# Install MongoDB in MacOS, and Setup replica set on localhost with only a single primary.

Steps:

#### Install via Home Brew:

```sh
# make sure brew is up to day
$ brew update

# install mongo
$ brew install mongo
```

#### Create the data directory.

Before you start MongoDB for the first time, create the directory to which the mongod process will write data. By default, the mongod process uses the `/data/db` directory (commands may need `sudo`).

```sh
# db directory
$ mkdir -p /data/mongodb/db

# db log directory (custom, only if SystemLog is modified with it)
$ mkdir -p /data/mongodb/log
```

ensure that the user account running mongod has read and write permissions for the directories.

#### Edit config file
Add the `replication` section to the mongod.conf file, i.e.:

```sh
# edit the configuration file
$ vim /usr/local/etc/mongod.conf
```

```
#! mongod.conf

# where to write logging data.
systemLog:
  destination: file
  path: /data/mongodb/log/mongo.log
  logAppend: true
  component:
    replication:
      verbosity: true

# Where and how to store data.
storage:
  dbPath: /data/mongodb/db

# network interfaces
net:
  port: 27017
  bindIp: 127.0.0.0 # Listen to local interface only, comment to listen on all interfaces.

#
replication:
  oplogSizeMB: 50
  replSetName: rs0
```

#### Restart mongo
Restart mongod using brew in this case:

```sh
$ brew services restart mongodb
```

#### Activate replica
Connect with local mongo shell and initiate the replica set:

```sh
$ mongo

MongoDB shell version: 3.2.9
connecting to: test
> rs.initiate({_id: "rs0", members: [{_id: 0, host: "127.0.0.1:27017"}] })
{ "ok" : 1 }
```

Now you'll be secondary, but then it will promote you to primary since you're the only one:


```
rs0:SECONDARY> rs.status
function () {
    return db._adminCommand("replSetGetStatus");
}

rs0:PRIMARY> rs.status()
{
  "set" : "rs0",
  "date" : ISODate("2017-01-06T16:16:27.323Z"),
  "myState" : 1,
  "term" : NumberLong(1),
  "heartbeatIntervalMillis" : NumberLong(2000),
  "members" : [
    {
      "_id" : 0,
      "name" : "127.0.0.1:27017",
      "health" : 1,
      "state" : 1,
      "stateStr" : "PRIMARY",
      "uptime" : 2022,
      "optime" : {
        "ts" : Timestamp(1483719372, 1),
        "t" : NumberLong(1)
      },
      "optimeDate" : ISODate("2017-01-06T16:16:12Z"),
      "infoMessage" : "could not find member to sync from",
      "electionTime" : Timestamp(1483719371, 2),
      "electionDate" : ISODate("2017-01-06T16:16:11Z"),
      "configVersion" : 1,
      "self" : true
    }
  ],
  "ok" : 1
}

```

You can tail the oplog at

```
rs0:PRIMARY> use local
switched to db local
rs0:PRIMARY> db.getCollection('oplog.rs').find()
```

...output here