What is hystrix?
Hystrix is a library developed by Netflix and is part of Spring via the Spring Cloud Netflix project. Hystrix is a fault tolerance library and is used as strategy against failures (at different levels) in a service-layer.
Why do we use Hystrix?
Hystrix can be used in situations where your application depends on remote services. In case one or more remote services are down you can handle the situation by using a circuit breaker in your application.
In simpler terms: How to allow one service to continue functioning – when it calls external services which are failing?
Hystrix is watching methods for failing calls to related services. If there is such a failing method, it will open the circuit, which means, it forwards the call to a fallback method. In case the services is restored it will close the circuit and the applications acts as expected again.
See this great article for more background.