Adding an Arbiter

Arbiters are nodes in a replica set that only participate in elections: they don't have a copy of the data and will never become the primary node (or even a readable secondary). They are mainly useful for breaking ties during elections (e.g. if a set only has two members).

You should only add an arbiter if you have an even number of full members to bring the set to an odd number of voters. As a corollary to this, having two (or more) arbiters is not (usually) safer than just one.

To add an arbiter, bring up a new node as a replica set member (--replSet on the command line) - just like when Adding a New Set Member.

Pre-v1.8: it is best to specify --oplogSize 1 on the arbiter's (mongod) command line so that 5% of available disk space isn't allocated to the oplog (arbiters do not need an oplog). This is handled automatically in v1.8 and beyond.

To start as an arbiter, we'll use rs.addArb() instead of rs.add(). While connected to the current primary:

> rs.addArb("broadway:27017");
{ "ok" : 1 }

When to add an arbiter

Two members with data : add an arbiter to have three voters. 2 out of 3 votes for a member establishes it as primary.
Three members with data : no need to add an arbiter. In fact having 4 voters is worse as 3 of 4 needed to elect a primary instead of 2 of 3. In theory one might add two arbiters thus making number of votes five, and 3 of 5 would be ok; however this is uncommon and generally not recommended.
Four members with data : add one arbiter.

Adding an Arbiter

When to add an arbiter

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