2012 mongo db_bangalore_replication

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Sridhar Nanjundeswaran, 10gen

[email protected]

@snanjund

October 26, 2012

mailto:[email protected]

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• Introduction to Replica Sets

• Developing with Replica Sets

• Operational Considerations

• Behind the Curtain

Introduction to Replica Sets

Why?

What is it?

Configuration Options

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• How many have faced node failures?

• How many have been woken to do fail overs?

• How many have experienced issues due to n/w latency?

• Different uses for data

– Normal processing

– Simple analytics

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Node 1

Node 2

Node 3

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Node 1

Node 2

Node 3

RS Init

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Node 1 Node 2

Node 3

Heartbeat

Election

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Node 1

Secondary

Node 2

Secondary

Node 3

Primary Replication Replication

Heartbeat

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Node 1

Secondary

Node 2

Secondary

Node 3

Primary

Heartbeat

Primary Election

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Node 1

Secondary

Node 2

Primary

Node 3

Primary

Heartbeat

Replication

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Node 1

Secondary

Node 2

Primary

Node 3

Recovery

Heartbeat

Replication

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Node 1

Secondary

Node 2

Primary

Node 3

Secondary Replication

Heartbeat

Replication

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Node 1

Secondary

Node 2

Arbiterr

Node 3

Primary Replication

Heartbeat

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> 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)

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> conf = {

_id : "mySet”,

members : [



{_id : 2, host : "C”},



]

}

> rs.initiate(conf)

Primary DC

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> conf = {

_id : "mySet”,

members : [



{_id : 2, host : "C”},



]

}

> rs.initiate(conf)

Secondary DC

Default priority = 1

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> conf = {

_id : "mySet”,

members : [



{_id : 2, host : "C”},



]

}

> rs.initiate(conf)

Analytics node

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> conf = {

_id : "mySet”,

members : [



{_id : 2, host : "C”},



]

}

> rs.initiate(conf) Backup node

Developing with Replica Sets

Consistency

Write Preference

Read Preference

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Application

Primary

Secondary

v1

Insert

Read

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Application 1

Primary

Secondary

v1

Insert

Read

Application 2

Read

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Application 1

Primary

Secondary

v1

Insert

Read

Application 2

Does not

exist Read

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Application 1

Primary

Secondary

v1

Insert

Read

Application 2

Read from primary is

always consistent

Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Reads V1 Read Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

Read

Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

Read

Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

v2

Replicate

Read

Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

v2

Replicate

Read

Read Read

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Application 1

Primary

Secondary

v1

v1

Insert

Read

Replicate

Application 2

Read Read

v2

Update

v2

Replicate

Read

Read Read

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• Fire and forget

• Wait for error

• Wait for journal sync

• Wait for replication

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Driver Primary

write

apply in memory

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Driver Primary

getLastError apply in memory

write

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Driver Primary


write

j:true

Write to journal

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Secondary

replicate

Driver Primary


write

W:majority

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• Since 2.0.0

• Control over where data is written to

• Each member can have one or more tags e.g.

– Dc : "ny”

– dc: "ny",ip: "192.168",rack: "row3rk7”

• Replica set defines rules for where data resides

• Rules can change without changing app code

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{

_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 : "allDCs"})

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Driver Primary (sf)

getLastError apply in

memory

write

W:allDCs

Secondary (ny)

replicate

Secondary

(cloud)

replicate

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• 5 modes (new in 2.2)

– PRIMARY(only) - Default

– PRIMARYPREFERRED

– SECONDARY (only)

– SECONDARYPREFERRED

– NEAREST

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• Custom read preferences

• Control where you read from

– E.g. { "disk": "ssd", "use": "reporting" }

• Use in conjunction with standard read

preferences

– Except primary

Operational Considerations

Upgrade/Maintenance

Common Deployment Scenarios

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• No downtime

• Rolling upgrade/maintenance

– Start with Secondary

– Primary last

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• Single datacenter

• Single switch & power

• Points of failure:

– Power

– Network

– Datacenter

– Two node failure

• Automatic recovery of

single node crash

Member 1

Member 2

Member 3

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• Multi datacenter

• DR node for safety

• Can’t do multi data

center durable write

safely since only 1

node in distant DC

Member 1

Member 2

Member 3

DC

1

DC

2

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• Three data centers

• Can survive full data

center loss

• Can do w= { dc : 2 } to

guarantee write in 2

data centers (with

tags)

Member 1

Member 2

Member 3

Member 4

DC

2

Member 5 -

DR

DC

1

DC

3

Behind the Curtain

Schema

Oplog

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• Local DB (not replicated)

– system.replset

– oplog.rs

• Capped collection

• Idempotent version of operation stored

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• Heartbeat every 2 seconds

– Times out in 10 seconds

• Missed heartbeat considered node down

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> db.replsettest.insert({_id:1,value:1})

{ "ts" : Timestamp(1350539727000, 1), "h" :

NumberLong("6375186941486301201"), "op" : "i", "ns" :

"test.replsettest", "o" : { "_id" : 1, "value" : 1 } }

> db.replsettest.update({_id:1},{$inc:{value:10}})

{ "ts" : Timestamp(1350539786000, 1), "h" :

NumberLong("5484673652472424968"), "op" : "u", "ns" :

"test.replsettest", "o2" : { "_id" : 1 }, "o" : { "$set" : { "value" :

11 } } }

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> db.replsettest.update({},{$set:{name : ”foo”}, false, true})

{ "ts" : Timestamp(1350540395000, 1), "h" : NumberLong("-4727576249368135876"), "op" : "u", "ns" : "test.replsettest", "o2" : { "_id" : 2 }, "o" : { "$set" : { "name" : "foo" } } }



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• Full read preference support in mongos

– Drivers too

• Improved RS lag logging

• rs.syncFrom command

• buildIndexes setting

• replIndexPrefetch setting

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• Use replica sets

• Easy to setup

– Try on a single machine

• Check doc page for RS tutorials

– http://docs.mongodb.org/manual/replication/#t

utorials

http://docs.mongodb.org/manual/replication/

http://docs.mongodb.org/manual/replication/

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2012 mongo db_bangalore_replication

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