#46 Add Prometheus exporter

lib/prometheus/controller.rb

@@ -0,0 +1,100 @@
+# frozen_string_literal: true
+
+require 'prometheus_exporter/client'
+
+module Prometheus
+  module Controller
+    # TODO: currently active users - as Event
+    # TODO: active_in_last_hour
+    # TODO: autosaves_per_minute
+
+    class << self
+      def initialize_metrics
+        register_metrics
+        initialize_instance_count
+        initialize_rfc_metrics
+      end
+
+      def register_metrics
+        prometheus = PrometheusExporter::Client.default
+
+        @instance_count = prometheus.register(:gauge, :instance_count, help: 'Instance count')
+        # counts solved, soft_solved, ongoing
+        @rfc_count = prometheus.register(:gauge, :rfc_count, help: 'Count of RfCs in each state')
+        # counts commented
+        @rfc_commented_count = prometheus.register(:gauge, :rfc_commented_count, help: 'Count of commented RfCs')
+      end
+
+      def initialize_instance_count
+        ApplicationRecord.descendants.reject(&:abstract_class).each do |each|
+          @instance_count.observe(each.count, class: each.name)
+        end
+      end
+
+      def initialize_rfc_metrics
+        # Initialize rfc metric
+        @rfc_count.observe(RequestForComment.unsolved.where(full_score_reached: false).count,
+                           state: RequestForComment::ONGOING)
+        @rfc_count.observe(RequestForComment.unsolved.where(full_score_reached: true).count,
+                           state: RequestForComment::SOFT_SOLVED)
+        @rfc_count.observe(RequestForComment.where(solved: true).count,
+                           state: RequestForComment::SOLVED)
+
+        # count of rfcs with comments
+        @rfc_commented_count.observe(RequestForComment.with_comments.count)
+      end
+
+      def update_notification(object)
+        Rails.logger.debug("Prometheus metric updated for #{object.class.name}")
+
+        case object
+        when RequestForComment
+          update_rfc(object)
+        end
+      end
+
+      def create_notification(object)
+        @instance_count.increment(class: object.class.name)
+        Rails.logger.debug("Prometheus instance count increased for #{object.class.name}")
+
+        case object
+        when RequestForComment
+          create_rfc(object)
+        when Comment
+          create_comment(object)
+        end
+      end
+
+      def destroy_notification(object)
+        @instance_count.decrement(class: object.class.name)
+        Rails.logger.debug("Prometheus instance count decreased for #{object.class.name}")
+
+        case object
+        when Comment
+          destroy_comment(object)
+        end
+      end
+
+      def create_rfc(rfc)
+        @rfc_count.increment(state: rfc.current_state)
+      end
+
+      def update_rfc(rfc)
+        @rfc_count.decrement(state: rfc.old_state)
+        # If the metrics are scraped when the execution is exactly at the place of this comment,
+        # the old state is already decremented while the new state is not yet incremented and
+        # the metric is therefore inconsistent. As this is only a temporarily off by one error
+        # in the metric and a solution (e.g. a mutex) would be complex, this is acceptable.
+        @rfc_count.increment(state: rfc.current_state)
+      end
+
+      def create_comment(comment)
+        @rfc_commented_count.increment if comment.only_comment_for_rfc?
+      end
+
+      def destroy_comment(comment)
+        @rfc_commented_count.decrement unless comment.request_for_comment.comments?
+      end
+    end
+  end
+end