Add built in variable called __range

- This variable can be used in a Prometheus query, when you want a value
equalling the range of the dashboard/chart.

- If the range of the dashboard is 8 hours, '28800s' will be
substituted in place of __range.

app/services/prometheus/proxy_variable_substitution_service.rb

@@ -58,7 +58,7 @@ module Prometheus
     def substitute_variables(result)
       return success(result) unless query(result)
 
-      result[:params][:query] = gsub(query(result), full_context)
+      result[:params][:query] = gsub(query(result), full_context(result))
 
       success(result)
     end
@@ -75,12 +75,16 @@ module Prometheus
       end
     end
 
-    def predefined_context
-      Gitlab::Prometheus::QueryVariables.call(@environment).stringify_keys
+    def predefined_context(result)
+      Gitlab::Prometheus::QueryVariables.call(
+        @environment,
+        start_time: start_timestamp(result),
+        end_time: end_timestamp(result)
+      ).stringify_keys
     end
 
-    def full_context
-      @full_context ||= predefined_context.reverse_merge(variables_hash)
+    def full_context(result)
+      @full_context ||= predefined_context(result).reverse_merge(variables_hash)
     end
 
     def variables
@@ -91,6 +95,16 @@ module Prometheus
       variables.to_h
     end
 
+    def start_timestamp(result)
+      Time.rfc3339(result[:params][:start])
+    rescue ArgumentError
+    end
+
+    def end_timestamp(result)
+      Time.rfc3339(result[:params][:end])
+    rescue ArgumentError
+    end
+
     def query(result)
       result[:params][:query]
     end

changelogs/unreleased/213923-add-range-var.yml

+---
+title: Provide `__range` variable for Prometheus queries
+merge_request: 33521
+author:
+type: added

lib/gitlab/prometheus/query_variables.rb

@@ -3,8 +3,10 @@
 module Gitlab
   module Prometheus
     module QueryVariables
-      def self.call(environment)
+      # start_time and end_time should be Time objects.
+      def self.call(environment, start_time: nil, end_time: nil)
         {
+          __range: range(start_time, end_time),
           ci_environment_slug: environment.slug,
           kube_namespace: environment.deployment_namespace || '',
           environment_filter: %{container_name!="POD",environment="#{environment.slug}"},
@@ -14,6 +16,16 @@ module Gitlab
           ci_environment_name: environment.name
         }
       end
+
+      private
+
+      def self.range(start_time, end_time)
+        if start_time && end_time
+          range_seconds = (end_time - start_time).to_i
+          "#{range_seconds}s"
+        end
+      end
+      private_class_method :range
     end
   end
 end

spec/lib/gitlab/prometheus/query_variables_spec.rb

@@ -7,8 +7,9 @@ describe Gitlab::Prometheus::QueryVariables do
     let(:project) { environment.project }
     let(:environment) { create(:environment) }
     let(:slug) { environment.slug }
+    let(:params) { {} }
 
-    subject { described_class.call(environment) }
+    subject { described_class.call(environment, params) }
 
     it { is_expected.to include(ci_environment_slug: slug) }
     it { is_expected.to include(ci_project_name: project.name) }
@@ -53,5 +54,42 @@ describe Gitlab::Prometheus::QueryVariables do
         it { is_expected.to include(kube_namespace: kube_namespace) }
       end
     end
+
+    context '__range' do
+      context 'when start_time and end_time are present' do
+        let(:params) do
+          {
+            start_time: Time.rfc3339('2020-05-29T07:23:05.008Z'),
+            end_time: Time.rfc3339('2020-05-29T15:23:05.008Z')
+          }
+        end
+
+        it { is_expected.to include(__range: "#{8.hours.to_i}s") }
+      end
+
+      context 'when start_time and end_time are not present' do
+        it { is_expected.to include(__range: nil) }
+      end
+
+      context 'when end_time is not present' do
+        let(:params) do
+          {
+            start_time: Time.rfc3339('2020-05-29T07:23:05.008Z')
+          }
+        end
+
+        it { is_expected.to include(__range: nil) }
+      end
+
+      context 'when start_time is not present' do
+        let(:params) do
+          {
+            end_time: Time.rfc3339('2020-05-29T07:23:05.008Z')
+          }
+        end
+
+        it { is_expected.to include(__range: nil) }
+      end
+    end
   end
 end

spec/services/prometheus/proxy_variable_substitution_service_spec.rb

@@ -186,5 +186,19 @@ describe Prometheus::ProxyVariableSubstitutionService do
         end
       end
     end
+
+    context '__range' do
+      let(:params_keys) do
+        {
+          query: 'topk(5, sum by (method) (rate(rest_client_requests_total[{{__range}}])))',
+          start_time: '2020-05-29T08:19:07.142Z',
+          end_time: '2020-05-29T16:19:07.142Z'
+        }
+      end
+
+      it_behaves_like 'success' do
+        let(:expected_query) { "topk(5, sum by (method) (rate(rest_client_requests_total[#{8.hours.to_i}s])))" }
+      end
+    end
   end
 end