There are transactions in MongoDB, they just wrap one operation on one document. But that one operation can be quite complex.<p>Example, one op can pick a money account, decrement its balance, and push an entry to the list of outbound transfers. After a few more similarly atomic steps, you have a safe and restartable money transfer.<p>The upside of working with this extra complication is that your data scales via sharding far beyond the maximum size of a MySQL installation.