Add ability to configure sampling rate based on transaction span name

docs/reference/configuration.md

@@ -677,6 +677,18 @@ Limits the amount of spans that are recorded per transaction. This is helpful in
 By default, the agent will sample every transaction (e.g. request to your service). To reduce overhead and storage requirements, you can set the sample rate to a value between `0.0` and `1.0`. We still record overall time and the result for unsampled transactions, but no context information, tags, or spans. The sample rate will be rounded to 4 digits of precision.
 
 
+### `transaction_sample_rate_by_name` [config-transaction-sample-rate-by-name]
+
+|     |     |     |     |
+| --- | --- | --- | --- |
+| Environment | `Config` key | Default | Example |
+| `ELASTIC_APM_TRANSACTION_SAMPLE_RATE_BY_NAME` | `transaction_sample_rate_by_name` | `{}` | `{"UsersController#index" => 1.0, "HealthController#ping" => 0.0}` |
+
+Configure specific sampling rates for transactions with particular names. This option takes a hash where the keys are transaction names and the values are sampling rates between `0.0` and `1.0`.
+
+This is useful when you want to ensure critical transactions are always sampled while reducing sampling for high-volume, less important transactions. When a transaction name matches an entry in this hash, the specified sampling rate takes precedence over the global `transaction_sample_rate`.
+
+
 ### `verify_server_cert` [config-verify-server-cert]
 
 |     |     |     |
@@ -731,4 +743,3 @@ Elastic APM patches `Kernel#require` to auto-detect and instrument supported thi
 To get around this patch, set the environment variable `ELASTIC_APM_SKIP_REQUIRE_PATCH` to `"1"`.
 
 The agent might need some additional tweaking to make sure the third-party libraries are picked up and instrumented. Make sure you require the agent *after* you require your other dependencies.
-

lib/elastic_apm/config.rb

@@ -22,6 +22,7 @@
 require 'elastic_apm/config/log_level_map'
 require 'elastic_apm/config/options'
 require 'elastic_apm/config/round_float'
+require 'elastic_apm/config/round_float_hash_value'
 require 'elastic_apm/config/regexp_list'
 require 'elastic_apm/config/wildcard_pattern_list'
 require 'elastic_apm/deprecations'
@@ -98,6 +99,7 @@ class Config
     option :transaction_ignore_urls,           type: :list,   default: [],      converter: WildcardPatternList.new
     option :transaction_max_spans,             type: :int,    default: 500
     option :transaction_sample_rate,           type: :float,  default: 1.0,     converter: RoundFloat.new
+    option :transaction_sample_rate_by_name,   type: :hash,   default: {},      converter: RoundFloatHashValue.new
     option :use_elastic_traceparent_header,    type: :bool,   default: true
     option :verify_server_cert,                type: :bool,   default: true
 

lib/elastic_apm/config/round_float_hash_value.rb

@@ -0,0 +1,35 @@
+# Licensed to Elasticsearch B.V. under one or more contributor
+# license agreements. See the NOTICE file distributed with
+# this work for additional information regarding copyright
+# ownership. Elasticsearch B.V. licenses this file to you under
+# the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# frozen_string_literal: true
+
+module ElasticAPM
+  class Config
+    # @api private
+    class RoundFloatHashValue
+      def initialize
+        @float_converter = RoundFloat.new
+      end
+
+      def call(hash)
+        return {} unless hash
+
+        hash.transform_values { |value| @float_converter.call(value) }
+      end
+    end
+  end
+end

lib/elastic_apm/instrumenter.rb

@@ -123,7 +123,7 @@ def start_transaction(
         sampled = trace_context.recorded?
         sample_rate = trace_context.tracestate.sample_rate
       else
-        sampled = random_sample?(config)
+        sampled = random_sample?(config.transaction_sample_rate)
         sample_rate = sampled ? config.transaction_sample_rate : 0
       end
 
@@ -195,6 +195,19 @@ def start_span(
           current_transaction
         end
       return unless transaction
+
+      unless trace_context
+        span_sample_rate = transaction_sample_rate_for_name(name, transaction.config)
+        # if the span sample rate is different from the transaction sample rate,
+        # we need to check if the span should be sampled
+        # and update the transaction's sample rate accordingly
+        if transaction.started_spans == 0 && span_sample_rate && span_sample_rate != transaction.sample_rate
+          span_sampled = random_sample?(span_sample_rate)
+          transaction.sampled = span_sampled
+          transaction.sample_rate = span_sampled ? span_sample_rate : 0
+        end
+      end
+
       return unless transaction.sampled?
       return unless transaction.inc_started_spans!
 
