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Prometheus supports four types of metrics, they are - Counter - Gauge - Histogram - Summary
Counter
Counter is a metric value which can only increase or reset i.e the value cannot reduce than the previous value. It can be used for metrics like number of requests, no of errors etc.
Type the below query in the query bar and click execute.
go_gc_duration_seconds_count
The rate() function in PromQL takes the history of metrics over a time frame and calculates how fast value is increasing per second. Rate is applicable on counter values only.
rate(go_gc_duration_seconds_count[5m]) 
Gauge
Gauge is a number which can either go up or down. It can be used for metrics like number of pods in a cluster, number of events in an queue etc.
go_memstats_heap_alloc_bytes 
PromQL functions like max_over_time, min_over_time and avg_over_time can be used on gauge metrics
Histogram
Histogram is a more complex metric type when compared to the previous two. Histogram can be used for any calculated value which is counted based on bucket values. Bucket boundaries can be configured by the developer. A common example would the time it takes to reply to a request, called latency.
Example: Lets assume we want to observe the time taken to process API requests. Instead of storing the request time for each request, histograms allow us to store them in buckets. We define buckets for time taken, for example lower or equal 0.3 , le 0.5, le 0.7, le 1, and le 1.2. So these are our buckets and once the time taken for a request is calculated it is added to the count of all the buckets whose bucket boundaries are higher than the measured value.
Lets say Request 1 for endpoint “/ping” takes 0.25 s. The count values for the buckets will be.
/ping
| Bucket | Count |
|---|---|
| 0 - 0.3 | 1 |
| 0 - 0.5 | 1 |
| 0 - 0.7 | 1 |
| 0 - 1 | 1 |
| 0 - 1.2 | 1 |
| 0 - +Inf | 1 |
Note: +Inf bucket is added by default.
(Since histogram is a cumulative frequency 1 is added to all the buckets which are greater than the value)
Request 2 for endpoint “/ping” takes 0.4s The count values for the buckets will be this.
/ping
| Bucket | Count |
|---|---|
| 0 - 0.3 | 1 |
| 0 - 0.5 | 2 |
| 0 - 0.7 | 2 |
| 0 - 1 | 2 |
| 0 - 1.2 | 2 |
| 0 - +Inf | 2 |
Since 0.4 is below 0.5, all buckets up to that boundary increase their counts.
Let’s explore a histogram metric from the Prometheus UI and apply few functions.
prometheus_http_request_duration_seconds_bucket{handler="/graph"}
histogram_quantile() function can be used to calculate calculate quantiles from histogram
histogram_quantile(0.9,prometheus_http_request_duration_seconds_bucket{handler="/graph"})
The graph shows that the 90th percentile is 0.09, To find the histogram_quantile over last 5m you can use the rate() and time frame
histogram_quantile(0.9, rate(prometheus_http_request_duration_seconds_bucket{handler="/graph"}[5m]))
Summary
Summaries also measure events and are an alternative to histograms. They are cheaper, but lose more data. They are calculated on the application level hence aggregation of metrics from multiple instances of the same process is not possible. They are used when the buckets of a metric is not known beforehand, but it is highly recommended to use histograms over summaries whenever possible.
In this tutorial we covered the types of metrics in detail and few PromQL operations like rate, histogram_quantile etc.
