What is a Prometheus rule? - A Comprehensive Guide

Prometheus rules are powerful configuration elements that enhance the capabilities of the Prometheus monitoring system. These rules allow you to precompute complex queries and define alert conditions, making your monitoring more efficient and responsive. In this comprehensive guide, you'll learn about the two main types of Prometheus rules: recording rules and alerting rules. We'll explore their importance, configuration, and best practices to help you optimize your monitoring setup.

What are Prometheus Rules and Why are They Important?

Prometheus rules are configuration elements that extend the functionality of the Prometheus monitoring system. They come in two main flavors: recording rules and alerting rules. Each type serves a distinct purpose in enhancing your monitoring capabilities.

Recording rules allow you to precompute frequently used or complex queries and save their results as new time series. This precomputation significantly improves query performance, especially for dashboards that need to load quickly.

Alerting rules, on the other hand, define conditions that trigger alerts when met. These rules automate the process of detecting critical issues in your systems, enabling faster response times to potential problems.

The importance of Prometheus rules lies in their ability to:

Optimize query performance by precomputing expensive calculations
Automate alert generation for critical conditions
Improve the efficiency of time series data management
Enhance the scalability of your monitoring system

Understanding Recording Rules in Prometheus

Recording rules in Prometheus are designed to precompute frequently used or resource-intensive queries. They save the results as new time series, which can be queried much faster than repeatedly executing the original complex query.

The syntax of a recording rule is straightforward:

- record: <new_metric_name>
  expr: <PromQL_expression>

Here's an example of a recording rule that calculates the average CPU usage across all instances:

- record: job:node_cpu_utilization:avg_rate5m
  expr: avg(rate(node_cpu_seconds_total{mode="user"}[5m])) by (job)

This rule computes the 5-minute average CPU utilization rate and stores it under a new metric name.

Benefits of using recording rules include:

Faster query execution for frequently accessed metrics
Reduced load on the Prometheus server
Simplified PromQL queries in dashboards and alerts

Best practices for naming and organizing recording rules:

Use a consistent naming convention (e.g., level:metric:operations)
Group related rules together
Add comments to explain complex calculations
Avoid creating too many recording rules — focus on the most valuable ones

Configuring Recording Rules: A Step-by-Step Guide

To set up recording rules in Prometheus, follow these steps:

Create a YAML file for your rules (e.g., recording_rules.yml):

groups:
  - name: cpu_utilization
    rules:
    - record: job:node_cpu_utilization:avg_rate5m
      expr: avg(rate(node_cpu_seconds_total{mode="user"}[5m])) by (job)

Add the rule file to your Prometheus configuration:

rule_files:
  - "recording_rules.yml"

Restart Prometheus to apply the changes.
Test your rules using the Prometheus expression browser or PromQL API.

To validate your rules before applying them, use the promtool command:

promtool check rules recording_rules.yml

This tool will catch syntax errors and other issues before you deploy your rules.

Mastering Alerting Rules in Prometheus

Alerting rules in Prometheus define conditions that, when met, trigger alerts. These rules allow you to automatically detect and respond to critical issues in your systems.

The anatomy of an alerting rule includes:

- alert: <alert_name>
  expr: <PromQL_expression>
  for: <duration>
  labels:
    <label_key>: <label_value>
  annotations:
    <annotation_key>: <annotation_value>

Here's an example of an alerting rule that triggers when CPU usage is above 90% for 5 minutes:

- alert: HighCPUUsage
  expr: 100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100) > 90
  for: 5m
  labels:
    severity: critical
  annotations:
    summary: "High CPU usage detected on {{ $labels.instance }}"
    description: "CPU usage is above 90% for 5 minutes on {{ $labels.instance }}"

Prometheus evaluates alerting rules at regular intervals. When an alert condition is met, it generates an alert and sends it to configured notification systems like Alertmanager.

Advanced Alerting Rule Techniques

To create more sophisticated alerting rules:

Use label matching and regex:

- alert: ServiceDown
  expr: up{job=~"important-service|critical-app"} == 0

Implement multi-condition alerts:

- alert: HighErrorRateAndLatency
  expr: (rate(http_requests_total{status=~"5.."}[5m]) / rate(http_requests_total[5m])) > 0.05 and http_request_duration_seconds{quantile="0.9"} > 1

Reduce alert noise by using the for clause and grouping related alerts:

- alert: PersistentHighCPU
  expr: 100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100) > 90
  for: 15m

Integrate with Alertmanager for advanced notification routing and grouping.

Best Practices for Prometheus Rule Management

To effectively manage your Prometheus rules:

Organize rules into logical groups based on service or function.
Use version control (e.g., Git) to track changes to rule files.
Document your rules with clear comments explaining their purpose and any complex logic.
Regularly review and update rules to ensure they remain relevant and effective.
Use promtool to validate rules before deployment.
Consider using a CI/CD pipeline to automate rule testing and deployment.

Troubleshooting Common Prometheus Rule Issues

When working with Prometheus rules, you might encounter these common issues:

Syntax errors: Use promtool to catch and fix syntax problems in your rule definitions.
Performance issues: Optimize inefficient rules by simplifying PromQL expressions or using recording rules for complex calculations.
Rule evaluation timeouts: Adjust the evaluation_interval in Prometheus configuration or optimize the rule expression.
Resource constraints: Monitor Prometheus itself and consider scaling your monitoring infrastructure if needed.

To optimize rule evaluation frequency, balance between timely alerts and system load. Start with longer intervals (e.g., 1 minute) and adjust based on your specific needs and resource availability.

Key Takeaways

Prometheus rules enhance monitoring efficiency through precomputation and automated alerting.
Recording rules improve query performance by precomputing expensive calculations.
Alerting rules automate the detection of critical conditions in your systems.
Proper rule management — including organization, version control, and regular review — is crucial for maintaining a robust monitoring system.

FAQs

What's the difference between recording rules and alerting rules?

Recording rules precompute and store query results as new time series, while alerting rules define conditions that trigger alerts when met.

How often does Prometheus evaluate rules?

Prometheus evaluates rules at intervals specified by the evaluation_interval in the Prometheus configuration. The default is usually 1 minute.

Can I use PromQL functions in Prometheus rules?

Yes, you can use any valid PromQL expression in both recording and alerting rules, including functions.

How do I ensure my Prometheus rules are working correctly?

Use the Prometheus expression browser to test your rules, monitor the prometheus_rule_evaluation_failures_total metric, and regularly review your alerting and recording rule effectiveness.

Monitoring Made Easy with SigNoz

While Prometheus offers powerful monitoring capabilities, setting up and managing rules can be complex. SigNoz provides a user-friendly alternative that simplifies monitoring and alerting for your applications and infrastructure.

With SigNoz, you can:

Easily set up alerts without complex PromQL queries
Visualize your metrics with intuitive dashboards
Correlate metrics, traces, and logs in a single platform
Scale your monitoring effortlessly with cloud-native architecture

To get started with SigNoz:

SigNoz cloud is the easiest way to run SigNoz. Sign up for a free account and get 30 days of unlimited access to all features. You can also install and self-host SigNoz yourself since it is open-source. With 18,000+ GitHub stars, open-source SigNoz is loved by developers. Find the instructions to self-host SigNoz.

SigNoz offers both cloud and open-source versions, giving you flexibility in your monitoring setup. Experience the ease of modern observability with SigNoz and take your monitoring to the next level.