The CouchDb documentation indicates that, by default, both reads and writes to single documents are quorum reads and writes (e.g. r=2 and w=2 in a 3-replica system).
However, the documentation for _find says r "defaults to 1, in which case the document found in the index is returned. If set to a higher value, each document is read from at least that many replicas before it is returned..." It is not 100% clear to me however what exactly that means. If I run _find with r=2 and I find a document in the index of a single node I think it's fairly clear that it will also fetch that document from a 2nd node and return the latest to me. However, I think it's still only checking the index on one node so consistency in a healthy cluster isn't guaranteed.
For example, suppose I have a healthy 3 node cluster with no network partitions. The DB in this cluster has a Mango index that includes field foo and I query, via _find, for all documents with foo=bar. Let's say that initially document X has value foo=baz so that X should not be returned. Now, I update X setting foo=bar and I do this with w=2. I then immediately re-run my _find with r=2. If the index is only consulted on one node then I'm not guaranteed to have X returned by my query even with r=2. So does r=2 mean only that documents found in one node's index will also be looked up on a 2nd node or does it mean that the index on 2 nodes will run the query and have their results merged.
Also, it seems like the same index and r=1 default would likely apply to JavaScript MapReduce views, but I see no equivalent documentation for that case. Do MapReduce view queries default to r=1 or r=2?
I posted a link to the above on the CouchDb Slack and got a response:
It's not quite a definitive answer so I'm not marking this correct but it is definitely more information that I had before.