Fly-Style opened a new pull request, #18819:
URL: https://github.com/apache/druid/pull/18819
# Cost-Based Autoscaler for Seekable Stream Supervisors
## Overview
Implements a cost-based autoscaling algorithm for seekable stream supervisor
tasks that optimizes task count by balancing lag reduction against resource
efficiency.
## Algorithm Design
### Cost Function
The autoscaler uses a weighted cost function to evaluate different task
count configurations:
```
cost = lagWeight × normalizedLag + idleWeight × predictedIdleRatio
```
**Components of cost function:**
- **Lag component**: Measures how quickly the system processes "backlog"
- **Idle component**: Measures resource efficiency (tasks waiting for data
or busy)
### Key Features
**1. Predictive Cost Calculation**
- Predicts lag and idle ratio for candidate task counts using linear scaling
- Evaluates multiple task count options within ±2 positions of current count
- Selects configuration with minimum cost
- Tracks historical observed lag values, provides stable normalization even
with varying lag magnitudes
**2. Lag-Aware Idle Estimation**
- **High lag (>2M)**: Inverse relationship - more tasks = less idle
(processing backlog)
- **Low lag (<2M)**: Normal relationship - more tasks = more idle (waiting
for data)
##### Key files Changed
**Core Implementation:**
- `WeightedCostFunction.java` - Cost function and adaptive bounds
- `CostBasedAutoScaler.java` - Autoscaler orchestration
- `CostMetrics.java` - Metrics data class
**Tests:**
- `WeightedCostFunctionTest.java` - Comprehensive unit tests
- `CostBasedAutoScalerTest.java` - Integration tests
<hr>
This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md)
(Remove this item if the PR doesn't have any relation to concurrency.)
- [ ] added documentation for new or modified features or behaviors.
- [ ] a release note entry in the PR description.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added or updated version, license, or notice information in
[licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md)
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [ ] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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