Prerequisites:

You need know the IP or hostname of the Mongo database which should be acted as a master, and you can ping the port 27017 from the mongo database which should be acted as a slave.

Here are some commands might be helpful:

• To know the external IP of the server:

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$ curl http://checkip.dyndns.org/

• To ping the server which you want to connect, on the specific port ( for example: 27017 ):

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$ telnet 1.2.3.4 27017

Practices:

Machine A (Master): Run Mongo Daemon as master:

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$ mongod --master

Machine B (Slave): If mongod is running, which is the default behavior when installed, you should stop it before the following steps:

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# Stop the mongodb which installed by default
$ sudo stop mongodb

Run Mongo Daemon as slave:

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$ mongod --slave

Probably you need to specify the dbpath, in case you have any disk space limitation:

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$ sudo mongod --slave --dbpath /data/mongodb/

Run mongo to open a mongo shell, then config for replication:

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> use local
> db.sources.find()
> db.sources.insert( { host: <masterhostname> <,only: databasename> } );

Then you run show dbs in the mongo shell:

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> show dbs

You will find the databasename which we typed behind the only key above displayed.

You may need to run the command multiple times to make sure the replication is going on. For example:

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> show dbs
databasename    17.9453125GB
local   0.078125GB
> show dbs
databasename    19.9443359375GB

You can also run the following commands to show the information of the replication on master and slave side:

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# Master side:
> rs.printReplicationInfo() # replace 'rs' to 'db' for the version prior to 2.6

# Slave side:
> rs.printSlaveReplicationInfo() # same thing as the above command, you need to replace 'rs' to 'db' if you run the mongodb which version prior to 2.6

Besides, you can run the following command to get a defail status of the server:

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> db.serverStatus()

You might neet to run resync to recover replication:

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> use admin
> db.runCommand( { resync: 1 } )

References:

1. http://docs.mongodb.org/manual/core/master-slave/

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