Record message size transmitted over action cable

Record action cable transmitted message size in bytes

Changelog: other

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -121,6 +121,7 @@ The following metrics are available:
 | `action_cable_single_client_transmissions_total`                 | Counter     | 13.10   | The number of ActionCable messages transmitted to any client in any channel                                           | `server_mode`                                             |
 | `action_cable_subscription_confirmations_total`                  | Counter     | 13.10   | The number of ActionCable subscriptions from clients confirmed                                                        | `server_mode`                                             |
 | `action_cable_subscription_rejections_total`                     | Counter     | 13.10   | The number of ActionCable subscriptions from clients rejected                                                         | `server_mode`                                             |
+| `action_cable_transmitted_bytes`                | Histogram     | 14.1   | Message size, in bytes, transmitted over action cable                                           | `operation`, `channel` |
 | `gitlab_issuable_fast_count_by_state_total`                      | Counter     | 13.5    | Total number of row count operations on issue/merge request list pages                                                |                                                           |
 | `gitlab_issuable_fast_count_by_state_failures_total`             | Counter     | 13.5    | Number of soft-failed row count operations on issue/merge request list pages                                          |                                                           |
 | `gitlab_external_http_total`                                     | Counter     | 13.8    | Total number of HTTP calls to external systems                                                                        | `controller`, `action`                                    |

lib/gitlab/metrics/subscribers/action_cable.rb

@@ -12,6 +12,7 @@ module Gitlab
         TRANSMIT_SUBSCRIPTION_CONFIRMATION = :action_cable_subscription_confirmations_total
         TRANSMIT_SUBSCRIPTION_REJECTION = :action_cable_subscription_rejections_total
         BROADCAST = :action_cable_broadcasts_total
+        DATA_TRANSMITTED_BYTES = :action_cable_transmitted_bytes
 
         def transmit_subscription_confirmation(event)
           confirm_subscription_counter.increment
@@ -23,6 +24,14 @@ module Gitlab
 
         def transmit(event)
           transmit_counter.increment
+
+          if event.payload.present?
+            channel = event.payload[:channel_class]
+            operation = operation_name_from(event.payload)
+            data_size = ::ActiveSupport::JSON.encode(event.payload[:data]).bytesize
+
+            transmitted_bytes_histogram.observe({ channel: channel, operation: operation }, data_size)
+          end
         end
 
         def broadcast(event)
@@ -31,6 +40,13 @@ module Gitlab
 
         private
 
+        # When possible tries to query operation name
+        def operation_name_from(payload)
+          data = payload.dig(:data, 'result', 'data') || {}
+
+          data.each_key.first
+        end
+
         def transmit_counter
           strong_memoize("transmission_counter") do
             ::Gitlab::Metrics.counter(
@@ -66,6 +82,12 @@ module Gitlab
             )
           end
         end
+
+        def transmitted_bytes_histogram
+          strong_memoize("transmitted_bytes_histogram") do
+            ::Gitlab::Metrics.histogram(DATA_TRANSMITTED_BYTES, 'Message size, in bytes, transmitted over action cable')
+          end
+        end
       end
     end
   end

spec/lib/gitlab/metrics/subscribers/action_cable_spec.rb

@@ -5,7 +5,7 @@ require 'spec_helper'
 RSpec.describe Gitlab::Metrics::Subscribers::ActionCable, :request_store do
   let(:subscriber) { described_class.new }
   let(:counter) { double(:counter) }
-  let(:data) { { data: { event: 'updated' } } }
+  let(:data) { { 'result' => { 'data' => { 'event' => 'updated' } } } }
   let(:channel_class) { 'IssuesChannel' }
   let(:event) do
     double(
@@ -35,6 +35,17 @@ RSpec.describe Gitlab::Metrics::Subscribers::ActionCable, :request_store do
 
       subscriber.transmit(event)
     end
+
+    it 'tracks size of payload as JSON' do
+      allow(::Gitlab::Metrics).to receive(:histogram).with(
+        :action_cable_transmitted_bytes, /transmit/
+      ).and_return(counter)
+      message_size = ::ActiveSupport::JSON.encode(data).bytesize
+
+      expect(counter).to receive(:observe).with({ channel: channel_class, operation: 'event' }, message_size)
+
+      subscriber.transmit(event)
+    end
   end
 
   describe '#broadcast' do