Merge branch '58839-automatically-set-prometheus-step-interval' into 'master'

Resolve "Automatically set Prometheus step interval"

Closes #58839

See merge request gitlab-org/gitlab-ce!26441

changelogs/unreleased/58839-automatically-set-prometheus-step-interval.yml

+---
+title: Automatically set Prometheus step interval
+merge_request: 26441
+author:
+type: changed

lib/gitlab/prometheus_client.rb

@@ -6,6 +6,14 @@ module Gitlab
     Error = Class.new(StandardError)
     QueryError = Class.new(Gitlab::PrometheusClient::Error)
 
+    # Target number of data points for `query_range`.
+    # Please don't exceed the limit of 11000 data points
+    # See https://github.com/prometheus/prometheus/blob/91306bdf24f5395e2601773316945a478b4b263d/web/api/v1/api.go#L347
+    QUERY_RANGE_DATA_POINTS = 600
+
+    # Minimal value of the `step` parameter for `query_range` in seconds.
+    QUERY_RANGE_MIN_STEP = 60
+
     attr_reader :rest_client, :headers
 
     def initialize(rest_client)
@@ -23,12 +31,18 @@ module Gitlab
     end
 
     def query_range(query, start: 8.hours.ago, stop: Time.now)
+      start = start.to_f
+      stop = stop.to_f
+      step = self.class.compute_step(start, stop)
+
       get_result('matrix') do
-        json_api_get('query_range',
-                     query: query,
-                     start: start.to_f,
-                     end: stop.to_f,
-                     step: 1.minute.to_i)
+        json_api_get(
+          'query_range',
+          query: query,
+          start: start,
+          end: stop,
+          step: step
+        )
       end
     end
 
@@ -40,6 +54,14 @@ module Gitlab
       json_api_get('series', 'match': matches, start: start.to_f, end: stop.to_f)
     end
 
+    def self.compute_step(start, stop)
+      diff = stop - start
+
+      step = (diff / QUERY_RANGE_DATA_POINTS).ceil
+
+      [QUERY_RANGE_MIN_STEP, step].max
+    end
+
     private
 
     def json_api_get(type, args = {})

spec/lib/gitlab/prometheus_client_spec.rb

@@ -230,4 +230,32 @@ describe Gitlab::PrometheusClient do
       let(:execute_query) { subject.query_range(prometheus_query) }
     end
   end
+
+  describe '.compute_step' do
+    using RSpec::Parameterized::TableSyntax
+
+    let(:now) { Time.now.utc }
+
+    subject { described_class.compute_step(start, stop) }
+
+    where(:time_interval_in_seconds, :step) do
+      0               | 60
+      10.hours        | 60
+      10.hours + 1    | 61
+      # frontend options
+      30.minutes      | 60
+      3.hours         | 60
+      8.hours         | 60
+      1.day           | 144
+      3.days          | 432
+      1.week          | 1008
+    end
+
+    with_them do
+      let(:start) { now - time_interval_in_seconds }
+      let(:stop) { now }
+
+      it { is_expected.to eq(step) }
+    end
+  end
 end

spec/support/helpers/prometheus_helpers.rb

@@ -25,12 +25,16 @@ module PrometheusHelpers
     "https://prometheus.example.com/api/v1/query?#{query}"
   end
 
-  def prometheus_query_range_url(prometheus_query, start: 8.hours.ago, stop: Time.now.to_f)
+  def prometheus_query_range_url(prometheus_query, start: 8.hours.ago, stop: Time.now, step: nil)
+    start = start.to_f
+    stop = stop.to_f
+    step ||= Gitlab::PrometheusClient.compute_step(start, stop)
+
     query = {
       query: prometheus_query,
-      start: start.to_f,
+      start: start,
       end: stop,
-      step: 1.minute.to_i
+      step: step
     }.to_query
 
     "https://prometheus.example.com/api/v1/query_range?#{query}"