From 0cef479db43dcadd63abe3f450b0f73fe9205c71 Mon Sep 17 00:00:00 2001
From: David Elner <david@davidelner.com>
Date: Fri, 14 Dec 2018 15:36:11 -0500
Subject: [PATCH 1/3] Changed: Enable priority_sampling by default.

---
 docs/GettingStarted.md     | 18 +++++++++---------
 lib/ddtrace/tracer.rb      | 10 ++++++++--
 test/context_flush_test.rb |  1 +
 test/sampler_test.rb       |  4 ++--
 4 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/docs/GettingStarted.md b/docs/GettingStarted.md
index a076565664b..e10626ec651 100644
--- a/docs/GettingStarted.md
+++ b/docs/GettingStarted.md
@@ -1356,15 +1356,7 @@ The sampler can set the priority to the following values:
  - `Datadog::Ext::Priority::AUTO_REJECT`: the sampler automatically decided to reject the trace.
  - `Datadog::Ext::Priority::AUTO_KEEP`: the sampler automatically decided to keep the trace.
 
-For now, priority sampling is disabled by default. Enabling it ensures that your sampled distributed traces will be complete. To enable the priority sampling:
-
-```ruby
-Datadog.configure do |c|
-  c.tracer priority_sampling: true
-end
-```
-
-Once enabled, the sampler will automatically assign a priority of 0 or 1 to traces, depending on their service and volume.
+Priority sampling is enabled by default. Enabling it ensures that your sampled distributed traces will be complete. Once enabled, the sampler will automatically assign a priority of 0 or 1 to traces, depending on their service and volume.
 
 You can also set this priority manually to either drop a non-interesting trace or to keep an important one. For that, set the `context#sampling_priority` to:
 
@@ -1383,6 +1375,14 @@ span.context.sampling_priority = Datadog::Ext::Priority::USER_REJECT
 span.context.sampling_priority = Datadog::Ext::Priority::USER_KEEP
 ```
 
+You can disable priority sampling via:
+
+```ruby
+Datadog.configure do |c|
+  c.tracer priority_sampling: false
+end
+```
+
 ### Distributed Tracing
 
 Distributed tracing allows traces to be propagated across multiple instrumented applications, so that a request can be presented as a single trace, rather than a separate trace per service.
diff --git a/lib/ddtrace/tracer.rb b/lib/ddtrace/tracer.rb
index 9aa97a6cd04..a7c19882aaa 100644
--- a/lib/ddtrace/tracer.rb
+++ b/lib/ddtrace/tracer.rb
@@ -126,7 +126,7 @@ def configure(options = {})
 
       # Those are rare "power-user" options.
       sampler = options.fetch(:sampler, nil)
-      priority_sampling = options[:priority_sampling]
+      priority_sampling = options.fetch(:priority_sampling, nil)
       max_spans_before_partial_flush = options.fetch(:max_spans_before_partial_flush, nil)
       min_spans_before_partial_flush = options.fetch(:min_spans_before_partial_flush, nil)
       partial_flush_timeout = options.fetch(:partial_flush_timeout, nil)
@@ -134,9 +134,15 @@ def configure(options = {})
       @enabled = enabled unless enabled.nil?
       @sampler = sampler unless sampler.nil?
 
-      if priority_sampling
+      # Re-build the sampler and writer if priority sampling is enabled,
+      # but neither are configured. Verify the sampler isn't already a
+      # priority sampler too, so we don't wrap one with another.
+      if priority_sampling != false && !@sampler.is_a?(PrioritySampler)
         @sampler = PrioritySampler.new(base_sampler: @sampler)
         @writer = Writer.new(priority_sampler: @sampler)
+      elsif priority_sampling == false
+        @sampler = sampler || Datadog::AllSampler.new if @sampler.is_a?(PrioritySampler)
+        @writer = Writer.new
       end
 
       @writer.transport.hostname = hostname unless hostname.nil?
diff --git a/test/context_flush_test.rb b/test/context_flush_test.rb
index c238ba0b411..de54cc4231d 100644
--- a/test/context_flush_test.rb
+++ b/test/context_flush_test.rb
@@ -366,6 +366,7 @@ def test_tracer_hard_limit_overrides_soft_limit
     tracer.configure(min_spans_before_partial_flush: context.max_length,
                      max_spans_before_partial_flush: context.max_length,
                      partial_flush_timeout: 3600)
+    tracer.writer = FauxWriter.new
 
     n = 1_000_000
     assert_operator(n, :>, context.max_length, 'need to send enough spans')
diff --git a/test/sampler_test.rb b/test/sampler_test.rb
index 84a62974db6..624918674b3 100644
--- a/test/sampler_test.rb
+++ b/test/sampler_test.rb
@@ -79,7 +79,7 @@ def test_tracer_with_rate_sampler
     prng = Random.new(123)
 
     tracer = get_test_tracer()
-    tracer.configure(sampler: Datadog::RateSampler.new(0.5))
+    tracer.configure(sampler: Datadog::RateSampler.new(0.5), priority_sampling: false)
 
     nb_spans = 10000
     nb_spans.times do
@@ -87,7 +87,7 @@ def test_tracer_with_rate_sampler
       span.finish()
     end
 
-    spans = tracer.writer.spans()
+    spans = tracer.writer.spans
     expected = nb_spans * 0.5
     assert_in_delta(spans.length, expected, 0.1 * expected)
 

From a8236efe0491564978f24e50d72547f3e9d2f431 Mon Sep 17 00:00:00 2001
From: David Elner <david@davidelner.com>
Date: Mon, 17 Dec 2018 11:56:45 -0500
Subject: [PATCH 2/3] Fixed: Priority sampling with minitests.

---
 docs/GettingStarted.md     |  8 --------
 test/context_flush_test.rb |  3 +--
 test/helper.rb             | 35 +++++++++++++++++++++++++++++++++--
 3 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/docs/GettingStarted.md b/docs/GettingStarted.md
index e10626ec651..2ab8796504a 100644
--- a/docs/GettingStarted.md
+++ b/docs/GettingStarted.md
@@ -1375,14 +1375,6 @@ span.context.sampling_priority = Datadog::Ext::Priority::USER_REJECT
 span.context.sampling_priority = Datadog::Ext::Priority::USER_KEEP
 ```
 
