SURYAS1306 opened a new pull request, #16048:
URL: https://github.com/apache/dubbo/pull/16048
## What is the purpose of the change?
This PR fixes an issue in AdaptiveLoadBalance / AdaptiveMetrics where
latency decay behaves incorrectly when provider metrics are not updated for a
period of time.
Currently, when no new provider metrics arrive, getLoad() may repeatedly
apply the penalty branch or aggressively right-shift lastLatency, which can
result in stale or extreme values dominating EWMA. This makes adaptive load
balancing unstable, especially in low-QPS or intermittent-update scenarios.
This PR ensures that latency decays safely and progressively instead of
collapsing or being stuck at penalty values.
Fixes #15810
## What is changed?
### 1. Improved decay logic in AdaptiveMetrics#getLoad()
- Prevents lastLatency from collapsing to zero.
- Ensures decay happens smoothly when provider metrics are not refreshed.
- Avoids repeatedly applying the penalty path when timestamps are equal.
### 2. Added unit test
Added `testAdaptiveMetricsDecayWithoutProviderUpdate`
Verifies that when provider metrics are not updated:
- latency decays over time
- penalty value is not stuck
- EWMA continues to evolve
## Why is this needed?
Adaptive load balancing relies on EWMA latency to reflect recent performance
trends.
Without this fix:
- old latency values can dominate indefinitely
- penalty values may be repeatedly re-applied
- low-traffic services become unfairly weighted
This change makes adaptive load balancing more stable, realistic, and robust
under real-world traffic patterns.
## Verifying this change
- Added new unit test covering the decay scenario
- All tests pass locally:
```bash
mvn -pl dubbo-cluster -am test
```
## Checklist
- [x] Related GitHub issue referenced (Fixes #15810)
- [x] Logic change explained clearly
- [x] Unit test added
- [x] All tests passing locally
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