Automatically set Prometheus step interval

By computing the step interval passed to the query_range Prometheus API
call we improve the performance on the Prometheus server and GitLab by
reducing the amount of data points sent back and prevent Prometheus
from sending errors when requesting longer intervals.

1 files changed, 28 insertions, 0 deletions

diff --git a/spec/lib/gitlab/prometheus_client_spec.rb b/spec/lib/gitlab/prometheus_client_spec.rb
index 4c3b8deefb9..2517ee71f24 100644
--- a/spec/lib/gitlab/prometheus_client_spec.rb
+++ b/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