Example "bank accounts" application using the Python eventsourcing library
There is an event sourced BankAccount aggregate which has a balance. Transactions can be appended, which affects the balance. Transactions are not allowed if the balance would go below the overdraft limit, or if the account has been closed. The overdraft limit can be adjusted.
There is a SimpleBankAccountApplication which allows accounts to be created and closed, deposits and withdraws on accounts, and transfers between accounts. All actions are atomic, including transfers between accounts.
There is also a BankAccountSystem which has similar functionality, but records client requests with Command aggregates, for maximum availability. These system commands are processed by the Sagas process application, which is followed by the Accounts process application.
Deposits and withdraws are processed by creating a command, which creates a saga, and then the account is adjusted. If an error occurs, the saga will not succeed and the account error will be recorded on the saga.
Transfers are processed by firstly debiting one account and then crediting the other account, in a multi-step process controlled by the TransferFundsSaga saga.
The test suite includes test cases for the simple application and the system with the eventsourcing library's POPO infrastructure, and extends the system test case to run with various combinations of runners and infrastructure, so that the system is tested running with a single threaded runner and POPO, with a single threaded runner with SQLAlchemy and SQLite in memory, with a single threaded runner with SQLAlchemy and MySQL, with multi-threaded runner with POPO infrastructure, with multi-threaded runner with SQLAlchemy and MySQL, with multiprocessing runner with SQLAlchemy and MySQL, with the Ray actor framework runner with SQLAlchemy and MySQL, and with the Ray actor framework runner with POPO.