-You can disable priority sampling via:
-
-```ruby
-Datadog.configure do |c|
-  c.tracer priority_sampling: false
-end
-```
-
 ### Distributed Tracing
 
 Distributed tracing allows traces to be propagated across multiple instrumented applications, so that a request can be presented as a single trace, rather than a separate trace per service.
diff --git a/test/context_flush_test.rb b/test/context_flush_test.rb
index de54cc4231d..80446434786 100644
--- a/test/context_flush_test.rb
+++ b/test/context_flush_test.rb
@@ -346,7 +346,7 @@ def test_tracer_configure
     assert_nil(tracer.context_flush)
 
     # If given a partial_flush option, then uses default context flush.
-    flush_tracer = Datadog::Tracer.new(writer: FauxWriter.new, partial_flush: true)
+    flush_tracer = get_test_tracer(partial_flush: true)
     refute_nil(flush_tracer.context_flush)
 
     # If not configured with any flush options, context flush still doesn't exist.
@@ -366,7 +366,6 @@ def test_tracer_hard_limit_overrides_soft_limit
     tracer.configure(min_spans_before_partial_flush: context.max_length,
                      max_spans_before_partial_flush: context.max_length,
                      partial_flush_timeout: 3600)
-    tracer.writer = FauxWriter.new
 
     n = 1_000_000
     assert_operator(n, :>, context.max_length, 'need to send enough spans')
diff --git a/test/helper.rb b/test/helper.rb
index 3f5fe96dbd1..937432b9e3c 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -35,8 +35,39 @@ class Span
 end
 
 # Return a test tracer instance with a faux writer.
-def get_test_tracer
-  Datadog::Tracer.new(writer: FauxWriter.new)
+def get_test_tracer(options = {})
+  options = { writer: FauxWriter.new }.merge(options)
+  Datadog::Tracer.new(options).tap do |tracer|
+    # TODO: Let's try to get rid of this override, which has too much
+    #       knowledge about the internal workings of the tracer.
+    #       It is done to prevent the activation of priority sampling
+    #       from wiping out the configured test writer, by replacing it.
+    tracer.define_singleton_method(:configure) do |opts = {}|
+      super(opts)
+
+      # Re-configure the tracer with a new test writer
+      # since priority sampling will wipe out the old test writer.
+      unless @writer.is_a?(FauxWriter)
+        @writer = if @sampler.is_a?(Datadog::PrioritySampler)
+                    FauxWriter.new(priority_sampler: @sampler)
+                  else
+                    FauxWriter.new
+                  end
+
+        hostname = opts.fetch(:hostname, nil)
+        port = opts.fetch(:port, nil)
+
+        @writer.transport.hostname = hostname unless hostname.nil?
+        @writer.transport.port = port unless port.nil?
+
+        statsd = opts.fetch(:statsd, nil)
+        unless statsd.nil?
+          @writer.statsd = statsd
+          @writer.transport.statsd = statsd
+        end
+      end
+    end
+  end
 end
 
