Circuit breaker is an established design pattern that applications implement to prevent a failure in one part of the system from cascading to the rest of the system. In an architecture with distributed applications, such as microservices, many services call other services running on remote servers. If the remote service is unavailable or the network is slow, the calling service may wait until a timeout occurs. During this time, the calling service continues to consume critical resources such as server threads and memory. If multiple services call the unresponsive or failing remote service, the impact of the remote service cascades throughout all the calling services, causing even more resources to be consumed and affected by a single failing service. Implementing a circuit breaker on the call to the remote service can prevent the impact of the failing or unresponsive service or network latency from cascading throughout the system.

The circuit breaker design pattern works much like an electrical circuit breaker which is intended to “trip” or open the circuit when failure is detected. This prevents the flow of electrical current through the circuit. After a time delay, the electrical circuit breaker resets and closes the circuit, which causes the flow of electricity to resume.

In a software application, a circuit breaker functions as a proxy that executes the remote services and monitors the remote service for failures. A failure can be an exception and/or a timeout. When the number of failures meets a predetermined threshold within a specified time period, the circuit breaker “trips” or opens the circuit. Subsequent requests for the service end with an error or result in execution of an alternative service. After a reset period elapses, the circuit breaker sets the circuit state to half-open and executes the next request for the service. By allowing a single request to execute and causing other requests to wait, the circuit breaker gauges the health of the service. Upon success of the service, the circuit breaker closes circuit and waiting requests proceed. However, if the service ends with another failure, the circuit breaker re-opens the circuit.

Circuit breakers can be especially useful in systems with a microservices architecture as these systems often feature a large number of distributed components. By configuring a circuit breaker on the invocation of the remote service, you can limit the impact the abnormal behavior of a remote service on other micorservices and critical resources in your system.