Blog Code: Monitoring Rails Requests

app_middleware_statsd_monitor.rb

# app/middleware/statsd_monitor.rb
class Middleware::StatsDMonitor
  def initialize(app)
    @app = app
  end

  def call(env)
    @app.call(env)
  end
end

application.rb

# config/application.rb
module DemoApp
  class Application < Rails::Application
    ...
    ...
    config.middleware.insert(0, "Middleware::StatsDMonitor")
  end
end

final.rb

# app/middleware/statsd_monitor.rb
class Middleware::StatsDMonitor
  def initialize(app)
    @app = app
  end

  def call(env)
    @app.call(env)
  end
end

# config/initializers/inflections.rb
ActiveSupport::Inflector.inflections(:en) do |inflect|
  # allows Middleware::StatsDMonitor to be loaded from middleware/statsd_monitor.rb
  inflect.acronym "StatsD"
end

# config/initializers/statsd.rb
STATSD_REQUEST_METRICS = {
  "request.success"               => 200,
  "request.redirect"              => 302,
  "request.bad_request"           => 400,
  "request.not_found"             => 404,
  "request.too_many_requests"     => 429,
  "request.internal_server_error" => 500,
  "request.bad_gateway"           => 502
}.freeze

STATSD_TAGS = ["env:#{Rails.env}"].freeze

StatsD.prefix              = "DemoApp"
StatsD.default_sample_rate = 1

Middleware::StatsDMonitor.extend(StatsD::Instrument)
Middleware::StatsDMonitor.statsd_measure(:call, "request.duration", tags: STATSD_TAGS)

STATSD_REQUEST_METRICS.each do |name, code|
  Middleware::StatsDMonitor.statsd_count_if(:call, name, tags: STATSD_TAGS) do |status, _env, _body|
    status.to_i == code
  end
end

# test/integration/statsd_request_monitoring_test.rb
require "test_helper"

class StatsDRequestMonitoringTest < ActionDispatch::IntegrationTest
  include StatsD::Instrument::Assertions

  test "request duration is measured" do
    assert_statsd_measure("#{StatsD.prefix}.request.duration", tags: STATSD_TAGS) do
      get "/"
    end
  end

  test "successful requests are counted" do
    assert_increment("request.success", 200)
  end

  test "redirects are counted" do
    assert_increment("request.redirect", 302)
  end

  test "bad requests are counted" do
    assert_increment("request.bad_request", 400)
  end

  test "not found requests are counted" do
    assert_increment("request.not_found", 404)
  end

  test "dropped requests are counted" do
    assert_increment("request.too_many_requests", 429)
  end

  test "internal server errors are counted" do
    assert_increment("request.internal_server_error", 500)
  end

  test "bad gateway requests are counted" do
    assert_increment("request.bad_gateway", 502)
  end

  private

  def assert_increment(name, status)
    assert_statsd_increment("#{StatsD.prefix}.#{name}", tags: STATSD_TAGS) do
      Rack::Runtime.any_instance.stubs(call: [status, {}, ""])
      get "/"
    end
  end
end

Gemfile

source "https://rubygems.org/"
...
...
gem "statsd-instrument"

inflections.rb

# config/initializers/inflections.rb
ActiveSupport::Inflector.inflections(:en) do |inflect|
  # allows Middleware::StatsDMonitor to be loaded from middleware/statsd_monitor.rb
  inflect.acronym "StatsD"
end

statsd_initializer_01.rb

# config/initializers/statsd.rb
StatsD.prefix              = "DemoApp"
StatsD.default_sample_rate = 1

statsd_initializer_02.rb

# config/initializers/statsd.rb
...
...
# send the environment as a tag
STATSD_TAGS = ["env:#{Rails.env}"].freeze

Middleware::StatsDMonitor.extend(StatsD::Instrument)
Middleware::StatsDMonitor.statsd_measure(:call, "request.duration", tags: STATSD_TAGS)

statsd_initializer_03.rb

# config/initializers/statsd.rb
STATSD_REQUEST_METRICS = {
  "request.success"               => 200,
  "request.redirect"              => 302,
  "request.bad_request"           => 400,
  "request.not_found"             => 404,
  "request.too_many_requests"     => 429,
  "request.internal_server_error" => 500,
  "request.bad_gateway"           => 502
}.freeze

STATSD_REQUEST_METRICS.each do |name, code|
  Middleware::StatsDMonitor.statsd_count_if(:call, name, tags: STATSD_TAGS) do |status, _env, _body|
    status.to_i == code
  end
end

test_counters.rb

# test/integration/statsd_request_monitoring_test.rb
class StatsDRequestMonitoringTest < ActionDispatch::IntegrationTest
...
...
  test "successful requests are counted" do
    assert_increment("request.success", 200)
  end

  test "redirects are counted" do
    assert_increment("request.redirect", 302)
  end

  test "bad requests are counted" do
    assert_increment("request.bad_request", 400)
  end

  test "not found requests are counted" do
    assert_increment("request.not_found", 404)
  end

  test "dropped requests are counted" do
    assert_increment("request.too_many_requests", 429)
  end

  test "internal server errors are counted" do
    assert_increment("request.internal_server_error", 500)
  end

  test "bad gateway requests are counted" do
    assert_increment("request.bad_gateway", 502)
  end

  private

  def assert_increment(name, status)
    assert_statsd_increment("#{StatsD.prefix}.#{name}", tags: STATSD_TAGS) do
      # skip the processing and just return the supplied status
      Rack::Runtime.any_instance.stubs(call: [status, {}, ""])
      get "/"
    end
  end
end

test_duration.rb

# test/integration/statsd_request_monitoring_test.rb
require "test_helper"

class StatsDRequestMonitoringTest < ActionDispatch::IntegrationTest
  include StatsD::Instrument::Assertions

  test "request duration is measured" do
    assert_statsd_measure("#{StatsD.prefix}.request.duration", tags: STATSD_TAGS) do
      get "/"
    end
  end
end