 # Return some test traces

From 956577f065b7e12005ff98bc49106da6348ff6d4 Mon Sep 17 00:00:00 2001
From: David Elner <david@davidelner.com>
Date: Mon, 17 Dec 2018 12:17:03 -0500
Subject: [PATCH 3/3] Fixed: Priority sampling with specs.

---
 spec/ddtrace/configuration/pin_setup_spec.rb  |  2 +-
 spec/ddtrace/context_spec.rb                  |  2 +-
 .../active_model_serializers/patcher_spec.rb  |  2 +-
 .../contrib/active_record/multi_db_spec.rb    |  2 +-
 .../contrib/active_record/performance_spec.rb |  2 +-
 .../contrib/active_record/tracer_spec.rb      |  2 +-
 .../notifications/subscription_spec.rb        |  2 +-
 .../contrib/aws/instrumentation_spec.rb       |  2 +-
 .../concurrent_ruby/integration_spec.rb       |  2 +-
 .../contrib/dalli/instrumentation_spec.rb     |  2 +-
 .../contrib/delayed_job/plugin_spec.rb        |  2 +-
 .../contrib/elasticsearch/patcher_spec.rb     |  2 +-
 .../contrib/elasticsearch/transport_spec.rb   |  2 +-
 .../contrib/excon/instrumentation_spec.rb     |  2 +-
 .../contrib/faraday/middleware_spec.rb        |  2 +-
 spec/ddtrace/contrib/graphql/tracer_spec.rb   |  2 +-
 spec/ddtrace/contrib/http/patcher_spec.rb     |  2 +-
 spec/ddtrace/contrib/mongodb/client_spec.rb   |  2 +-
 spec/ddtrace/contrib/mysql2/patcher_spec.rb   |  2 +-
 spec/ddtrace/contrib/racecar/patcher_spec.rb  |  2 +-
 spec/ddtrace/contrib/rack/distributed_spec.rb |  2 +-
 spec/ddtrace/contrib/rack/integration_spec.rb |  2 +-
 spec/ddtrace/contrib/rack/middleware_spec.rb  |  2 +-
 .../contrib/rack/request_queuing_spec.rb      |  2 +-
 .../contrib/rack/resource_name_spec.rb        |  2 +-
 .../contrib/rails/action_controller_spec.rb   |  2 +-
 spec/ddtrace/contrib/rails/middleware_spec.rb |  2 +-
 spec/ddtrace/contrib/rails/railtie_spec.rb    |  2 +-
 .../contrib/rake/instrumentation_spec.rb      |  2 +-
 spec/ddtrace/contrib/redis/miniapp_spec.rb    |  2 +-
 spec/ddtrace/contrib/redis/redis_spec.rb      |  2 +-
 .../contrib/resque/instrumentation_spec.rb    |  2 +-
 .../contrib/rest_client/request_patch_spec.rb |  2 +-
 .../contrib/sequel/configuration_spec.rb      |  2 +-
 .../contrib/sequel/instrumentation_spec.rb    |  2 +-
 spec/ddtrace/contrib/shoryuken/tracer_spec.rb |  2 +-
 .../contrib/sinatra/activerecord_spec.rb      |  2 +-
 .../ddtrace/contrib/sinatra/multi_app_spec.rb |  2 +-
 spec/ddtrace/contrib/sinatra/tracer_spec.rb   |  4 +--
 spec/ddtrace/integration_spec.rb              | 12 ++-----
 spec/ddtrace/pin_spec.rb                      |  4 +--
 .../propagation/http_propagator_spec.rb       |  2 +-
 .../propagation_integration_spec.rb           |  2 +-
 spec/support/tracer_helpers.rb                | 35 +++++++++++++++++--
 44 files changed, 80 insertions(+), 55 deletions(-)

diff --git a/spec/ddtrace/configuration/pin_setup_spec.rb b/spec/ddtrace/configuration/pin_setup_spec.rb
index ab3bbf732f0..d9d3d89bf0f 100644
--- a/spec/ddtrace/configuration/pin_setup_spec.rb
+++ b/spec/ddtrace/configuration/pin_setup_spec.rb
@@ -24,7 +24,7 @@
         }
       end
 
