feat: disable-server-timing config with YAML/env/agent priority

lib/instana/config.rb

@@ -83,6 +83,10 @@
       # W3C Trace Context Support
       @config[:w3c_trace_correlation] = ENV.fetch('INSTANA_DISABLE_W3C_TRACE_CORRELATION', nil).nil?
 
+      # Server-Timing response header injection (default: injected)
+      # Priority: YAML > env var > agent config > default (false)
+      read_disable_server_timing_config
+
       # HTTP exit span 4xx error classification (opt-in, default: off)
       # Priority: env vars > agent config > default (false / [])
       read_http_exit_config
@@ -151,6 +155,8 @@
       read_otlp_config_from_agent(tracing_config) if should_read_from_agent?(:otlp)
       # Read HTTP exit 4xx classification from agent if env vars did not already set it
       read_http_exit_classification_from_agent(tracing_config) unless env_http_exit_classification_set?
+      # Read Server-Timing disable flag from agent if not already set from YAML or env
+      read_disable_server_timing_from_agent(tracing_config) if @config[:disable_server_timing_source] == 'default'
       # Read span filtering configuration from agent
       ::Instana.span_filtering_config&.read_config_from_agent(discovery)
     rescue => e
@@ -268,6 +274,49 @@
 
     private
 
+    # Read disable-server-timing from YAML (tracing.global) or INSTANA_TRACING_DISABLE_SERVER_TIMING.
+    # Agent value is applied later via read_config_from_agent if neither is set.
+    def read_disable_server_timing_config
+      @config[:disable_server_timing] = false
+      @config[:disable_server_timing_source] = 'default'
+
+      yaml_value = parse_disable_server_timing_from_yaml
+      unless yaml_value.nil?
+        @config[:disable_server_timing] = truthy?(yaml_value)
+        @config[:disable_server_timing_source] = 'yaml'
+        return
+      end
+
+      env_value = ENV.fetch('INSTANA_TRACING_DISABLE_SERVER_TIMING', nil)
+      return if env_value.nil? || env_value.strip.empty?
+
+      @config[:disable_server_timing] = truthy?(env_value.strip)
+      @config[:disable_server_timing_source] = 'env'
+    end
+
+    # @return [Object, nil] raw YAML value of tracing.global.disable-server-timing, or nil if not set
+    def parse_disable_server_timing_from_yaml
+      config_path = ENV.fetch('INSTANA_CONFIG_PATH', nil)
+      return nil unless config_path && File.exist?(config_path)
+
+      yaml_content = YAML.safe_load(File.read(config_path))
+      tracing_config = yaml_content[TRACING_KEY] || yaml_content[LEGACY_TRACING_KEY]
+      global_config = tracing_config && tracing_config['global']
+      global_config.is_a?(Hash) ? global_config['disable-server-timing'] : nil
+    rescue => e
+      ::Instana.logger.warn("Failed to load disable-server-timing configuration from YAML: #{e.message}")
+      nil
+    end
+
+    # @param tracing_config [Hash] the tracing sub-hash from the discovery payload
+    def read_disable_server_timing_from_agent(tracing_config)
+      global_config = tracing_config['global']
+      return unless global_config.is_a?(Hash) && !global_config['disable-server-timing'].nil?
+
+      @config[:disable_server_timing] = truthy?(global_config['disable-server-timing'])
+      @config[:disable_server_timing_source] = 'agent'
+    end
+
     # Read HTTP exit 4xx error classification config (wrapper — same pattern as read_span_stack_config / read_otlp_config)
     def read_http_exit_config
       @config[:http_exit_classify_all_4xx_as_errors] = false

lib/instana/instrumentation/rack.rb

@@ -139,6 +139,8 @@ def set_response_headers(headers, trace_context)
       end
 
       headers['Traceparent'] = trace_context.trace_parent_header
+      return if ::Instana.config[:disable_server_timing]
+
       headers['Server-Timing'] = "intid;desc=#{trace_context.trace_id_header}"
     end
   end

