ZODB ( http://www.zodb.org/en/latest/ ) is a NoSQL database that has been around for almost twenty years now. It's a Python-specific NoSQL database that allows persisting of arbitrary objects in a tree structure. It not only has multi-object transactions, transaction savepoints for memory efficiency's sake (both optimistic and normal), a pluggable 3-way conflict resolver, two-phase commit and support for long-running branches which contain multiple transactions.