# MongoDB Replica Set 101
# Introduction
A replica set is a group of **mongod** instances that maintain the same data set. A replica set contains several data bearing nodes and optionally one arbiter node. of the data bearing nodes, one and only one member is deemed the primary node, while the other nodes are deemed secondary nodes.
The **primary node** receives all write operations and records all changes to its data sets in its operation log, i.e. oplog
**Secondaries** replicate the primary's oplog and apply the operations to their data sets such that the secondaries' data sets reflect the primary's data set.
Replication provides redundancy and increases data availability. With multiple copies of data on different database servers, replication provides a level of fault tolerance against the loss of a single database server.
In some cases, replication can provide increased read capacity as clients can send read operations to different servers. Maintaining copies of data in different data centers can increase data locality and availability for distributed applications. You can also maintain additional copies for dedicated purposes, such as disaster recovery, reporting, or backup.
.
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668805289529/dWPpDRAIB.png align="left")
# Basic setup
**First of let’s create 3 EC2 instances on AWS and install mongoDB on it.**
**Make sure that mongodb service is running on all instances.**
![Screenshot from 2022-11-18 23-01-07.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668805527278/eDx9H5RC6.png align="left")
# MongoDB primary server setup
**Login to the server which you want to make primary member of the replica set**
## Step - 1 : Make the required changes for replica setup in MongoDB config file
```bash
vi /etc/mongodb.conf
```
Make the changes as follows
```yaml
# mongod.conf
# for documentation of all options, see:
# http://docs.mongodb.org/manual/reference/configuration-options/
# Where and how to store data.
storage:
dbPath: /var/lib/mongodb
journal:
enabled: true
# engine:
# mmapv1:
# wiredTiger:
# where to write logging data.
systemLog:
destination: file
logAppend: true
path: /var/log/mongodb/mongod.log
# network interfaces
net:
port: 27017
bindIp: 0.0.0.0
# how the process runs
processManagement:
timeZoneInfo: /usr/share/zoneinfo
#security:
#operationProfiling:
replication:
replSetName: "myReplicaSet"
#sharding:
## Enterprise-Only Options:
#auditLog:
#snmp:
```
**Note : Here i am setting the replica-set name and also binding the MongoDB with 0.0.0.0 to make it listen on all available interfaces.**
After making above changes restart the MongoDB service.
```bash
sudo systemctl restart mongod
```
## Step - 2 : Initializing Replica Set
**Login to mongo shell and Check the status of replica set**
```bash
#Login to mongo shell
mongo
#check the status of replica-set
rs.status()
```
You will get the following output
![Screenshot from 2022-11-19 00-24-58.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668805832524/1UizmG_7k.png align="left")
It shows that still replica set has not been initialized.
**Now let’s initialize the replica set**
```bash
rs.initiate()
```
You will get the following output
![initiate.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668805895498/bQ1v9x8eK.png align="left")
**Note : As you can observer in image still the `myReplicaSet` has tag `SECONDARY` but once you logout and again re-login to the mongo shell then it will change to `myReplicaSet:PRIMARY`**
## Step - 3 : Adding secondary servers as replica-set member
**Note : We will be using private IP address of the secondary servers while adding it as replica-set member.**
```bash
rs.add("172.31.25.135")
```
You will get the following output
![add.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668805931497/GwEWG3mau.png align="left")
**The same way add another server entry too as replica set member.**
**After adding server entries again check the status of replica-set. You will get the output similar to following -**
```json
ReplicaSet:PRIMARY> rs.status()
{
"set" : "myReplicaSet",
"date" : ISODate("2022-11-18T19:06:15.606Z"),
"myState" : 1,
"term" : NumberLong(1),
"syncSourceHost" : "",
"syncSourceId" : -1,
"heartbeatIntervalMillis" : NumberLong(2000),
"majorityVoteCount" : 2,
"writeMajorityCount" : 2,
"votingMembersCount" : 3,
"writableVotingMembersCount" : 3,
"optimes" : {
"lastCommittedOpTime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"lastCommittedWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"readConcernMajorityOpTime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"readConcernMajorityWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"appliedOpTime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"durableOpTime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"lastAppliedWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"lastDurableWallTime" : ISODate("2022-11-18T19:06:06.681Z")
