How to work around the lack of transactions in MongoDB?

As of 4.0, MongoDB will have multi-document ACID transactions. The plan is to enable those in replica set deployments first, followed by the sharded clusters. Transactions in MongoDB will feel just like transactions developers are familiar with from relational databases - they'll be multi-statement, with similar semantics and syntax (like start_transaction and commit_transaction). Importantly, the changes to MongoDB that enable transactions do not impact performance for workloads that do not require them.

For more details see here.

Having distributed transactions, doesn't mean that you should model your data like in tabular relational databases. Embrace the power of the document model and follow the good and recommended practices of data modeling.

Check this out, by Tokutek. They develop a plugin for Mongo that promises not only transactions but also a boosting in performance.

Bring it to the point: if transactional integrity is a must then don't use MongoDB but use only components in the system supporting transactions. It is extremely hard to build something on top of component in order to provide ACID-similar functionality for non-ACID compliant components. Depending on the individual usecases it may make sense to separate actions into transactional and non-transactional actions in some way...