The following was not clear from the task description, so I had to make assumptions:
- Transaction IDs are globally unique;
- Only 'deposit' transactions can be reversed;
- Negative balance is not allowed;
All of the data storage containers, as well as the engine, provide detailed errors. It's possible to tell what exactly went wrong during transaction processing.
On the application level the errors are printed to stderr, which shouldn't interfere with the automated testing and data output.
- All transaction types;
- All transaction errors;
- All balance operations;
- All balance operation errors;
Unit tests are nested into parent modules for simplicity.