Community Day Trivia #11: What is the minimum number of members in a replica set?

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What is the minimum number of members in a replica set?

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Hello Stennie,
you can convert a standalone server to a replica set. That way you have 1 member in a RS.

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From a production standpoint the minimum odd number would be three data-bearing nodes on three separate hosts preferably in separate availability zones, at the least, to maintain quorum.

But, technically just one like Michael mentioned.

Thanks,
Mahi

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Hi @mahisatya
for later readers: I completely agree with @mahisatya, never run a converted stand-alone server as RS in production. Ok you gain the chance to have an oplog but I would not sleep well with a 1 on node RS.
Regards,
Michael

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Hi Folks,

As @michael_hoeller pointed out, 1 is the minimum number of members in a replica set and as @mahisatya mentioned 3 data-bearing members is the minimum number recommended for a production replica set.

Can anyone think of good use cases for a single member replica set?

Regards,
Stennie

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While writing, testing and POC-ing transaction related code on local/dev/testing instances.

Mahi

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You want to migrate a single instance to MongoDB Atlas.
Since a live migration requires a replicaset resp. an oplog you convert the single instance to a RS, activate the oplog and use mongomirror (Atlas live migration) to migrate the data to Atlas…

I would not claim it a day to day use case, but the simplest paths to migrate a single instance - so somehow this cold be the use case.

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Good use cases for a 1 node replica set:

-Testing out change streams without needing a full replica set
-Point-In-Time Backup Requirement for a non-prod instance
-Proof of Concept that will eventually be expanded to a full replica set with 3+ members

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Adding to @Michael_Grayson’s first point (Testing out change streams without needing a full replica set), testing out Transactions. I recently had a use case and found it useful and I had used the handy mtools for creating my RS.

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