@@ -278,10 +291,17 @@ def inspect
         '>'
     end
 
+    def transaction_sample_rate_for_name(name, config)
+      return if !name || config.transaction_sample_rate_by_name.empty?
+      return unless config.transaction_sample_rate_by_name.key?(name)
+
+      config.transaction_sample_rate_by_name[name]
+    end
+
     private
 
-    def random_sample?(config)
-      rand <= config.transaction_sample_rate
+    def random_sample?(sample_rate)
+      rand <= sample_rate
     end
 
     def update_transaction_metrics(transaction)

lib/elastic_apm/transaction.rb

@@ -87,7 +87,15 @@ def initialize(
     end
     # rubocop:enable Metrics/ParameterLists
 
-    attr_accessor :name, :type, :result, :outcome
+    attr_accessor(
+      :name,
+      :type,
+      :result,
+      :outcome,
+      :sampled,
+      :sample_rate,
+      :started_spans
+    )
 
     attr_reader(
       :breakdown_metrics,
@@ -98,12 +106,11 @@ def initialize(
       :framework_name,
       :notifications,
       :self_time,
-      :sample_rate,
       :span_frames_min_duration,
-      :started_spans,
       :timestamp,
       :trace_context,
-      :transaction_max_spans
+      :transaction_max_spans,
+      :config
     )
 
     alias :collect_metrics? :collect_metrics

spec/elastic_apm/instrumenter_spec.rb

@@ -469,5 +469,104 @@ module ElasticAPM
         end
       end
     end
+
+    describe '#transaction_sample_rate_for_name' do
+      let(:default_rate) { 0.5 }
+      let(:special_rate) { 1.0 }
+
+      context 'with nil name' do
+        it 'returns nil' do
+          config = Config.new(transaction_sample_rate: default_rate)
+          expect(subject.transaction_sample_rate_for_name(nil, config)).to be_nil
+        end
+      end
+
+      context 'with empty transaction_sample_rate_by_name' do
+        it 'returns nil' do
+          config = Config.new(transaction_sample_rate_by_name: {})
+          expect(subject.transaction_sample_rate_for_name('Something', config)).to be_nil
+        end
+      end
+
+      context 'with matching name in transaction_sample_rate_by_name' do
+        it 'returns matching sample rate' do
+          config = Config.new(
+            transaction_sample_rate: default_rate,
+            transaction_sample_rate_by_name: { 'Something' => special_rate }
+          )
+          expect(subject.transaction_sample_rate_for_name('Something', config)).to eq(special_rate)
+        end
+      end
+
+      context 'with non-matching name in transaction_sample_rate_by_name' do
+        it 'returns default sample rate' do
+          config = Config.new(
+            transaction_sample_rate_by_name: { 'SomethingElse' => special_rate }
+          )
+          expect(subject.transaction_sample_rate_for_name('Something', config)).to be_nil
+        end
+      end
+    end
+
+    describe 'span-based sampling' do
+      context 'when starting a span with a name that has a different sampling rate' do
+        it 'can change the sampling decision of the transaction' do
+          config = Config.new(
+            transaction_sample_rate: 0.0,
+            transaction_sample_rate_by_name: { 'ImportantOperation' => 1.0 }
+          )
+
+          # Force predictable random sampling
+          allow(subject).to receive(:rand).and_return(0.5)
+
+          transaction = subject.start_transaction('Test', config: config)
+          expect(transaction).not_to be_sampled
+
+          # Verify sampling is updated when first span is created
+          allow(subject).to receive(:random_sample?).and_return(true)
+          span = subject.start_span('ImportantOperation')
+
+          # The span should exist (since the transaction is now sampled)
+          expect(span).not_to be_nil
+          expect(transaction).to be_sampled
+        end
+
+        it 'only considers the first span for changing the sampling decision' do
+          config = Config.new(
+            transaction_sample_rate: 0.0,
+            transaction_sample_rate_by_name: {
+              'FirstSpan' => 1.0,
+              'SecondSpan' => 0.0,
+              'ThirdSpan' => 0.5
+            }
+          )
+
+          # Force predictable random sampling
+          allow(subject).to receive(:rand).and_return(0.5)
+
+          transaction = subject.start_transaction('Test', config: config)
+          expect(transaction).not_to be_sampled
+
+          # First span changes sampling because it's the first span
+          first_span = subject.start_span('FirstSpan')
+          expect(first_span).not_to be_nil
+          expect(transaction).to be_sampled
+          expect(transaction.sample_rate).to eq(1.0)
+
+          # Initial sampling rate stored
+          original_sample_rate = transaction.sample_rate
+
+          # Second span should not change sampling even though it has a different rate
+          second_span = subject.start_span('SecondSpan')
+          expect(second_span).not_to be_nil
+          expect(transaction.sample_rate).to eq(original_sample_rate)
+
+          # Third span should not change sampling either
+          third_span = subject.start_span('ThirdSpan')
+          expect(third_span).not_to be_nil
+          expect(transaction.sample_rate).to eq(original_sample_rate)
+        end
+      end
+    end
   end
 end