Architecture

Circuit breaker

A state machine that fails fast after a downstream is unhealthy.

In plain terms

Closed → Open → Half-open. Stops you from beating up an already-broken service. Hystrix popularised it; Resilience4j is the modern Java incarnation.

Origin

Michael Nygard introduced the term in "Release It!" (2007). Netflix Hystrix (2012) made it the default microservices pattern; Resilience4j replaced it after Hystrix went into maintenance in 2018.

Where it shows up in production

Netflix Hystrix The original library. Dominant for ~6 years, now archived but still widely deployed.
Resilience4j Modern Java circuit breaker; default in Spring Cloud since 2019.
Envoy circuit breakers Per-cluster connection / pending / retry limits at the proxy level.

On Semicolony

Simulator circuit breaker

Sources & further reading

Book Nygard — Release It! (book)
Docs Circuit breaker simulator (Semicolony)

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