A software example
The django-observer package [j.mp/django-obs] is a third-party Django package that can be used to register callback functions that are executed when there are changes in several Django fields. Many different types of fields are supported (CharField, IntegerField, and so forth).
RabbitMQ is a library that can be used to add asynchronous messaging support to an application. Several messaging protocols are supported, such as HTTP and AMQP. RabbitMQ can be used in a Python application to implement a publish-subscribe pattern, which is nothing more than the Observer design pattern [j.mp/rabbitmqobs].
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