Prometheus Rate Function: A Practical Guide to Using It

Prometheus is a powerful and flexible tool for observability and monitoring, with the rate() function standing out as a key feature for tracking system behavior over time.
This guide will walk you through how to use the rate() function effectively, covering its mechanics, use cases, and best practices to boost your monitoring and observability efforts, helping you build more reliable systems.

Understanding the Prometheus Rate Function

The rate() function is a key component of PromQL (Prometheus Query Language) used for analyzing the rate of change in counter metrics over time. At its core, rate() calculates the per-second average rate of increase of time series in a range vector.

rate() helps answer critical questions such as:

• "What is the current request rate for a service?"
• "How rapidly is CPU usage increasing over a given time period?"
• "What's the rate of errors occurring in our application?"

These insights help in understanding system performance and behavior.

The Importance of the Rate Function in Monitoring

Understanding rates of change is crucial in monitoring systems for a few key reasons:

1. Performance Monitoring: It helps identify sudden spikes or drops in system performance, allowing for quick detection of anomalies or potential issues.
2. Capacity Planning: Analyzing trends in rates allows for predicting future resource needs and planning for scaling accordingly.
3. Alerting: Rate-based alerts can catch issues before they become critical, enabling proactive problem-solving.
4. SLO/SLA Tracking: Rates are often key components of service-level objectives and agreements, making them crucial for ensuring compliance and maintaining service quality.
5. Trend Analysis: Rates provide valuable insights into long-term trends, helping in strategic decision-making and system optimization.

Using the Rate Function: Syntax and Basic Examples

The basic syntax of the rate() function is straightforward:

rate(metric[time_range])

Example:

Assume there's a counter metric, http_requests_total, which tracks the total number of HTTP requests to a service.

To calculate the rate of requests per second over the last 5 minutes, use the following query:

rate(http_requests_total[5m])

This query returns the per-second rate of increase for http_requests_total over the last 5 minutes.

Note: It's crucial to use rate() only counter-metrics. Applying it to gauge metrics will result in incorrect data.

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Real-World Example: Monitoring API Request Rates

Let's consider a scenario where you need to monitor request rates for different API endpoints. Here's how it can be set up:

1. Instrument the API: Expose a counter metric, api_requests_total, with labels for the endpoint and method.
2. Create a Grafana Dashboard: Use the following PromQL query to visualize the request rates:

sum by (endpoint) (rate(api_requests_total{job="api-server"}[5m]))

This query calculates the request rate for each endpoint over the last 5 minutes and sums up the rates for all methods.

3. Set Up Alerts: Trigger an alert if the request rate for any endpoint exceeds 100 requests per second:

sum by (endpoint) (rate(api_requests_total{job="api-server"}[5m])) > 100

This setup provides a real-time view of API usage patterns, enabling quick identification of bottlenecks and helping optimize heavily used endpoints.

Rate vs. Irate: When to Use Each

Prometheus provides two similar functions, rate() and irate(), but they serve different purposes:

• rate() calculates the average rate of increase over the specified time range.
• irate() calculates the instant rate of increase using only the last two data points.

When to Use Each:

• rate() is ideal for stable metrics and alerting, as it smooths out short-term fluctuations and gives you a more consistent view of the rate over time.
• irate() is better for graphing highly variable metrics, where you want to capture rapid changes and can tolerate more noise.

# Stable rate over 5 minutes
rate(http_requests_total[5m])

# Instant rate, more prone to spikes
irate(http_requests_total[5m])

Use rate() when you want a stable, long-term view of your metrics.
Use irate() when you need to see short-term spikes or rapid changes.

Advanced Usage: Combining rate() with Other Functions

The true potential of the rate() function in Prometheus comes when it’s combined with other PromQL functions.

Below are a few advanced techniques to enhance your metrics analysis:

1. Calculating Request Error Rates

To calculate the ratio of 5xx errors to total requests:

sum(rate(http_requests_total{status=~"5.."}[5m])) / sum(rate(http_requests_total[5m]))

This gives a clear view of your error rate by comparing 5xx errors to the total number of requests.

2. Using histogram_quantile() with Rates

To calculate the 95th percentile of request durations over the last 5 minutes:

histogram_quantile(0.95, sum by (le) (rate(http_request_duration_seconds_bucket[5m])))

This is especially useful for tracking performance SLOs by providing a detailed view of latency.