-      let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+      let(:tracer) { get_test_tracer }
 
       it do
         expect(target.datadog_pin.service).to eq('my-service')
diff --git a/spec/ddtrace/context_spec.rb b/spec/ddtrace/context_spec.rb
index acf986d598b..677df748037 100644
--- a/spec/ddtrace/context_spec.rb
+++ b/spec/ddtrace/context_spec.rb
@@ -5,7 +5,7 @@
 RSpec.describe Datadog::Context do
   subject(:context) { described_class.new(options) }
   let(:options) { {} }
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   describe '#current_root_span' do
     subject(:current_root_span) { context.current_root_span }
diff --git a/spec/ddtrace/contrib/active_model_serializers/patcher_spec.rb b/spec/ddtrace/contrib/active_model_serializers/patcher_spec.rb
index eb17b8d37d2..28ddf0c5ff8 100644
--- a/spec/ddtrace/contrib/active_model_serializers/patcher_spec.rb
+++ b/spec/ddtrace/contrib/active_model_serializers/patcher_spec.rb
@@ -10,7 +10,7 @@
 RSpec.describe 'ActiveModelSerializers patcher' do
   include_context 'AMS serializer'
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   def all_spans
     tracer.writer.spans(:keep)
diff --git a/spec/ddtrace/contrib/active_record/multi_db_spec.rb b/spec/ddtrace/contrib/active_record/multi_db_spec.rb
index 18dbf955f70..924b2df1a1c 100644
--- a/spec/ddtrace/contrib/active_record/multi_db_spec.rb
+++ b/spec/ddtrace/contrib/active_record/multi_db_spec.rb
@@ -6,7 +6,7 @@
 require 'sqlite3'
 
 RSpec.describe 'ActiveRecord multi-database implementation' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:configuration_options) { { tracer: tracer, service_name: default_db_service_name } }
   let(:default_db_service_name) { 'default-db' }
 
diff --git a/spec/ddtrace/contrib/active_record/performance_spec.rb b/spec/ddtrace/contrib/active_record/performance_spec.rb
index a1b4aac1f3d..aee3eaba788 100644
--- a/spec/ddtrace/contrib/active_record/performance_spec.rb
+++ b/spec/ddtrace/contrib/active_record/performance_spec.rb
@@ -5,7 +5,7 @@
 require 'sqlite3'
 
 RSpec.describe 'ActiveRecord tracing performance' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:options) { { tracer: tracer } }
   let(:spans) { tracer.writer.spans }
 
diff --git a/spec/ddtrace/contrib/active_record/tracer_spec.rb b/spec/ddtrace/contrib/active_record/tracer_spec.rb
index 8ea9f479951..decef947731 100644
--- a/spec/ddtrace/contrib/active_record/tracer_spec.rb
+++ b/spec/ddtrace/contrib/active_record/tracer_spec.rb
@@ -4,7 +4,7 @@
 require_relative 'app'
 
 RSpec.describe 'ActiveRecord instrumentation' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:configuration_options) { { tracer: tracer } }
 
   before(:each) do
diff --git a/spec/ddtrace/contrib/active_support/notifications/subscription_spec.rb b/spec/ddtrace/contrib/active_support/notifications/subscription_spec.rb
index afee9f296e1..3e6fd6b27db 100644
--- a/spec/ddtrace/contrib/active_support/notifications/subscription_spec.rb
+++ b/spec/ddtrace/contrib/active_support/notifications/subscription_spec.rb
@@ -7,7 +7,7 @@
 RSpec.describe Datadog::Contrib::ActiveSupport::Notifications::Subscription do
   describe 'instance' do
     subject(:subscription) { described_class.new(tracer, span_name, options, &block) }
-    let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+    let(:tracer) { get_test_tracer }
     let(:span_name) { double('span_name') }
     let(:options) { {} }
     let(:payload) { {} }
diff --git a/spec/ddtrace/contrib/aws/instrumentation_spec.rb b/spec/ddtrace/contrib/aws/instrumentation_spec.rb
index a7eed337119..36a46f61aea 100644
--- a/spec/ddtrace/contrib/aws/instrumentation_spec.rb
+++ b/spec/ddtrace/contrib/aws/instrumentation_spec.rb
@@ -6,7 +6,7 @@
 require 'ddtrace/ext/http'
 
