This replica set used to have a couple more hidden secondaries for backups at different times (not sure if it may be relevant), but they were removed.
I think this is the relevant point, if a secondary or primary goes down you do have a majority of nodes that can vote which is why you see the primary is still available (or elected) and you can read. But when you are trying to write with a write concern of majority you don’t have a majority because arbiters don’t hold data. So you can get a majority write concern because only 1 data bearing nodes is available.
With the hidden secondaries it would work because they held data so you could have a write concern that went to multiple nodes.
If the arbiter is down then you would still be able to read/write as normal because you have the majority of data bearing nodes.
1 Like