Resilience means that microservices can deal with the failure of other microservices. Even if a called microservice is not available, they will still work. Section 9.5 presented this topic.
The example application implements resilience with Hystrix.11 This library protects calls so that no problems arise if a system fails. When a call is protected by Hystrix, it is executed in a different thread than the call itself. This thread is taken from a distinct thread pool. This makes it comparatively easy to implement a timeout during a call.
In addition, Hystrix implements a Circuit Breaker. If a call causes an error, the Circuit Breaker will open after a certain number of errors. In that case subsequent calls are not directed to the called system anymore, but generate an error immediately. After a sleep window the Circuit Breaker closes so that calls are directed to the actual system again. The exact behavior can be configured.  In the configuration the error threshold percentage can be determined. That is the percentage of calls that have to cause an error within the time window for the circuit breaker to open. Also the sleep window can be defined, in which the Circuit Breaker is open and not sending calls to the system.