 RSpec.describe 'AWS instrumentation' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:client) { ::Aws::S3::Client.new(stub_responses: responses) }
   let(:responses) { true }
diff --git a/spec/ddtrace/contrib/concurrent_ruby/integration_spec.rb b/spec/ddtrace/contrib/concurrent_ruby/integration_spec.rb
index b32bae667ad..0ea383c421b 100644
--- a/spec/ddtrace/contrib/concurrent_ruby/integration_spec.rb
+++ b/spec/ddtrace/contrib/concurrent_ruby/integration_spec.rb
@@ -12,7 +12,7 @@
     remove_patch!(:concurrent_ruby)
   end
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:configuration_options) { { tracer: tracer } }
 
   subject(:deferred_execution) do
diff --git a/spec/ddtrace/contrib/dalli/instrumentation_spec.rb b/spec/ddtrace/contrib/dalli/instrumentation_spec.rb
index f6592f73c46..a626ecb6b26 100644
--- a/spec/ddtrace/contrib/dalli/instrumentation_spec.rb
+++ b/spec/ddtrace/contrib/dalli/instrumentation_spec.rb
@@ -9,7 +9,7 @@
   let(:test_port) { ENV.fetch('TEST_MEMCACHED_PORT', '11211') }
 
   let(:client) { ::Dalli::Client.new("#{test_host}:#{test_port}") }
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:pin) { ::Dalli.datadog_pin }
 
   def all_spans
diff --git a/spec/ddtrace/contrib/delayed_job/plugin_spec.rb b/spec/ddtrace/contrib/delayed_job/plugin_spec.rb
index 23a70b01f69..1157c02bb07 100644
--- a/spec/ddtrace/contrib/delayed_job/plugin_spec.rb
+++ b/spec/ddtrace/contrib/delayed_job/plugin_spec.rb
@@ -24,7 +24,7 @@ def job_data
     end)
   end
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   before do
     Datadog.configure { |c| c.use :delayed_job, tracer: tracer }
diff --git a/spec/ddtrace/contrib/elasticsearch/patcher_spec.rb b/spec/ddtrace/contrib/elasticsearch/patcher_spec.rb
index e8798fea60e..2ad06417758 100644
--- a/spec/ddtrace/contrib/elasticsearch/patcher_spec.rb
+++ b/spec/ddtrace/contrib/elasticsearch/patcher_spec.rb
@@ -9,7 +9,7 @@
 
   let(:client) { Elasticsearch::Client.new(url: server) }
   let(:pin) { Datadog::Pin.get_from(client) }
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   before do
     Datadog.configure do |c|
diff --git a/spec/ddtrace/contrib/elasticsearch/transport_spec.rb b/spec/ddtrace/contrib/elasticsearch/transport_spec.rb
index dbe5206a475..e93b747b0f2 100644
--- a/spec/ddtrace/contrib/elasticsearch/transport_spec.rb
+++ b/spec/ddtrace/contrib/elasticsearch/transport_spec.rb
@@ -22,7 +22,7 @@
   let(:server) { "http://#{host}:#{port}" }
 
   let(:client) { Elasticsearch::Client.new(url: server) }
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:spans) { tracer.writer.spans }
   let(:span) { spans.first }
diff --git a/spec/ddtrace/contrib/excon/instrumentation_spec.rb b/spec/ddtrace/contrib/excon/instrumentation_spec.rb
index 9ab1d64abf0..452c2b7f73a 100644
--- a/spec/ddtrace/contrib/excon/instrumentation_spec.rb
+++ b/spec/ddtrace/contrib/excon/instrumentation_spec.rb
@@ -5,7 +5,7 @@
 require 'ddtrace/contrib/excon/middleware'
 
 RSpec.describe Datadog::Contrib::Excon::Middleware do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:connection_options) { { mock: true } }
   let(:middleware_options) { {} }
diff --git a/spec/ddtrace/contrib/faraday/middleware_spec.rb b/spec/ddtrace/contrib/faraday/middleware_spec.rb
index 04dc8bc6547..469c0dfa2bb 100644
--- a/spec/ddtrace/contrib/faraday/middleware_spec.rb
+++ b/spec/ddtrace/contrib/faraday/middleware_spec.rb
@@ -5,7 +5,7 @@
 require 'ddtrace/ext/distributed'
 
