fix: correct EWMA decay formula and reject sample count of 0#27
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Pull request overview
Fixes the EWMA breaker’s smoothing/decay factor to use the canonical span-based formula and ensures invalid configuration (sampleCount == 0) is rejected during circuit construction, preventing out-of-range EWMA weights.
Changes:
- Update EWMA decay formula to
2/(N+1)soN==1yields decay1and allN>=1stay within(0,1]. - Add validation in
EWMABreaker.applyto reject configurations that producedecay > 1(i.e.,sampleCount == 0). - Add tests covering the
sampleCount == 1decay behavior andsampleCount == 0rejection viaNewCircuit.
Reviewed changes
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| File | Description |
|---|---|
| breaker.go | Corrects EWMA decay computation and adds sampleCount==0 rejection via apply() validation. |
| breaker_test.go | Adds regression tests for decay correctness at sampleCount==1 and rejection at sampleCount==0. |
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The smoothing factor was computed as 2/(N/2+1), which is the span formula with N halved. For sampleCount == 1 this yields decay == 1.333 - a smoothing factor greater than 1, giving the previous failure rate a negative weight so the rate can go below 0 or above 1. It also contradicted the documented "a value of 1 causes a single sample to be considered": that single-sample behavior (decay == 1) actually only occurred at sampleCount == 2. Use the canonical span-based factor 2/(N+1) (the Wikipedia exponential- smoothing link in the code): it equals exactly 1 at N == 1 (matching the docs) and stays in (0,1] for every N >= 1. Only N == 0 remains out of range (decay == 2), so apply now rejects it. The zero-value breaker (decay == 0) stays allowed and never opens, as documented. This changes convergence speed for existing sample counts, so it is a behavior change. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What
Corrects the EWMA smoothing factor and rejects
sampleCount == 0.Why
The decay was
2/(N/2+1)— the span formula withNhalved:sampleCount == 1→ decay 1.333 (> 1), giving the previous rate a negative weight, so the failure rate can go<0or>1. Mathematically broken.sampleCount == 2.Anyone who read the doc and passed
sampleCount=1is running a breaker with a broken update rule.How
2/(N+1): exactly1atN==1(matches the docs), and in(0,1]for allN≥1.N==0is left out of range (decay 2) →applyrejects it with a clear error.Notes
feat!type-parameter change.🤖 Generated with Claude Code