It doesn't use real node-local/shard-local MVCC AFAICT. It's O_APPEND on flat files.

It _is_ append only. But doesn’t use flat files. The main storage files have 2 different immutable btrees to find documents (by id and by sequence num). And it does have mvcc for both storage and secondary indexes. Internally it has transactions but they aren’t exposed to the user, as multi document transactions don’t make sense for the replicated document model.

Thanks for the clarification. Can I ask how the MVCC is implemented (e.g. WAL, MVTO, MVRC, etc.), or if there's documentation around that might give some additional insight?

Here’s a link that describes the Couchstore file format. It’s the storage engine for Couchbase and is essentially the same design as CouchDB. https://github.com/couchbaselabs/couchstore/wiki/Format

Thanks a lot; this is super-helpful.

Sorry, wasn't clear if you were talking about MVCC in a replication context or in a node-local context. It does the former, it definitely does not do the latter.

I don't know that much about databases, could you go into more detail?

From the point of view of a single cluster or node (ignoring replication for a moment), it just doesn't have any strong notion of transactions. There's no write-ahead log, no rollback, and no situation where you'd be able to operate in a mode equivalent to something like `SERIALIZABLE` on a relational database.

oh right, I understand what you mean now.

I guess most people should know that going in. Are their good master-master systems out there with local transactions?

The claim from daimenkatz was that transactions and MVCC are supported internally, but not externally. It's unclear to me if any of this is accurate. Here's the information that was offered: https://github.com/couchbaselabs/couchstore/wiki/Format. Let me know if you see MVCC or transactional behavior in there.