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Spring / Resilience4j Interview questions

What are the states of a CircuitBreaker?

A Resilience4j CircuitBreaker moves through several distinct states rather than just "on" or "off".

  • CLOSED - calls pass through normally and results feed the failure-rate calculation.
  • OPEN - calls are rejected immediately without touching the dependency.
  • HALF_OPEN - a limited number of trial calls are permitted to test recovery.
  • DISABLED - the breaker is switched off and always allows calls through.
  • FORCED_OPEN - the breaker is manually forced to reject every call.
  • METRICS_ONLY - calls are always permitted but metrics are still recorded, useful for observing behavior before enforcing it.

State transitions:

stateDiagram-v2
  [*] --> CLOSED
  CLOSED --> OPEN: failure rate exceeded
  OPEN --> HALF_OPEN: wait duration elapses
  HALF_OPEN --> CLOSED: trial calls succeed
  HALF_OPEN --> OPEN: trial calls fail
  CLOSED --> FORCED_OPEN: manual override
  CLOSED --> DISABLED: manual override
Which state always allows calls through but still records metrics?
From HALF_OPEN, a CircuitBreaker moves to CLOSED when:

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