 RSpec.describe 'Faraday middleware' do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:client) do
     ::Faraday.new('http://example.com') do |builder|
diff --git a/spec/ddtrace/contrib/graphql/tracer_spec.rb b/spec/ddtrace/contrib/graphql/tracer_spec.rb
index 32c2e8af07a..9cb6651538f 100644
--- a/spec/ddtrace/contrib/graphql/tracer_spec.rb
+++ b/spec/ddtrace/contrib/graphql/tracer_spec.rb
@@ -14,7 +14,7 @@
     end
   end
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   def pop_spans
     tracer.writer.spans(:keep)
diff --git a/spec/ddtrace/contrib/http/patcher_spec.rb b/spec/ddtrace/contrib/http/patcher_spec.rb
index 69dd7ff040d..e65b596f497 100644
--- a/spec/ddtrace/contrib/http/patcher_spec.rb
+++ b/spec/ddtrace/contrib/http/patcher_spec.rb
@@ -3,7 +3,7 @@
 require 'net/http'
 
 RSpec.describe 'net/http patcher' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:host) { 'example.com' }
 
   before do
diff --git a/spec/ddtrace/contrib/mongodb/client_spec.rb b/spec/ddtrace/contrib/mongodb/client_spec.rb
index 04d83b8864f..7a7990c1b7c 100644
--- a/spec/ddtrace/contrib/mongodb/client_spec.rb
+++ b/spec/ddtrace/contrib/mongodb/client_spec.rb
@@ -4,7 +4,7 @@
 require 'mongo'
 
 RSpec.describe 'Mongo::Client instrumentation' do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:client) { Mongo::Client.new(*client_options) }
   let(:client_options) { [["#{host}:#{port}"], { database: database }] }
diff --git a/spec/ddtrace/contrib/mysql2/patcher_spec.rb b/spec/ddtrace/contrib/mysql2/patcher_spec.rb
index 44924f94219..c68e67f6691 100644
--- a/spec/ddtrace/contrib/mysql2/patcher_spec.rb
+++ b/spec/ddtrace/contrib/mysql2/patcher_spec.rb
@@ -4,7 +4,7 @@
 require 'mysql2'
 
 RSpec.describe 'Mysql2::Client patcher' do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:client) do
     Mysql2::Client.new(
diff --git a/spec/ddtrace/contrib/racecar/patcher_spec.rb b/spec/ddtrace/contrib/racecar/patcher_spec.rb
index 2679b4f87f0..e1a039e17a5 100644
--- a/spec/ddtrace/contrib/racecar/patcher_spec.rb
+++ b/spec/ddtrace/contrib/racecar/patcher_spec.rb
@@ -5,7 +5,7 @@
 require 'active_support'
 require 'ddtrace'
 RSpec.describe 'Racecar patcher' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   def all_spans
     tracer.writer.spans(:keep)
diff --git a/spec/ddtrace/contrib/rack/distributed_spec.rb b/spec/ddtrace/contrib/rack/distributed_spec.rb
index 63bef3fbb5f..de13aa804b4 100644
--- a/spec/ddtrace/contrib/rack/distributed_spec.rb
+++ b/spec/ddtrace/contrib/rack/distributed_spec.rb
@@ -8,7 +8,7 @@
 RSpec.describe 'Rack integration distributed tracing' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:rack_options) { { tracer: tracer } }
 
   let(:spans) { tracer.writer.spans }
diff --git a/spec/ddtrace/contrib/rack/integration_spec.rb b/spec/ddtrace/contrib/rack/integration_spec.rb
index 900161789cc..e2e9aa1a745 100644
--- a/spec/ddtrace/contrib/rack/integration_spec.rb
+++ b/spec/ddtrace/contrib/rack/integration_spec.rb
@@ -9,7 +9,7 @@
 RSpec.describe 'Rack integration tests' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:rack_options) { { tracer: tracer } }
 
   let(:spans) { tracer.writer.spans }
diff --git a/spec/ddtrace/contrib/rack/middleware_spec.rb b/spec/ddtrace/contrib/rack/middleware_spec.rb
index 337155a94cb..fd8c20bf363 100644
--- a/spec/ddtrace/contrib/rack/middleware_spec.rb
+++ b/spec/ddtrace/contrib/rack/middleware_spec.rb
@@ -8,7 +8,7 @@
   subject(:middleware) { described_class.new(app) }
   let(:app) { instance_double(Rack::Builder) }
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:configuration_options) { { tracer: tracer } }
 
