If you have have replicaset with 4 data nodes and 1 arbiter, what is the expected behavior if 2 data nodes are down? will the remaining 2 data-nodes + arbiter elect one of the data nodes as a primary so the cluster can still operate?
A primary can be elected, since majority (3 nodes) can still be met. (arbiter can vote). But majority writes can not be performed as at most 2 can ack on a write (arbiter can’t ack any write).
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Important but easy to miss. I know, I did.
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Thanks! The sentence you wrote should be put as an important note on the MongoDB docs!
Hi @AmitG,
There are several warnings related to arbiters in the MongoDB documentation.
Under Add an Arbiter to a Replica Set:
If you are using a three-member primary-secondary-arbiter (PSA) architecture, consider the following:
- The write concern
"majority"can cause performance issues if a secondary is unavailable or lagging. For advice on how to mitigate these issues, see Mitigate Performance Issues with PSA Replica Set.- If you are using a global default
"majority"and the write concern is less than the size of the majority, your queries may return stale (not fully replicated) data.
There is also a warning under Calculating Majority for Write Concern:
There is more detail in the documentation links above, but in general it is better to avoid the use of arbiters where possible.
Regards,
Stennie
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Thanks! I had actually read that section a couple times but it was not clear to me what the ramifications would be for a 4-1 replicaset. The post from Kobe succinctly described the downside in a concrete way that I could understand. One of our other lead engineers missed the downstream effect of majority write concern even though a primary can be elected.
Thanks for the clarification on avoiding arbiters as much as possible!
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