# MongoDB Basic Cluster Administration<br />ch2 Replication

###### tags: `MongoDB University M103`

## MongoDB Replica Set

* Primary
* Secondary
* arbiter
* Holds no data
* Can vote in an election
* Cannot become primary

* Replica set can be up to **50** members
* only **7** voting members

:::info
* Replica set are groups of mongod
* High availability and Failover
* Members can have different roles and specific purposes
:::

election trigger

* Adding a new node to the replica set,
* initiating a replica set,
* performing replica set maintenance using methods such as rs.stepDown() or rs.reconfig()
* the secondary members losing connectivity to the primary for more than the configured timeout (10 seconds by default).

## Setting Up a Replica Set

Use setting file demo

storage:
    dbPath: /var/mongod/db/node1
net:
    bindIp: 127.0.0.1,192.168.103.100
    port: 27011
security:
    authorization: enabled
    keyFile: /var/mongod/kpi/m103-keyfile
systemLog:
    destination: file
    path: /var/mongodb/db/node1/mongod.log
    logAppend: true
processManagement:
    fork: true
replication:
    replSetName: m103-example

## Replication Configuration Document

### Replication Configuration
* JSON Object that defines the configuration options of our replica set
* Can be configured manually from the shell
* There are set of mongo shell replication help make it easyer to manage
* rs.add
* rs.initiate
* rs.remove
* rs.status
* rs.config
* rs.reconfig

## Replication Commands

* rs.status()
* Reports health on replica set nodes
* Uses data from heartbeats

* rs.isMaster()
* Describes a node's role in the replica set
* Shorter output than rs.status()

* db.serverStatus('repl')
* section of the db.serverStatus() output
* Similar to the output of rs.isMaster()

* rs.printReplicationInfo()
* only returns the oplog data relative to current node
* content first and last oplog events timestamps

## Local DB

use local

* me
* oplog.rs
* replset.election
* replset.minvalid
* startup_log
* system.replset
* system.rollback_id

:::warning
### Usually, do not modify them.

Any data written to **local** database is not written to oplog.rs or changing any of the system configuration collections will stay there and will not be replicated
:::

#### oplog.rs
* capped collection
* takes 5% of free disk by default

var stats = db.oplog.rs.stats()

stats.capped
// true

stats.size

stats.maxSize

## Reads and Writes on a Replica Set

To enable to read on secondary node

rs.slaveOk()

:::info
We can never write to secondary node
:::