   before(:each) do
diff --git a/spec/ddtrace/contrib/rack/request_queuing_spec.rb b/spec/ddtrace/contrib/rack/request_queuing_spec.rb
index c6f9f9b014d..4fe5aa06d7c 100644
--- a/spec/ddtrace/contrib/rack/request_queuing_spec.rb
+++ b/spec/ddtrace/contrib/rack/request_queuing_spec.rb
@@ -8,7 +8,7 @@
 RSpec.describe 'Rack integration request queuing' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:rack_options) { { tracer: tracer } }
 
   let(:spans) { tracer.writer.spans }
diff --git a/spec/ddtrace/contrib/rack/resource_name_spec.rb b/spec/ddtrace/contrib/rack/resource_name_spec.rb
index f9bc60f650d..190b7e1624d 100644
--- a/spec/ddtrace/contrib/rack/resource_name_spec.rb
+++ b/spec/ddtrace/contrib/rack/resource_name_spec.rb
@@ -8,7 +8,7 @@
 RSpec.describe 'Rack integration with other middleware' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:rack_options) do
     {
       application: app,
diff --git a/spec/ddtrace/contrib/rails/action_controller_spec.rb b/spec/ddtrace/contrib/rails/action_controller_spec.rb
index f6f65f1ec5e..af852c237aa 100644
--- a/spec/ddtrace/contrib/rails/action_controller_spec.rb
+++ b/spec/ddtrace/contrib/rails/action_controller_spec.rb
@@ -1,7 +1,7 @@
 require 'ddtrace/contrib/rails/rails_helper'
 
 RSpec.describe 'ActionController tracing' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:rails_options) { { tracer: tracer } }
 
   before(:each) do
diff --git a/spec/ddtrace/contrib/rails/middleware_spec.rb b/spec/ddtrace/contrib/rails/middleware_spec.rb
index d9baf2dc101..e20fe8b7bfb 100644
--- a/spec/ddtrace/contrib/rails/middleware_spec.rb
+++ b/spec/ddtrace/contrib/rails/middleware_spec.rb
@@ -15,7 +15,7 @@ def index
     end)
   end
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   def all_spans
     tracer.writer.spans(:keep)
diff --git a/spec/ddtrace/contrib/rails/railtie_spec.rb b/spec/ddtrace/contrib/rails/railtie_spec.rb
index 29e0c33f9e7..fe134fec204 100644
--- a/spec/ddtrace/contrib/rails/railtie_spec.rb
+++ b/spec/ddtrace/contrib/rails/railtie_spec.rb
@@ -7,7 +7,7 @@
   before(:each) { skip 'Test not compatible with Rails < 4.0' if Rails.version < '4.0' }
   include_context 'Rails test application'
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   let(:routes) { { '/' => 'test#index' } }
   let(:controllers) { [controller] }
diff --git a/spec/ddtrace/contrib/rake/instrumentation_spec.rb b/spec/ddtrace/contrib/rake/instrumentation_spec.rb
index 5e8568f8d59..ab9c8e3893f 100644
--- a/spec/ddtrace/contrib/rake/instrumentation_spec.rb
+++ b/spec/ddtrace/contrib/rake/instrumentation_spec.rb
@@ -7,7 +7,7 @@
 require 'ddtrace/contrib/rake/patcher'
 
 RSpec.describe Datadog::Contrib::Rake::Instrumentation do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:configuration_options) { { tracer: tracer, enabled: true } }
   let(:spans) { tracer.writer.spans }
   let(:span) { spans.first }
diff --git a/spec/ddtrace/contrib/redis/miniapp_spec.rb b/spec/ddtrace/contrib/redis/miniapp_spec.rb
index 3c77353b521..1e6e9b674f8 100644
--- a/spec/ddtrace/contrib/redis/miniapp_spec.rb
+++ b/spec/ddtrace/contrib/redis/miniapp_spec.rb
@@ -6,7 +6,7 @@
 require 'ddtrace'
 
 RSpec.describe 'Redis mini app test' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   def all_spans
     tracer.writer.spans(:keep)
diff --git a/spec/ddtrace/contrib/redis/redis_spec.rb b/spec/ddtrace/contrib/redis/redis_spec.rb
index 8e4314820d4..accdb9e496c 100644
--- a/spec/ddtrace/contrib/redis/redis_spec.rb
+++ b/spec/ddtrace/contrib/redis/redis_spec.rb
@@ -6,7 +6,7 @@
 require 'ddtrace'
 
