chapter 13. Danger of using single UOW instance.  #312

In chapter 13, Unit of work instance is no longer initialized in each api request as it did in chapter 12.
https://github.com/cosmicpython/code/blob/chapter_12_cqrs/src/allocation/entrypoints/flask_app.py

The message bus keeps using a single UOW instance.
This is not a problem if all messages are handled by a single thread.

However, on a multi-threading environment where each api request may be handled by different concurrent thread, there is possibility of losing events before events are collected to the queue in the message bus.

Before a thread execute bellow statement after it handles a message, https://github.com/cosmicpython/code/blob/chapter_13_dependency_injection/src/allocation/service_layer/messagebus.py#L44
if another thread executes with uow: statement, the repository instance in the singleton UOW is replaced to a new instance and all aggregate objects stored in the old instance are lost.
https://github.com/cosmicpython/code/blob/chapter_13_dependency_injection/src/allocation/service_layer/unit_of_work.py#L52

Found this problem while developing a GRPC server which is multi-threaded by default.
I fixed this issue by making message bus to keep a UOW instance generator function instead of the UOW instance.