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Is it? The front page of CouchDB says "CouchDB is highly available and partition tolerant, but is also eventually consistent"

IIRC you can't update multiple documents atomically.

I’m only aware of one NoSQL database that could do multi-document /multi-collection transactions.

ArangoDB (document database)

postgresql xc / postgresql xl

Oracle Berkeley DB, wiredtiger, lmdb, titandb, neo4j and orientdb.