Canary Metric Judge

Decide whether a canary release should promote, hold, or roll back.

Requirements

• Define judge_canary(baseline, candidate, thresholds).
• Return {"decision": ..., "reasons": [...]}.
• Decisions are "promote", "hold", or "rollback".
• If candidate requests are below thresholds["min_requests"], return hold with exactly ["not_enough_traffic"]; rollback reasons are excluded.
• Roll back if candidate error_rate is missing or exceeds max_error_rate → reason token "error_rate".
• Roll back if baseline/candidate latency is missing, baseline latency is zero, or candidate latency divided by baseline latency exceeds max_latency_ratio → reason token "latency".
• Quality is higher-is-better. Define quality_delta = candidate["quality"] - baseline["quality"]. Roll back if either quality is missing or quality_delta < min_quality_delta → reason token "quality".
• Comparisons are strict: traffic holds when requests < min_requests; error and latency roll back on >; quality rolls back on <. Equality passes each gate.
• Rollback reason tokens must be exactly "error_rate", "latency", and "quality" (not "error", "latency_ratio", or "quality_delta").
• Promote only when no rollback reason exists.
• Reason order: not_enough_traffic (hold only), then rollback reasons in order error_rate, latency, quality.

Example

Constraints

• Don't divide by zero.
• Missing metrics should fail closed with rollback reasons.
• Keep reason ordering deterministic.