3. Smoothing Out Spikes with avg_over_time()

To smooth out short-term fluctuations by averaging the request rate over a longer period:

avg_over_time(rate(http_requests_total[5m])[1h:])

This gives the average rate over the last hour, updated every 5 minutes, helping to track longer-term trends.

4. Comparing Rates Across Different Time Ranges

To detect sudden traffic spikes by comparing the short-term and long-term request rates:

rate(http_requests_total[5m]) / rate(http_requests_total[1h]) > 1.2

This query identifies when the short-term rate is 20% higher than the long-term rate.

5. Calculating the Rate of Change for a Gauge Metric

For calculating the rate of change in gauge metrics (like memory usage) using deriv():

deriv(process_resident_memory_bytes{job="app"}[1h])

This tracks how quickly memory usage is changing over an hour.

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Common Pitfalls and How to Avoid Them

When working with the rate() function, keep an eye on these common issues:

1. Too Short Time Ranges: Using a time range that's shorter than the scrape interval can result in inaccurate data. A good rule of thumb is to use a time range at least 4x the scrape interval. For instance, if your scrape interval is 15 seconds, set the time range in your rate() function to at least 1 minute.
2. Ignoring Counter Resets: The rate() function can handle counter resets (e.g., when a service restarts), but these resets can still cause temporary spikes. Be mindful of this when interpreting data, as the rate is calculated from the available information.
3. Misunderstanding Aggregation: Since rate() returns a per-second value, summing or averaging these rates won’t give an accurate total. Instead, sum the underlying counters and then apply the rate() function.
4. Inappropriate Use with Gauges: The rate() function is specifically for counters. Using it with gauge metrics will yield incorrect results, so avoid this combination.
5. Neglecting Label Changes: Frequent changes in label values can create gaps in your data, leading to inaccurate rate calculations. Always account for potential label changes when working with metrics.

Best Practices for Using the rate() Function

To make the most of the rate() function in Prometheus, keep these best practices in mind:

• Choose the Right Time Range: Ensure the time range is long enough to capture meaningful trends while remaining short enough to respond to real-time changes.
• Alerting with Care: When using rate() in alerts, opt for longer time ranges to reduce the risk of false positives caused by short-term fluctuations.
• Leverage Other Functions: As highlighted in advanced examples, combining rate() with other PromQL functions can provide richer insights into your data.
• Know Your Metrics: Understand the nature of your data—such as how often metrics update and their variability—to ensure accurate monitoring.
• Test Thoroughly: Validate your queries with historical data to ensure they're reliable under different conditions and scenarios.

Conclusion

The Prometheus rate() function is a fundamental tool for monitoring, offering versatility from tracking request rates to analyzing performance and error metrics. Its strength lies in revealing the rate of change across various metrics, making it vital for any observability strategy.

To truly master the rate() function, practice is key. Experimenting with different queries and time ranges can uncover the best approaches for your specific use cases. As systems grow in complexity, proficiency with functions like rate() becomes essential to ensure smooth performance, reliability, and a positive user experience.

FAQs

Q: How does the Prometheus rate() function differ from increase()?
A: While rate() calculates the per-second average rate of increase, increase() calculates the total increase in the counter's value over the time range. rate() is generally more useful for ongoing monitoring, while increase() can help understand total change over a specific period.

Q: How do you calculate request rates using the Prometheus rate function?
A: To calculate request rates, use a query like rate(http_requests_total[5m]). This will give the per-second rate of requests over the last 5 minutes. These rates can be summed or grouped as needed, e.g., sum(rate(http_requests_total[5m])) for the total request rate across all instances.

Q: Are there approaches for capturing spikes with PromQL?
A: Yes, max_over_time() can be used with rate() to capture spikes. For example, max_over_time(rate(http_requests_total[5m])[1h:]) will show the maximum rate observed in 5-minute windows over the last hour.

Q: How do you calculate the increase of a counter over time using Prometheus functions?
A: To calculate the total increase of a counter, use the increase() function. For example, increase(http_requests_total[1h]) will show the total increase in the number of requests over the last hour.

Q: Can rate() be used with all types of Prometheus metrics?
A: No, rate() should only be used with counter-metrics. It doesn't make sense to use rate() with gauge metrics, as they don't represent cumulative values.