asmhatre
320733265f
Companion module for the rewritten post 'Resilience4j Circuit Breaker in Spring Boot 4.1: What It's Still For', reworked around Framework 7 now shipping @Retryable/@ConcurrencyLimit in core. Covers what's still Resilience4j's job (circuit breaker, rate limiter, bulkhead's bounded wait, fallback methods, Actuator/Micrometer metrics), the off-by-one between maxAttempts and maxRetries, and two Boot-4.1 build breaks: spring-boot-starter-aop no longer exists (renamed to spring-boot-starter-aspectj, proven with Maven Central metadata and the renamed starter's own POM -- see resilience/docs/08-starter-aop-renamed-to-starter-aspectj.md in this same repo), and the resulting fix uses that renamed starter directly rather than assembling spring-aop + aspectjweaver by hand. Kept as its own module rather than a new top-level repository, alongside the existing resilience/ module for the sibling Framework-7 post.
60 lines
3.4 KiB
Markdown
60 lines
3.4 KiB
Markdown
# 2. The circuit breaker, verified
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[Previous: 01-two-resilience-stacks.md](01-two-resilience-stacks.md) | [README](../README.md) | Next: [03-spring-retryable.md](03-spring-retryable.md)
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Source: [`R4jPaymentService.java`](../src/main/java/com/ankurm/resilience/r4j/R4jPaymentService.java).
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Config: [`application.yml`](../src/main/resources/application.yml).
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Test: [`CircuitBreakerTripAndRecoverTest.java`](../src/test/java/com/ankurm/resilience/r4j/CircuitBreakerTripAndRecoverTest.java).
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Transcript: [`docs/output/01-circuitbreaker-trip.txt`](output/01-circuitbreaker-trip.txt).
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## The state machine
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```
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CLOSED --(failure rate >= threshold, over >= minimumNumberOfCalls)--> OPEN
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OPEN --(waitDurationInOpenState elapses)--> HALF_OPEN
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HALF_OPEN --(permittedNumberOfCallsInHalfOpenState calls all succeed)--> CLOSED
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HALF_OPEN --(a probe call fails)--> OPEN
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```
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This repo's config: `slidingWindowSize=10`, `minimumNumberOfCalls=5`, `failureRateThreshold=50`,
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`waitDurationInOpenState=2s`, `permittedNumberOfCallsInHalfOpenState=2`.
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## What the transcript actually shows
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Calls 1-5 against a dead downstream: each one really calls the downstream, gets a
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`DownstreamUnavailableException`, and the fallback method runs. By call 5 the breaker has seen
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5 calls (the configured minimum) at a 100% failure rate and flips to `OPEN`. Call 6 is the
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first call that does **not** touch the downstream at all — it gets `CallNotPermittedException`
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instead, straight from the breaker, before `R4jPaymentService.pay()`'s body ever runs.
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Calls 7-9 (phase 2 in the transcript) confirm the downstream call counter is frozen at 5 for
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the rest of the OPEN period — three more calls, zero more downstream traffic. This is the
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property `@Retryable` cannot offer on its own (see [chapter 3](03-spring-retryable.md)): a
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circuit breaker remembers that the dependency is down and stops asking.
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After `waitDurationInOpenState` (2s; the test waits 2.2s to be safely past it) the breaker
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allows exactly `permittedNumberOfCallsInHalfOpenState` probe calls through. The downstream was
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reconfigured to succeed by then, both probes pass, and the breaker closes.
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## `minimumNumberOfCalls` and why it exists
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A fresh breaker with one failed call and no `minimumNumberOfCalls` floor would trip on pure
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noise — one deployment-time connection refused, one cold JVM. `minimumNumberOfCalls=5` here
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means the first four failures cannot trip anything by themselves; the breaker needs a real
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sample before it judges the dependency.
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## Actuator health vs the circuitbreakers endpoint
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With `management.endpoint.health.show-details: always`, `GET /actuator/health` reports only an
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aggregate `"circuitBreakers":{"status":"UNKNOWN"}` in this version — no per-instance detail.
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The full breakdown (`state`, `failureRate`, `bufferedCalls`, `notPermittedCalls`, ...) is at
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`GET /actuator/circuitbreakers` instead. Both captured live, breaker actually OPEN, via
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[`ActuatorHealthTest.java`](../src/test/java/com/ankurm/resilience/r4j/ActuatorHealthTest.java):
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[`05-actuator-health-tripped.txt`](output/05-actuator-health-tripped.txt).
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- Resilience4j reference: [CircuitBreaker](https://resilience4j.readme.io/docs/circuitbreaker) (`rel=nofollow`)
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- The actual states and transition rules, from the CircuitBreaker interface itself, are worth
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reading directly rather than from a diagram: `io.github.resilience4j.circuitbreaker.CircuitBreaker.State`.
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Next: [03-spring-retryable.md](03-spring-retryable.md).
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