The project hierarchy:

    - services:
        system microservices

    - common:
        enums, objects, utils, external queues interface, and all the logic that is common to the services.

    - tests
        tests of system flow


OOD and OOP:
    - Singleton: ThreadEventNotifier uses a Singleton pattern to create only one class instance.
    - CachedInstance: similar to Singleton design, the difference is that the class can create only one object of each type(based on the parameters sent to the constructor).
    - TemplateMethod: BaseObserver created a different sensor type and used its template methods in each child.
    - Interface: create ObserverInterface to presume the structure that all Observers will use in the system.
    - Abstract: use ThreadManager to create an abstraction to manage threads and create multiple thread managers in the ThreadEventNotifier to manage multiple threads in one place.
    - Observer: use message notifier to send a notification when a new message has arrived in the queue.
        send the message to the other message observer to observe the message and decide if to send an alert

Common:
    The common directory holds all the mutual logic of the system.
    I used Pydantic to validate the message data to make the flow clean and decrease human mistakes.

Requirements:
    python3.9+
    pydantic==1.10.12

Notes:
    the DeadLetterQueue implementation is supposed to demonstrate the queue logic that happens behind the scenes.
     to simplify it I did not implement any retry of the message processing, it's only to test the general behavior expected of the system.

    check out the SystemDesign.pdf file to get a better understanding of the system design.