Hystrix is a fault tolerance library that is very useful for managing failures in a distributed environment like microservices. Suppose we have a service A dependent on service B, which is in turn dependent on service C.
A -> B -> C
Let’s say a call is made from A to B. To serve this request, B needs to call C but there’s a communication failure between them. If the call from B to C is wrapped in Hystrix mechanism, we prevent the failure being propagated to A. Because B couldn’t fetch the actual information from C, Hystrix gives us the option of declaring a fallback value to be used in this case, if feasible.
In fact, Hystrix goes one step further. If we have a high volume of requests going from B to C per unit of time, and the rate of failure reaches some particular value say 50%, then Hystrix will stop any further requests for a period of time and instead return the fallback value immediately. In Hystrix terminology the circuit is said to be open in this case.
After a pre-configured time has elapsed, Hyxtrix will attempt to process the next request by calling service C. This is the half-open state. If the call still fails, the system revert backs to open state. But if it is successful, the circuit is closed and for all subsequent requests, service C is called.