Behavioral GoF design pattern that enables us to notify a collection of objects about changes made to a different central entity.
This pattern basically mimics publish/subscribe paradigm. Best used when we have a logical "producer" which generates some content and multiple subscribers that want to be actively notified, so they can somehow react.
TL;DR behaviour - there is a central object (Producer) that produces some content/events. This object has a list of references to other objects (Observer_1, Observer_2, ...) which subscribe to being notified when the producer makes some changes. These subscribers (Observer_1, Observer_2, ...) can then be registered/deregistered dynamically during runtime.
Let's imagine following situation in the context of aforementioned prototype. The scraper which is responsible for acquiring data from the web (i.e. running several web crawlers), receives scraping requests through Kafka pipeline (topic) and upon their receival triggers multiple web crawlers for various sources (e.g. Google, Facebook, ...).
One way to implement the mentioned example is to use an Observer. Instead of manually triggering individual web crawlers we can make KafkaConsumer class which will act as a producer and upon receiving scraper request will notify all registered crawlers (observers) to begin their crawling process.
Dummy implementation of this example/solution and how to use it is part of this directory.
The ability to dynamically change the pool of subscribers and quickly add new ones is a major benefit of this design pattern.
The only slightly negative aspect is that individual subscribers don't receive notifications simultaneously, and in rare circumstances, there may be a large delay between the first and the last.