v1.9.9

v1.9.9 (2025-11-19)

In this release we've introduced availability-based scaling enabled by default.

About availability-based scaling and how it can be configured

The scaler monitors agent availability to handle situations where EC2 instances are healthy but Buildkite agents aren't connecting. This can happen due to network issues, agent configuration problems, or instance startup delays.

AVAILABILITY_THRESHOLD (default: 0.5)

When jobs are queued, the scaler checks if the percentage of connected agents meets this threshold. For example, with 4 agents per instance and 2 instances running (8 expected agents), if only 3 agents are online, that's 37.5% availability.

When availability drops below the threshold and the ASG has converged (actual instances match desired), the scaler adds one instance to help recover availability.

Set AVAILABILITY_THRESHOLD=0 to disable availability-based scaling. The scaler will then scale based only on job count.

Threshold tuning:

• Lower threshold (e.g., 0.3): Tolerates slower agent connection times, reduces instance churn
• Higher threshold (e.g., 0.8): Aggressive scaling to maintain high availability when agents are expected to connect quickly
• Disabled (0): Job-based scaling only, suitable when agents connect reliably

Changed

• Fix availability-based scaling and simplify threshold logic by @scadu in #257

Dependencies

• chore(deps): bump github.com/aws/aws-sdk-go-v2/config from 1.31.12 to 1.31.19 by @dependabot[bot] in #259
• chore(deps): bump github.com/aws/aws-sdk-go-v2/service/ec2 from 1.261.0 to 1.266.0 by @dependabot[bot] in #260
• chore(deps): bump github.com/aws/aws-sdk-go-v2/service/ssm from 1.66.3 to 1.67.1 by @dependabot[bot] in #262
• chore(deps): bump github.com/aws/aws-sdk-go-v2/service/cloudwatch from 1.52.0 to 1.52.2 by @dependabot[bot] in #261
• chore(deps): bump github.com/aws/aws-sdk-go-v2/service/autoscaling from 1.60.2 to 1.60.4 by @dependabot[bot] in #264
• chore(deps): bump the aws group with 5 updates by @dependabot[bot] in #266

Internal

• Dependabot weekly, add agent-stewards to CODEOWNERS by @DrJosh9000 in #258
• Group AWS library updates together by @DrJosh9000 in #265