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mongodbmongodb-replica-set
Ezeon
Ezeon

Reputation: 166

MongoDB Replica Set - 5 data centers - are two arbiters possible?

Is it possible to deploy MongoDB Replica Set with two arbiters? The documentation states that a replica set should contain a maximum of one arbiter, but it doesn't specify if this is a real limitation or just a recommendation.

The planned deployment would be in five data centers, where three of the data centers would hold the actual data and run on high performance hardware, and the last two data centers would hold no data and only run arbiters.

The goal is to achieve high-availability and allow the system to operate even with the loss of any two data centers.

The other option is a 4+1 setup which would of course cost more, but would that really provide any benefits over 3+2 in this case?

Upvotes: 0

Views: 405

Answers (1)

kevinadi
kevinadi

Reputation: 13805

More than one arbiter is not recommended, because arbiters are considered voting nodes. You need to consider what happens when the two data-bearing nodes go offline, where you are left with a primary and two arbiters. This is not ideal because:

  • Majority write concern will wait until writes propagate to the majority of voting nodes. Since the arbiters cannot do writes, the write will hang. This is especially a problem if the replica set is part of a sharded cluster, since chunk moves requires majority write concern.

  • Having two arbiters and a single active data-bearing nodes means that you don't have high availability anymore. If the primary is subsequently corrupted, you have no other node that have a copy of the data.

  • Losing two datacenters typically means that you have a more pressing issue compared to the database not allowing writes (e.g. reliability of your hosting company). You have to wonder if a hosting company allows two datacenter to be offline for an extended period of time, what are the chances of them corrupting your data?

If you envision that two of your data-bearing nodes can be offline at the same time (due to maintenance, disasters, etc.), then the best thing to do is to have a replica set with five data-bearing nodes.

If five data-bearing nodes is not ideal for your situation, I would recommend you to go with only one arbiter (the 4+1 topology you mentioned).

Upvotes: 2

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