Prometheus remoteWrite configuration causing unexpected load balancer traffic and cost

Issue

A misconfigured Prometheus remoteWrite endpoint resulted in excessive egress traffic through the Kubernetes load balancer, exceeding allocated quotas and incurring unexpected costs. The issue manifested as sustained high traffic (e.g., 1.3 GB/hour) for an extended period, despite the remoteWrite destination being unreachable. This led to repeated retry storms, overwhelming the load balancer and triggering quota alerts.

The problem persisted even after the destination failed to respond, as aggressive retry settings in the Prometheus configuration continued to send metric batches through the load balancer, accumulating significant egress volume.

Culprit

The root cause was a combination of misconfiguration and aggressive retry settings:

1. Invalid Destination URL: The remoteWrite endpoint was configured with an incorrect or unresolved address, causing DNS or connection failures.
2. Excessive Retry Configuration: The retry parameters were overly aggressive, with short backoff intervals and high queue capacity, leading to a continuous storm of failed retry attempts.
3. Lack of Validation: The configuration was not validated before deployment, allowing the misconfiguration to persist undetected.
4. Egress Path Dependency: Each failed retry was routed through the Kubernetes load balancer, contributing to the egress traffic volume.

This resulted in:

• High egress traffic volumes exceeding quota limits.
• Increased costs due to overage charges.
• Persistent errors in Prometheus logs, impacting observability.

Solution

Immediate Remediation

1. Disable Misconfigured RemoteWrite: Temporarily disable the problematic remoteWrite configuration to halt excessive traffic.
2. Validate Endpoints: Ensure all remoteWrite destinations are reachable and correctly configured before deployment.
3. Monitor Traffic: Implement alerts for unusual egress patterns to detect similar issues early.

Correct Configuration

To properly configure remoteWrite for Azure Monitor or other external destinations:

1. Use Valid Endpoints: Ensure the remoteWrite URL points to a valid and accessible destination (e.g., Azure Monitor Data Collection Rule).
2. Adjust Retry Settings: Configure retry parameters to balance resilience with traffic control (e.g., longer backoff intervals, limited queue capacity).
3. Implement Authentication: Use workload identity or other secure authentication methods to avoid connection issues.

Cost Prevention Measures

1. Quota Alerts: Set up alerts to notify when egress traffic approaches or exceeds quota limits.
2. CI/CD Validation: Add validation steps in the pipeline to check for misconfigured remoteWrite endpoints before deployment.
3. Alternative Storage: For non-critical metrics, consider retaining data locally within the cluster to avoid egress costs.

Verification

After resolving the issue:

• Traffic Levels: Confirm egress traffic returns to baseline levels.
• Cost Impact: Verify no overage charges or unexpected costs.
• Log Integrity: Ensure Prometheus logs no longer contain retry errors.
• Functional Testing: Validate that metrics are correctly exported to the intended destination.

This approach ensures a secure, cost-effective, and resilient monitoring setup while preventing similar incidents in the future.