of members during
failover

54. Priorities - example
A B C D E
p:10 p:10 p:1 p:1 p:0

55. Priorities - example
A B C D E
p:10 p:10 p:1 p:1 p:0
• Assuming all members are up to date

56. Priorities - example
A B C D E
p:10 p:10 p:1 p:1 p:0
• Assuming all members are up to date
• Members A or B will be chosen first
• Highest priority

57. Priorities - example
A B C D E
p:10 p:10 p:1 p:1 p:0
• Assuming all members are up to date
• Members A or B will be chosen first
• Highest priority
• Members C or D will be chosen when:
• A and B are unavailable
• A and B are not up to date

58. Priorities - example
A B C D E
p:10 p:10 p:1 p:1 p:0
• Assuming all members are up to date
• Members A or B will be chosen first
• Highest priority
• Members C or D will be chosen when:
• A and B are unavailable
• A and B are not up to date
• Member E is never chosen
• priority:0 means it cannot be elected

59. Durabilit
y

60. Durability Options

61. Durability Options
•Fire and forget

62. Durability Options
•Fire and forget
•Write Concern

63. Write Concern
&
If a write requires a
return trip
What the return trip
should depend on

64. Write Concern

65. Write Concern
w:
the number of servers to replicate to (or
majority)

66. Write Concern
w:
the number of servers to replicate to (or
majority)
wtimeout:
timeout in ms waiting for replication

67. Write Concern
w:
the number of servers to replicate to (or
majority)
wtimeout:
timeout in ms waiting for replication
j:
wait for journal sync

68. Write Concern
w:
the number of servers to replicate to (or
majority)
wtimeout:
timeout in ms waiting for replication
j:
wait for journal sync
tags:
ensure replication to n nodes of given tag

69. Fire and Forget
Driver Primary
write
apply in memory
•Operations are applied in memory
•No waiting for persistence to disk
•MongoDB clients do not block waiting to confirm
the operation completed

70. Wait for error
Driver Primary
write
getLastError apply in memory
•Operations are applied in memory
•No waiting for persistence to disk
•MongoDB clients do block waiting to confirm the
operation completed

71. Wait for journal
sync
Driver
write
Primary
getLastError
apply in memory
j:true
Write to journal
•Operations are applied in memory
•Wait for persistence to journal
•MongoDB clients do block waiting to confirm the
operation completed

72. Wait for fsync
Driver Primary
write
getLastError
apply in memory
fsync:true
write to journal (if enabled)
fsync
•Operations are applied in memory
•Wait for persistence to journal
•Wait for persistence to disk
•MongoDB clients do block waiting to confirm the
operation completed

73. Wait for replication
Driver Primary Secondary
write
getLastError
apply in memory
w:2
replicate
•Operations are applied in memory
•No waiting for persistence to disk
•Waiting for replication to n nodes
•MongoDB clients do block waiting to confirm the
operation completed

74. Tagging
• Control over where data is written to.
• Each member can have one or more tags:
tags: {dc: "stockholm"}
tags: {dc: "stockholm",
ip: "192.168",
rack: "row3-rk7"}
• Replica set defines rules for where data resides
• Rules defined in RS config... can change
without change application code

75. Tagging - example
{
_id : "someSet",
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 : {
veryImportant : {"dc" : 3},
sortOfImportant : {"dc" : 2}
}
}
}

76. High
Availability
Scenarios

77. Single Node
• Downtime inevitable
• If node crashes human
intervention might be
needed
• Should absolutely run
with journaling to
prevent data loss /

78. Replica Set 1
• Single datacenter
Arbiter
• Single switch & power
• One node failure
• Automatic recovery of
single node crash
• Points of failure:
• Power
• Network
• Datacenter

79. Replica Set 2
• Single datacenter
Arbiter
• Multiple power/network
zones
• Automatic recovery of single
node crash
• w=2 not viable as losing 1
node means no writes
• Points of failure:
• Datacenter
• Two node failure

80. Replica Set 3
• Single datacenter
• Multiple power/network
zones
• Automatic recovery of
single node crash
• w=2 viable as 2/3 online
• Points of failure:
• Datacenter
• Two node failure

81. When disaster

82. Replica Set 4
• Multi datacenter
• DR node for safety
• Can't do multi data
center durable write
safely since only 1 node
in distant DC

83. Replica Set 5
• Three data centers
• Can survive full data
center loss
• Can do w= { dc : 2 } to
guarantee write in 2
data centers

84. Set
Use? Data Protection High Availability Notes
size
Must use --journal to protect
X One No No
against crashes
On loss of one member, surviving
Two Yes No
member is read only
On loss of one member, surviving
Three Yes Yes - 1 failure two members can elect a new
primary
* On loss of two members,
X Four Yes Yes - 1 failure* surviving two members are read
only
On loss of two members, surviving
Five Yes Yes - 2 failures three members can elect a new
primary
Typical

85. http://spf13.com
http://github.com/s
@spf13
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