# How does the Prometheus rate() function work?

In Prometheus, the rate() function is used to calculate the per-second average rate of increase in a time series over a given time range. The rate() function can be used to calculate the rate of increase in metrics such as requests per second, bytes per second, or errors per second.

The rate() function works by taking a time series as input and calculating the slope of the linear regression line that best fits the data points over the specified time range. The slope of this line represents the per-second rate of change in the time series.

The rate() function can also take an optional second argument, which specifies the time window over which to calculate the rate. This can be useful for calculating short-term or long-term rates, depending on the needs of the user.

For example, the following query calculates the per-second rate of increase in the "requests_total" metric over the past 5 minutes:

rate(requests_total[5m])

This query returns the per-second average rate of increase in the "requests_total" metric over the past 5 minutes.

The rate() function is a powerful tool for monitoring and alerting in Prometheus, as it enables users to quickly identify trends and patterns in their metrics data and respond to issues in a timely manner.