Make metrics tracking async via a job

app/controllers/file_controller.rb

@@ -3,8 +3,6 @@
 ##
 # API for delivering files from stacks
 class FileController < ApplicationController
-  include MetricsConcern
-
   rescue_from ActionController::MissingFile do
     render plain: 'File not found', status: :not_found
   end
@@ -19,7 +17,13 @@ def show
     response.headers['Content-Length'] = current_file.content_length
     response.headers.delete('X-Frame-Options')
 
-    track_download current_file.id, file: current_file.file_name
+    TrackDownloadJob.perform_later(
+      druid: current_file.id,
+      file: current_file.file_name,
+      user_agent: request.user_agent,
+      ip: request.remote_ip
+    )
+
     send_file current_file.path, disposition:
   end
   # rubocop:enable Metrics/AbcSize

app/controllers/iiif_controller.rb

@@ -3,8 +3,6 @@
 ##
 # API for delivering IIIF-compatible images and image tiles
 class IiifController < ApplicationController
-  include MetricsConcern
-
   skip_forgery_protection
 
   before_action :add_iiif_profile_header
@@ -26,7 +24,12 @@ def show
 
     set_image_response_headers
 
-    track_download(identifier_params[:id], file: download_filename)
+    TrackDownloadJob.perform_later(
+      druid: identifier_params[:id],
+      file: download_filename,
+      user_agent: request.user_agent,
+      ip: request.remote_ip
+    )
 
     self.content_type = Mime::Type.lookup_by_extension(iiif_params[:format]).to_s
     self.status = projection.response.status

app/controllers/object_controller.rb

@@ -4,7 +4,6 @@
 # API for delivering whole objects from stacks
 class ObjectController < ApplicationController
   include Zipline
-  include MetricsConcern
 
   # Return a zip of all the files if they have access to all the files.
   # This will force a login if any of the files is not access=world
@@ -23,7 +22,12 @@ def show
       ]
     end
 
-    track_download druid
+    TrackDownloadJob.perform_later(
+      druid:,
+      user_agent: request.user_agent,
+      ip: request.remote_ip
+    )
+
     zipline(zip_contents, "#{druid}.zip")
   end
 

app/jobs/application_job.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class ApplicationJob < ActiveJob::Base
+  # Automatically retry jobs that encountered a deadlock
+  # retry_on ActiveRecord::Deadlocked
+
+  # Most jobs are safe to ignore if the underlying records are no longer available
+  # discard_on ActiveJob::DeserializationError
+end

app/jobs/track_download_job.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+# Track a single download event by sending a request to the metrics API
+class TrackDownloadJob < ApplicationJob
+  queue_as :default
+
+  rescue_from StandardError do |exception|
+    Rails.logger.error("Error sending metrics: #{exception}")
+  end
+
+  def perform(druid:, user_agent:, ip:, file: nil)
+    return true unless Settings.features.metrics == true
+
+    properties = { druid:, file: }.compact
+
+    MetricsService.new.track_event('download', properties, user_agent:, ip:)
+  end
+end

app/services/metrics_service.rb

@@ -12,23 +12,41 @@ def initialize(base_url: Settings.metrics_api_url)
     @base_url = base_url
   end
 
-  def track_event(data, original_request)
-    post_json('/ahoy/events', data, original_request)
+  def track_event(name, properties, user_agent:, ip:)
+    headers = {
+      'User-Agent': user_agent,
+      'X-Forwarded-For': ip
+    }
+
+    post_json('/ahoy/events', event_data(name, properties), headers)
   end
 
   private
 
-  def post_json(url, data, original_request)
+  # Schema: https://github.com/ankane/ahoy#events-1
+  # NOTE: it's possible to batch events this way.
+  def event_data(name, properties)
+    {
+      events: [
+        {
+          id: SecureRandom.uuid,
+          time: Time.current,
+          name:,
+          properties:
+        }
+      ]
+    }
+  end
+
+  def default_headers
+    { 'Content-Type': 'application/json' }
+  end
+
+  def post_json(url, data, headers)
     connection.post(url) do |req|
-      # Pass the original browser info and IP along to the metrics API
-      req.headers['User-Agent'] = original_request.user_agent
-      req.headers['X-Forwarded-For'] = original_request.remote_ip
-      req.headers['Content-Type'] = 'application/json'
+      req.headers = default_headers.merge(headers)
       req.body = data.to_json
     end
-  rescue Faraday::ConnectionFailed => e
-    Rails.logger.error("Error sending metrics: #{e}")
-    nil
   end
 
   def connection

config/application.rb

@@ -43,9 +43,13 @@ class Application < Rails::Application
                              "172.20.21.192/28" # foa_lb_mgmt_prod_nets
                            ].map { |proxy| IPAddr.new(proxy) }
 
-
     # IIIF Auth v2 makes a request in one window to login and then opens a iframe to get a token.
     # In order for this second request to know who the user is, the session token must created with SameSite=None
     config.action_dispatch.cookies_same_site_protection = :none
+
+    # Use ActiveJob async adapter for all environments – our only jobs are
+    # for tracking metrics and they execute very quickly, so there's no need
+    # for a dedicated redis instance or similar
+    config.active_job.queue_adapter = :async
   end
 end

spec/requests/metrics_spec.rb

@@ -2,7 +2,9 @@
 
 require 'rails_helper'
 
-RSpec.describe 'Metrics tracking', type: :request do
+RSpec.describe 'Metrics tracking' do
+  include ActiveJob::TestHelper
+
   let(:xml) do
     <<-XML
     <publicObject id="druid:xf680rd3068" published="2019-12-19T17:58:11Z" publishVersion="dor-services/8.1.1">
@@ -83,6 +85,8 @@
           headers: { 'User-Agent' => 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko)' },
           env: { 'REMOTE_ADDR' => '73.235.188.148' }
 
+      perform_enqueued_jobs
+
       expect(a_request(:post, 'https://example.com/ahoy/events').with do |req|
         expect(req.body).to include '"name":"download"'
         expect(req.body).to include '"druid":"xf680rd3068"'
@@ -98,6 +102,8 @@
           headers: { 'User-Agent' => 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko)' },
           env: { 'REMOTE_ADDR' => '73.235.188.148' }
 
+      perform_enqueued_jobs
+
       expect(a_request(:post, 'https://example.com/ahoy/events').with do |req|
         expect(req.body).to include '"name":"download"'
         expect(req.body).to include '"druid":"xf680rd3068"'
@@ -151,6 +157,8 @@
       ), headers: { 'User-Agent' => 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko)' },
          env: { 'REMOTE_ADDR' => '73.235.188.148' }
 
+      perform_enqueued_jobs
+
       expect(a_request(:post, 'https://example.com/ahoy/events').with do |req|
         expect(req.body).to include '"name":"download"'
         expect(req.body).to include '"druid":"nr349ct7889"'