Why is an arbiter needed for an election in a primary - secondary - arbiter MongoDB replica set?

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Mongo docs list this three-member configuration: primary, secondary, arbiter, as the minimal architecture of a replica set.

Why would an arbiter be necessary there? If the primary fails, the secondary won't see the heartbeat, so it needs to become primary. In other words, why wouldn't a primary + secondary configuration be sufficient? This related question doesn't seem to address the issue, as it discusses larger numbers of nodes.

asked 43 secs ago

Dan Dascalescu

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