Interesting new direction of TokuDB pushing their storage engine based on Fractal Tree Indexes to MongoDB:
Running MongoDB with Fractal Tree Indexes (used today in the MySQL storage
engine TokuDB) is fully transactional. Each statement is transactional. If
an update is to modify ten rows, then either all rows are modified, or none
are. Queries use multi-versioning concurrency control (MVCC) to return
results from a snapshot of the system, thereby not being affected by write
operations that may happen concurrently.
Original title and link: MongoDB Transactions With TokuDB’s Fractal Tree Indexes Engine ( ©myNoSQL)