If a circuit breaker detects too many faults, it will open its circuit, that is, not allow new calls.

When the circuit is open, a circuit breaker will perform fail-fast logic. This means that it doesn't wait for a new fault, for example, a timeout, to happen on subsequent calls. Instead, it directly redirects the call to a fallback method. The fallback method can apply various business logic to produce a best-effort response. For example, a fallback method can return data from a local cache or simply return an immediate error message. This will prevent a microservice from becoming unresponsive if the services it depends on stop responding normally. This is specifically useful under high load.

After a while, the circuit breaker will be half-open, allowing new calls to see whether the issue that caused the failures is gone. If new failures are detected by the circuit breaker, it will open the circuit again and go back to the fail-fast logic. Otherwise, it will close the circuit and go back to normal operation. This makes a microservice resilient to faults, or self-healing, a capability that is indispensable in a system landscape of microservices that communicate synchronously with each other.