},
"lastStableRecoveryTimestamp" : Timestamp(1668798366, 1),
"electionCandidateMetrics" : {
"lastElectionReason" : "electionTimeout",
"lastElectionDate" : ISODate("2022-11-18T19:01:06.590Z"),
"electionTerm" : NumberLong(1),
"lastCommittedOpTimeAtElection" : {
"ts" : Timestamp(0, 0),
"t" : NumberLong(-1)
},
"lastSeenOpTimeAtElection" : {
"ts" : Timestamp(1668798066, 1),
"t" : NumberLong(-1)
},
"numVotesNeeded" : 1,
"priorityAtElection" : 1,
"electionTimeoutMillis" : NumberLong(10000),
"newTermStartDate" : ISODate("2022-11-18T19:01:06.626Z"),
"wMajorityWriteAvailabilityDate" : ISODate("2022-11-18T19:01:06.685Z")
},
"members" : [
{
"_id" : 0,
"name" : "ip-172-31-90-148:27017",
"health" : 1,
"state" : 1,
"stateStr" : "PRIMARY",
"uptime" : 1318,
"optime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2022-11-18T19:06:06Z"),
"lastAppliedWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"lastDurableWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"syncSourceHost" : "",
"syncSourceId" : -1,
"infoMessage" : "",
"electionTime" : Timestamp(1668798066, 2),
"electionDate" : ISODate("2022-11-18T19:01:06Z"),
"configVersion" : 3,
"configTerm" : 1,
"self" : true,
"lastHeartbeatMessage" : ""
},
{
"_id" : 1,
"name" : "172.31.25.135:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 193,
"optime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"optimeDurable" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2022-11-18T19:06:06Z"),
"optimeDurableDate" : ISODate("2022-11-18T19:06:06Z"),
"lastAppliedWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"lastDurableWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"lastHeartbeat" : ISODate("2022-11-18T19:06:14.505Z"),
"lastHeartbeatRecv" : ISODate("2022-11-18T19:06:14.504Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncSourceHost" : "ip-172-31-90-148:27017",
"syncSourceId" : 0,
"infoMessage" : "",
"configVersion" : 3,
"configTerm" : 1
},
{
"_id" : 2,
"name" : "172.31.26.100:27017",
"health" : 1,
"state" : 2,
"stateStr" : "SECONDARY",
"uptime" : 193,
"optime" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"optimeDurable" : {
"ts" : Timestamp(1668798366, 1),
"t" : NumberLong(1)
},
"optimeDate" : ISODate("2022-11-18T19:06:06Z"),
"optimeDurableDate" : ISODate("2022-11-18T19:06:06Z"),
"lastAppliedWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"lastDurableWallTime" : ISODate("2022-11-18T19:06:06.681Z"),
"lastHeartbeat" : ISODate("2022-11-18T19:06:14.505Z"),
"lastHeartbeatRecv" : ISODate("2022-11-18T19:06:14.504Z"),
"pingMs" : NumberLong(0),
"lastHeartbeatMessage" : "",
"syncSourceHost" : "ip-172-31-90-148:27017",
"syncSourceId" : 0,
"infoMessage" : "",
"configVersion" : 3,
"configTerm" : 1
}
],
"ok" : 1,
"$clusterTime" : {
"clusterTime" : Timestamp(1668798366, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1668798366, 1)
}
```
Here you can observe in `Members` section we have 3 entries in which 1st one has `"stateStr"` as `PRIMARY` but for other 2 entries have `"stateStr"` as `SECONDARY` .
## Step - 3 : Creating database and adding some data
**Let’s create a database name testDB and add some data.**
```bash
#Creating databse
use testDB
#Adding one entry
db.testcollection.insertOne({"Name":"Shubham"})
```
You will get the following output
```json
ReplicaSet:PRIMARY> db.testcollection.insertOne({"Name":"Shubham"}) {
acknowledged: true,
insertedId: ObjectId("6377d95f06bc220aae79d510")
}
ReplicaSet:PRIMARY>
```
# MongoDB secondary server setup
**Now login to the secondary member of the replica-set**
Now let’s try to fetch the data which we have added on our primary server.
```bash
#Login to mongo shell
mongo
#Fetch all available databses
show dbs
```
You will get the following error
![slaveError.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668805956827/Fnu55Kqv3.png align="left")
Now Let’s allow read operations to run on secondary member.
```bash
#in mongo shell run the following command
rs.slaveOk()
```
Now try to fetch the data. You will get the output as following
![Screenshot from 2022-11-19 02-00-18.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1668806005906/SRTrRRS9f.png align="left")
Congrats 🎉 now we are able to read data on secondary member of replica set.
## Connection string URI format
### Standard connection string format -
Normally we connect to MongoDB with the connection string as follows -
```bash
mongodb://mongodb0.example.com:27017
```
### Replica Set connection string format -
For replica set , we define the connection string as follows -
```bash
mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017,mongodb2.example.com:27017/?replicaSet=myReplicaSet
```
Reference - [MongoDB - Replication](https://www.mongodb.com/docs/manual/replication/)
**Thanks for your valuable time. Please drop the feedback.**