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A circuit breaker in a Rust reverse proxy can fail even when its basic Closed, Open, and HalfOpen logic looks right. Late request results can overwrite newer state, health checks can hide real traffic failures, and configuration reloads can strand in-flight outcomes. A September 28, 2026 report on the Rust proxy ferryman describes these pitfalls and several implementation choices; its details are attributed to that report rather than independently verified project source.
How a circuit breaker should control proxy traffic
A circuit breaker limits repeated calls to an unhealthy upstream. In the common three-state model described in the conproxy Rust documentation, Closed allows ordinary calls, Open blocks them, and HalfOpen admits limited test calls to check whether the upstream has recovered.
Those states are only the outline. A reverse proxy must also account for when a request was admitted, when its result arrives, whether the result is a recovery probe, and whether the breaker instance is still the one associated with that request.
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Bug 1: A late request result can undo a newer transition
Suppose a request is admitted while the breaker is Closed. Other requests then fail and move it to Open. If the earlier request later succeeds and its completion handler blindly sets the breaker to Closed, the upstream gets treated as healthy even though the breaker opened after that request began.
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The reverse race is possible during recovery: a late failure from ordinary traffic can reopen the breaker after a HalfOpen recovery probe has been admitted. The matching ferryman account describes both stale-result problems. It reports that the implementation tags admissions as either Normal or Probe, so a stale ordinary outcome cannot override the Open state and probe outcomes govern recovery.
The general lesson is to treat an outcome as evidence about the state in which its request was admitted, not as an unconditional command to set the current state. A design should distinguish ordinary calls from recovery probes and ensure that only outcomes valid for the current transition can change that transition.
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Publish transition data in a safe order
The report says ferryman tracks state, consecutive failures, and an open timestamp in monotonic milliseconds. It also says the implementation publishes the open timestamp before publishing the Open state. That ordering matters when another task observes the new state: it should not see Open before the information needed to decide when recovery may be tested is available.
According to the same account, a compare-and-swap admits one recovery probe during the Open-to-HalfOpen transition. This is one way to prevent multiple concurrent callers from all treating themselves as the single test request. The appropriate synchronization strategy depends on the implementation, but state and associated timing data need a coherent publication order.
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Bug 2: A healthy health check can mask failing requests
A health endpoint may succeed while the proxy’s ordinary request path to the same upstream is failing. If that health-check success resets the breaker’s ordinary failure evidence while it is Closed, the breaker may never reach its failure threshold. The September 28, 2026 ferryman report identifies this as a bug: health-check success should not erase ordinary failure evidence in the Closed state.
Keep health-check observations distinct from the results that govern the traffic circuit unless the health check faithfully represents the path and behavior users depend on. A check that only proves the process is alive, for example, does not establish that the relevant route, dependency, or request handling is healthy.
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Bug 3: Replacing breakers on reload can orphan outcomes
A request can begin under one breaker instance and complete after configuration reload has replaced that instance. The outcome then updates an object no longer used for new requests. The replacement breaker lacks that in-flight result, so its view of the upstream may not reflect failures or recovery already observed by requests that were underway during reload.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThe ferryman report says the project reuses breaker instances per upstream across reloads. That preserves continuity of the breaker’s state while other configuration changes are applied. If an implementation instead replaces instances, it needs an explicit handoff strategy for state and outstanding outcomes; merely swapping the configuration reference does not account for work already in flight.
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Timeouts are part of proxy correctness too
The ferryman report also flags a hyper-util configuration trap: it says the header-read timeout is ignored unless a timer is attached to the builder. Treat this as version-specific implementation guidance, not a universal statement about every hyper-util release. Check the API documentation and builder configuration for the exact version in use, and verify that a timeout is actually active rather than assuming that setting a duration is sufficient.
Timeouts should be placed on the relevant part of the proxy transaction. A limit while reading client headers, for example, answers a different question from a limit while waiting for an upstream response. A separate Rust reverse-proxy article discusses timeout placement alongside protocol-version translation and half-open concurrency; those broader topics are distinct from the three ferryman bugs and should not be mistaken for findings about that project.
Configuration choices that change breaker behavior
Failure counting and recovery policy are configuration choices, not universal constants. The conproxy documentation exposes dimensions such as failure threshold, failure window, open duration, and the number of successes required in HalfOpen. Its documented defaults are 25 failures, a 60-second window, 30 seconds open, and two HalfOpen successes; those figures apply to that crate’s documentation and are not general circuit-breaker defaults.
When evaluating or designing a breaker, compare the policy and lifecycle behavior rather than relying on a label such as “circuit breaker.” In particular, establish what counts as a failure and over what interval, how many HalfOpen probes may run, how late outcomes interact with newer transitions, whether state survives reloads, and whether health checks reflect the real traffic path. The cited sources do not establish a benchmark or head-to-head performance result for alternative implementations.
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