test/config_test.rb

@@ -1283,3 +1283,99 @@ def test_agent_config_ignores_invalid_codes
     assert_equal [401], subject[:http_exit_classify_as_errors]
   end
 end
+
+class DisableServerTimingConfigTest < Minitest::Test
+  YAML_FILE = 'test_disable_server_timing.yaml'.freeze
+
+  def logger
+    Logger.new('/dev/null')
+  end
+
+  def setup
+    ENV.delete('INSTANA_TRACING_DISABLE_SERVER_TIMING')
+    ENV.delete('INSTANA_CONFIG_PATH')
+  end
+
+  def teardown
+    ENV.delete('INSTANA_TRACING_DISABLE_SERVER_TIMING')
+    ENV.delete('INSTANA_CONFIG_PATH')
+    File.unlink(YAML_FILE) if File.exist?(YAML_FILE)
+  end
+
+  def write_yaml(top_key, value)
+    File.write(YAML_FILE, "#{top_key}:\n  global:\n    disable-server-timing: #{value}\n")
+    ENV['INSTANA_CONFIG_PATH'] = YAML_FILE
+  end
+
+  def agent_discovery(value)
+    { 'tracing' => { 'global' => { 'disable-server-timing' => value } } }
+  end
+
+  def test_default_is_false
+    subject = Instana::Config.new(logger: logger)
+    assert_equal false, subject[:disable_server_timing]
+  end
+
+  def test_env_truthy_values
+    %w[true 1 TRUE yes].each do |value|
+      ENV['INSTANA_TRACING_DISABLE_SERVER_TIMING'] = value
+      assert_equal true, Instana::Config.new(logger: logger)[:disable_server_timing], "expected #{value} to be truthy"
+    end
+  end
+
+  def test_env_falsy_or_empty_values
+    ['false', '0', ''].each do |value|
+      ENV['INSTANA_TRACING_DISABLE_SERVER_TIMING'] = value
+      assert_equal false, Instana::Config.new(logger: logger)[:disable_server_timing], "expected #{value.inspect} to be falsy"
+    end
+  end
+
+  def test_yaml_tracing_key
+    write_yaml('tracing', 'true')
+    assert_equal true, Instana::Config.new(logger: logger)[:disable_server_timing]
+  end
+
+  def test_yaml_legacy_key
+    write_yaml('com.instana.tracing', 'true')
+    assert_equal true, Instana::Config.new(logger: logger)[:disable_server_timing]
+  end
+
+  def test_yaml_takes_precedence_over_env
+    write_yaml('tracing', 'false')
+    ENV['INSTANA_TRACING_DISABLE_SERVER_TIMING'] = 'true'
+    assert_equal false, Instana::Config.new(logger: logger)[:disable_server_timing]
+  end
+
+  def test_invalid_yaml_falls_back_to_env
+    File.write(YAML_FILE, "tracing: [unclosed")
+    ENV['INSTANA_CONFIG_PATH'] = YAML_FILE
+    ENV['INSTANA_TRACING_DISABLE_SERVER_TIMING'] = 'true'
+    assert_equal true, Instana::Config.new(logger: logger)[:disable_server_timing]
+  end
+
+  def test_agent_sets_value_when_nothing_else_configured
+    subject = Instana::Config.new(logger: logger)
+    subject.read_config_from_agent(agent_discovery(true))
+    assert_equal true, subject[:disable_server_timing]
+  end
+
+  def test_env_takes_precedence_over_agent
+    ENV['INSTANA_TRACING_DISABLE_SERVER_TIMING'] = 'true'
+    subject = Instana::Config.new(logger: logger)
+    subject.read_config_from_agent(agent_discovery(false))
+    assert_equal true, subject[:disable_server_timing]
+  end
+
+  def test_yaml_takes_precedence_over_agent
+    write_yaml('tracing', 'true')
+    subject = Instana::Config.new(logger: logger)
+    subject.read_config_from_agent(agent_discovery(false))
+    assert_equal true, subject[:disable_server_timing]
+  end
+
+  def test_agent_without_key_keeps_default
+    subject = Instana::Config.new(logger: logger)
+    subject.read_config_from_agent('tracing' => { 'global' => { 'stack-trace' => 'all' } })
+    assert_equal false, subject[:disable_server_timing]
+  end
+end

test/instrumentation/rack_test.rb

@@ -33,6 +33,12 @@ def call(_env)
     end
   end
 
+  class ServerTimingApp
+    def call(_env)
+      [200, { 'Server-Timing' => 'db;dur=53' }, ['Ok']]
+    end
+  end
+
   class NoHeadersApp
     def call(_env)
       [501, nil, ['No']]
@@ -53,9 +59,36 @@ def app
       map("/path_tpl") { run PathTemplateApp.new }
       map("/error") { run ErrorApp.new }
       map("/five_zero_one") { run FiveZeroOneApp.new }
+      map("/server_timing") { run ServerTimingApp.new }
     end
   end
 
+  def test_server_timing_not_added_when_disabled
+    clear_all!
+    ::Instana.config[:disable_server_timing] = true
+
+    get '/mrlobster'
+    assert last_response.ok?
+
+    assert !last_response.headers.key?("Server-Timing")
+    assert last_response.headers.key?("X-Instana-T")
+    assert last_response.headers.key?("Traceparent")
+  ensure
+    ::Instana.config[:disable_server_timing] = false
+  end
+
+  def test_existing_server_timing_preserved_when_disabled
+    clear_all!
+    ::Instana.config[:disable_server_timing] = true
+
+    get '/server_timing'
+    assert last_response.ok?
+
+    assert_equal 'db;dur=53', last_response.headers["Server-Timing"]
+  ensure
+    ::Instana.config[:disable_server_timing] = false
+  end
+
   def test_basic_get
     clear_all!
     Instana.config[:back_trace][:stack_trace_level] = "all"