MongoDB is to me a great database. However there are cases where I really need atomic multi-document transactions. For example to transfer things (like money or reputation) between accounts and this needs to either succeed completely or fail completely.
I wonder if it would be possible to interact with MongoDB through a library implementing the MultiVersion Concurrency Control pattern.
How bad would it be concerning performances? Would it be possible and profitable to use a hybrid approach, using the 'mongo-mvcc' library only when necessary and the traditional db connection when working only on a single document or would this break the mvcc stuff ?
The simplest way is to use locks (two-phase commit), although this is not very efficient in some cases. For higher concurrency some kind of MVCC can be implemented on the top of Mongo. This article provides a good description:
http://highlyscalable.wordpress.com/2012/01/07/mvcc-transactions-key-value/