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Overview of the circuit breaker in Hystrix

Libraries provided by Netflix, usually look simple, but after a deep dive, you will realize this stuff is pretty complicated. In this article, I want to explain behavior and usage of the circuit-breaker pattern being a part of the Hystrix.

Following the Wikipedia definition "Circuit breaker is used to detect failures and encapsulates logic of preventing a failure to reoccur constantly (during maintenance, temporary external system failure or unexpected system difficulties)." What does it mean?
For example, let's consider the circuit as a connection between two services. It operates normally (aka stays closed) when the connection is stable, and the target service is healthy. We can execute our requests (assume 100 per second) without any problems.
But when our target service is down (what we know after a few unsuccessful tries), it no longer makes any sense to try 100 times per second. So we open our circuit and serve responses from a predefined fallback, as we know that …