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Thanos ruler: failed to evaluate rule
Understanding Thanos and Its Purpose
Thanos is an open-source project that provides a highly available, long-term storage solution for Prometheus metrics. It is designed to seamlessly integrate with existing Prometheus deployments, offering features such as global querying, unlimited storage, and downsampling of metrics. Thanos is widely used in cloud-native environments to ensure that metrics are stored reliably and can be queried efficiently across multiple clusters.
Identifying the Symptom: Ruler Evaluation Failure
One common issue users may encounter when using Thanos is the error message: ruler: failed to evaluate rule. This error indicates that the Thanos Ruler component has encountered a problem while attempting to evaluate a rule. The symptom is typically observed in the logs of the Thanos Ruler service, and it can disrupt the expected alerting and recording rule functionalities.
Exploring the Issue: Why Does This Error Occur?
The error ruler: failed to evaluate rule can arise due to several reasons. The most common causes include:
- Syntax Errors: Mistakes in the rule syntax can prevent successful evaluation. This includes incorrect expressions or missing fields in the rule definition.
- Missing Data: The rule may depend on metrics or labels that are not available in the data source, leading to evaluation failures.
Understanding the root cause is crucial for resolving the issue effectively.
Steps to Fix the Issue
1. Verify Rule Syntax
Start by checking the syntax of your Prometheus rules. Ensure that all expressions are correctly formatted and adhere to the Prometheus rule syntax. You can use the Prometheus documentation for reference.
# Example of a simple rule
- alert: HighRequestLatency
expr: job:request_latency_seconds:mean5m{job="myjob"} > 0.5
for: 10m
labels:
severity: page
annotations:
summary: "High request latency detected"
2. Check Data Availability
Ensure that the metrics required by the rule are available in your Prometheus data source. You can query Prometheus directly to verify the presence of the necessary metrics:
up{job="myjob"}
If the data is missing, investigate the data collection and ingestion pipeline to resolve any issues.
3. Review Logs for Additional Clues
Examine the logs of the Thanos Ruler service for any additional error messages or warnings that might provide more context about the failure. Logs can often reveal underlying issues that are not immediately apparent.
4. Test Rules in Isolation
If possible, test the problematic rule in isolation using a local Prometheus setup. This can help identify whether the issue is specific to the rule itself or related to the Thanos environment.
Conclusion
By following these steps, you should be able to diagnose and resolve the ruler: failed to evaluate rule error in Thanos. Ensuring correct rule syntax and data availability are key to maintaining a reliable alerting and monitoring setup. For further assistance, consider visiting the Thanos troubleshooting guide.
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