Mongodb needs at least 2 servers, preferably 3, to setup a proper mongodb replication. In this article, we will use below hostname as our mongodb nodes:
192.168.0.10 mongo-1 (primary)
192.168.0.11 mongo-2
192.168.0.12 mongo-3
Make sure mongodb is installed in all servers.
Set mongodb repo:
mongo-1: $ cat > mongodb.repo << EOF
>[mongodb]
>name=MongoDB Repository
>baseurl=http://downloads-distro.mongodb.org/repo/redhat/os/x86_64/
>gpgcheck=0
>enabled=1
>EOF
mongo-1: $ sudo mv mongodb.repo /etc/yum.repos.d/
Install mongodb:
mongo-1: $ sudo yum install -y mongodb-org
Set /etc/hosts for each server as below:
mongo-1: $ cat >> hosts << EOF
192.168.0.10 mongo-1
192.168.0.11 mongo-2
192.168.0.12 mongo-3
EOF
mongo-1: $ sudo mv /etc/hosts /etc/hosts.original
mongo-1: $ sudo mv hosts /etc/
To ease up this installation, turn off firewall and set selinux to permissive mode, temporarily, in all servers.
mongo-1: $ sudo systemctl stop firewalld
mongo-1: $ sudo setenforce 0
Edit /etc/mongod.conf in every server, to be similar as below (assuming we are using myreplica as our replSet)
mongo-1: $ sudo cat /etc/mongod.conf
logpath=/var/log/mongodb/mongod.log
logappend=true
fork=true
dbpath=/var/lib/mongo
pidfilepath=/var/run/mongodb/mongod.pid
replSet=myreplica
Once editing is done, restart mongodb in each server
mongo-1: $ sudo systemctl restart mongod
On the first server initiate mongo replica:
mongo-1: $ sudo mongo
MongoDB shell version: x.x.x
connecting to: test
Server has startup warnings:
2018-05-28T04:39:22.580+0000 [initandlisten]
2018-05-28T04:39:22.580+0000 [initandlisten] ** WARNING: Readahead for /var/lib/mongo is set to 4096KB
2018-05-28T04:39:22.580+0000 [initandlisten] ** We suggest setting it to 256KB (512 sectors) or less
2018-05-28T04:39:22.580+0000 [initandlisten] ** http://dochub.mongodb.org/core/readahead
myreplica:PRIMARY> rs.initiate()
Add the other server, namely mongo-2 and mongo-3 to the replicaset
myreplica:PRIMARY> rs.add("mongo-2")
myreplica:PRIMARY> rs.add("mongo-3")
Run rs.status() to see the status of our replica
myreplica:PRIMARY> rs.status() {
"set" : "myreplica",
"date" : ISODate("2018-05-28T05:32:10Z"),
"myState" : 1,
"members" : [
{
"_id" : 0,
"name" : "mongodb01.novalocal.local:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 11,
"optime" : Timestamp(1604978545, 5),
"optimeDate" : ISODate("2018-05-28T05:32:10Z"),
"electionTime" : Timestamp(1604626688, 1),
"electionDate" : ISODate("2018-05-28T05:32:10Z"),
"self" : true
},
In order to rectify the "stateStr: UNKNOWN" and "lastHeartbeatMessage: still initializing", simply add the name of the primary server, as given by mongodb in /etc/hosts of all secondary servers
mongo-2: $ cat /etc/hosts
192.168.0.10 mongo-1 mongodb-1.novalocal
192.168.0.11 mongo-2
192.168.0.12 mongo-3
mongo-3: $ cat /etc/hosts
192.168.0.10 mongo-1 mongodb-1.novalocal
192.168.0.11 mongo-2
192.168.0.12 mongo-3
You should be getting "syncingTo : mongodb-1.novalocal:27017", and "stateStr: SECONDARY" when you run rs.status() in primary server
myreplica:PRIMARY> rs.status()
...
{
"_id" : 2,
"name" : "mongo-3:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 368,
"optime" : Timestamp(1527485519, 1),
"optimeDate" : ISODate("2018-05-28T05:31:59Z"),
"lastHeartbeat" : ISODate("2018-05-28T05:38:06Z"),
"lastHeartbeatRecv" : ISODate("2018-05-28T05:38:06Z"),
"pingMs" : 1,
"syncingTo" : "mongodb-1.novalocal:27017"
}
...
Your replica is now complete. To test it out:
Create new database in primary server, and fill up with data
myreplica:PRIMARY> use mynewdb
myreplica:PRIMARY> db.stack.save(
... {
... "name": "myreplica",
... "description": "this is my new mongodb replica",
... "hosts" : [ "mongo-1", "mongo-2", "mongo-3" ],
... })
WriteResult({ "nInserted" : 1 })
myreplica:PRIMARY> show dbs
admin (empty)
local 2.077GB
mynewdb 0.078GB
myreplica:PRIMARY> show collections;
stack
system.indexes
myreplica:PRIMARY> db.stack.find()
{ "_id" : ObjectId("5b0b97f9aca2dd0afb9d86a5"), "name" : "myreplica", "description" : "this is my new mongodb replica", "hosts" : [ "mongo-1", "mongo-2", "mongo-3" ] }
Login to secondary servers, sync (by running "rs.slaveOk()" ) and check whether the data gets replicated
myreplica:SECONDARY> use mynewdb
switched to db mynewdb
myreplica:SECONDARY> show collections
2018-05-28T05:51:42.601+0000 error: { "$err" : "not master and slaveOk=false", "code" : 13435 } at src/mongo/shell/query.js:131
myreplica:SECONDARY> rs.slaveOk()
myreplica:SECONDARY> show collections
stack
system.indexes
myreplica:SECONDARY> db.stack.find()
{ "_id" : ObjectId("5b0b97f9aca2dd0afb9d86a5"), "name" : "myreplica", "description" : "this is my new mongodb replica", "hosts" : [ "mongo-1", "mongo-2", "mongo-3" ] }
Done :)
