mongodb database replication
TRANSCRIPT
Replication
Process of keeping IDENTICAL COPY of your data on different servers.
Replication offersMinimum Downtimein case of hardware failure, software failure or maintenance
Replication is highly recommended for all databases in production from the very first day.
Replica SetMongoDB’s Terminology for Replication
A replica set is a group of servers with one primary, the server taking client requests, and multiple secondaries, servers that keep copies of the primary’s data.
Give A Go To MongoDB’s Replica SetIn60 Seconds!
#ToBeTriedOnlyAtHome
Step 1: Setup Replica Set
$: mongo --nodb
$: myReplica = new ReplSetTest({nodes: 3});
$: myReplica.startSet();
$: myReplica.initiate();
Congratulations!3 member of replica set running on ports 31000, 31001 and 31002
Step 2: Connect To Primary
$: mongo --port 31000
testReplSet:PRIMARY> use surryhills_db
testReplSet:PRIMARY> db.offices.insert({name: ‘MongoDB’});
testReplSet:PRIMARY> show dbs
testReplSet:PRIMARY> show collections
testReplSet:PRIMARY> db.offices.find().pretty()
Pro Tip :] Auto Format Query Response
Add DBQuery.prototype._prettyShell = true;to your ~/.mongorc.js
Step 3: Connect To A Secondary
$: mongo --port 31001
testReplSet:SECONDARY> db.slaveOk()
testReplSet:SECONDARY> show dbs
testReplSet:SECONDARY> use surryhills_db
testReplSet:SECONDARY> show collections
testReplSet:SECONDARY> db.offices.find()
Step 4: Shutdown Replica Set
$: myReplica.stopSet()
Setup Your Replica SetPart 1: Run replica members
$: mongod --port 31000 --fork --logpath ./path/to/log --dbpath /path/to/srv1 --replSet myReplica
$: mongod --port 31001 --fork --logpath ./path/to/log --dbpath /path/to/srv2 --replSet myReplica
$: mongod --port 31002 --fork --logpath ./path/to/log --dbpath /path/to/srv3 --replSet myReplica
Setup Your Replica SetPart 2: Configure the set
$: mongo --port 31000
$: rs.initiate();
$: rs.add(‘my-host:31001’);
$: rs.add(‘my-host:31002’);
Setup Your Replica SetPart 2: Example Configuration$: rs.config();{ "_id" : "myReplica",
"version": 3, "members" : [ { "_id" : 0, "host" : "my-host:31000" }, { "_id" : 1, "host" : "my-host:31001" }, { "_id" : 2, "host" : "my-host:31002" } ]}
Auto Connect To Current Primary
$: mongo --host myReplica/my-host:31000,my-host:31001,my-host:31002
Replica Set Good To Knows
1. Always start your instance with `replSet`
2. You can’t initiate a replica set with more than one member with existing data on it.
3. If one of set members has data, you must run rs.initiate() against that member
4. `system.replset` collection on `local` database of all members contains set configuration.
5. Replica sets can have up to 50 members, 7 voting
6. By default, MongoDB reads from primary
Replica Set ReconfigurationAdd new member
$: mongo --port 31000 # current primary
$: rs.add(‘my-host:31003’) # accepts object as well.
Replica Set ReconfigurationRemove existing member
$: mongo --port 31000 # current primary
$: rs.remove(‘my-host:31003’)
Replica Set ReconfigurationUpdate existing member
$: mongo --port 31000 # current primary
$: config = rs.config()
$: config.members[2].priority = 0;
$: rs.reconfig(config);
How Replication WorksMongoDB keeps a log of every write operations that primary does in database
Logs are saved in “oplog.rs” collection of “local” db
Secondaries apply changes by reading from oplog.rs and applieing to their data
Secondaries maintain their own “oplog.rs” as well.
“oplog.rs” Facts“oplog.rs” is a capped collection which means it can save limited amount of operation logs.
Bulk operations fill “oplog” quickly. A bulk removal of 1K documents adds 1K documents to “oplog”
It’s important to have a “oplog.rs” to keep logs for atleast last 24 hrs.
“Stale” Secondary :(Secondary misses a copy of last operation it applied in syncing source.
Secondary queries other members to find another source to continue syncing.
Secondary missed some operations permanently. Admin must resyncs from scratch.
HeartbeatMembers PING each other every 2 seconds.
Members should reply within 10 seconds otherwise they’re counted as unreachable.
Change in replica set configuration
Current primary steps down
Replica Set ElectionWhen It Happens?
Current primary is unreachable
MajorityThe Single Most Important Fact About Replica Set
More than half of All Voting Members in the set must be reachable.
You need a majority of voting members to elect a primary.
A primary can only stay primary so long as it can reach a majority of voting members.
Majority
Replica Set with two members.
Three Common Replica Set Design Mistakes :(
Even number of members in two data center.
A set with more than one arbiter
Keep a majority of members in preferred data center and rest in the other one.
Two Common Replica Set Design Patterns :)
Keep even number of members in two data center and a tie breaker in third one.
Meet Arbiter Budget Friendly Member Of Replica Set
Does not hold data
It’s a lightweight process
Can vote on elections
Start An Arbiter Process
$: mongod --port 31005 --replSet myReplica --nojournal --smallfiles
Arbiter does not save data, no need for journaling
Don’t waste disk space and use --smallfiles
Add An Arbiter To Replica Set
$: rs.addArb(‘arbiter-host-name:port’);
ArbiterGood To Know
You can’t change an arbiter to non-arbiter member
If possible, use normal members instead of arbiters
At most you need one arbiter in your set
Do not add an arbiter to a set with odd members
Replica SetUseful Commands
$: rs.config()
$: rs.reconfig()
$: rs.isMaster()
$: rs.status()
$: rs.stepDown()
Rollback :(Primary does a write and goes down before at least one secondary has a chance to replicate the write.
RollbackMain Reasons
Network Partitions
Different write rates between primary and secondary
Rollback Process
1. Former primary reverts write operations
2. Saves affected documents into /rollback directory
3. Admin browses documents using mongorestore
4. Admin applies changes to the current primary
DurabilityWrite Concerns And Replica Set
1. Unacknowledged
2. Acknowledged
3. Journaled
4. Replica Acknowledged
5. Custom Writer Concern
Improve Durabilitydb.runCommand({getLastError:1}) options
Returns the error status of the preceding write operation on the current connection.
j: boolean
w: number | majority | custom role
wtimeout: number (milliseconds)
fsync: boolean
“wtimeout” GOOD TO KNOW
MongoDB does not “rollback” or ”undo modifications” made before the “wtimeout” interval expired.
Custom Write ConcernsStep 1: Tags
$: config = rs.config()
$: config.members[0].tags = {dc: ‘dc-1’};
$: config.members[3].tags = {dc: ‘dc-2’};
$: rs.reconfig(config);
Custom Write ConcernsStep 2: Custom Last Error Mode
$: config = rs.config()
$: config.settings.lastErrorModes = [{
“each-dc”: {dc: 2}
}];
$: rs.reconfig(config);
Custom Write ConcernsStep 3: Use it!
$: db.runCommand({getLastError:1, w: ‘each-dc’})
Replica Set & Mixed Storage Engines
You can have both WiredTiger & MMAPv1
● Due to migration● Due to different use cases
Keen To Go Further?
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Keen To Go Further?docs.mongodb.org/manual/replication
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