 RSpec.describe 'Redis test' do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   def all_spans
     tracer.writer.spans(:keep)
diff --git a/spec/ddtrace/contrib/resque/instrumentation_spec.rb b/spec/ddtrace/contrib/resque/instrumentation_spec.rb
index 67594b55607..39ff4f0319a 100644
--- a/spec/ddtrace/contrib/resque/instrumentation_spec.rb
+++ b/spec/ddtrace/contrib/resque/instrumentation_spec.rb
@@ -6,7 +6,7 @@
 RSpec.describe 'Resque instrumentation' do
   include_context 'Resque job'
 
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:pin) { ::Resque.datadog_pin }
   let(:spans) { tracer.writer.spans }
   let(:span) { spans.first }
diff --git a/spec/ddtrace/contrib/rest_client/request_patch_spec.rb b/spec/ddtrace/contrib/rest_client/request_patch_spec.rb
index b5a73641d5a..6abaf01a14f 100644
--- a/spec/ddtrace/contrib/rest_client/request_patch_spec.rb
+++ b/spec/ddtrace/contrib/rest_client/request_patch_spec.rb
@@ -5,7 +5,7 @@
 require 'restclient/request'
 
 RSpec.describe Datadog::Contrib::RestClient::RequestPatch do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:rest_client_options) { {} }
 
   before do
diff --git a/spec/ddtrace/contrib/sequel/configuration_spec.rb b/spec/ddtrace/contrib/sequel/configuration_spec.rb
index c94f6423c12..de5e5a1dc50 100644
--- a/spec/ddtrace/contrib/sequel/configuration_spec.rb
+++ b/spec/ddtrace/contrib/sequel/configuration_spec.rb
@@ -6,7 +6,7 @@
 require 'ddtrace/contrib/sequel/patcher'
 
 RSpec.describe 'Sequel configuration' do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:spans) { tracer.writer.spans }
   let(:span) { spans.first }
 
diff --git a/spec/ddtrace/contrib/sequel/instrumentation_spec.rb b/spec/ddtrace/contrib/sequel/instrumentation_spec.rb
index fc1c890d110..f1f39dba60f 100644
--- a/spec/ddtrace/contrib/sequel/instrumentation_spec.rb
+++ b/spec/ddtrace/contrib/sequel/instrumentation_spec.rb
@@ -6,7 +6,7 @@
 require 'ddtrace/contrib/sequel/integration'
 
 RSpec.describe 'Sequel instrumentation' do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:configuration_options) { { tracer: tracer } }
   let(:sequel) do
     Sequel.sqlite(':memory:').tap do |s|
diff --git a/spec/ddtrace/contrib/shoryuken/tracer_spec.rb b/spec/ddtrace/contrib/shoryuken/tracer_spec.rb
index 00b8be4423d..afa15f5c60b 100644
--- a/spec/ddtrace/contrib/shoryuken/tracer_spec.rb
+++ b/spec/ddtrace/contrib/shoryuken/tracer_spec.rb
@@ -3,7 +3,7 @@
 require 'shoryuken'
 
 RSpec.describe Datadog::Contrib::Shoryuken::Tracer do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:options) { { tracer: tracer } }
   let(:spans) { tracer.writer.spans }
   let(:span) { spans.first }
diff --git a/spec/ddtrace/contrib/sinatra/activerecord_spec.rb b/spec/ddtrace/contrib/sinatra/activerecord_spec.rb
index 0ad21d92ad0..96501580945 100644
--- a/spec/ddtrace/contrib/sinatra/activerecord_spec.rb
+++ b/spec/ddtrace/contrib/sinatra/activerecord_spec.rb
@@ -11,7 +11,7 @@
 RSpec.describe 'Sinatra instrumentation with ActiveRecord' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:options) { { tracer: tracer } }
 
   let(:span) { spans.first }
diff --git a/spec/ddtrace/contrib/sinatra/multi_app_spec.rb b/spec/ddtrace/contrib/sinatra/multi_app_spec.rb
index 925f070a740..580f7a63966 100644
--- a/spec/ddtrace/contrib/sinatra/multi_app_spec.rb
+++ b/spec/ddtrace/contrib/sinatra/multi_app_spec.rb
@@ -9,7 +9,7 @@
 RSpec.describe 'Sinatra instrumentation for multi-apps' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:options) { { tracer: tracer } }
 
   let(:span) { spans.first }
diff --git a/spec/ddtrace/contrib/sinatra/tracer_spec.rb b/spec/ddtrace/contrib/sinatra/tracer_spec.rb
index 38bb8949620..8bd41ec3b36 100644
--- a/spec/ddtrace/contrib/sinatra/tracer_spec.rb
+++ b/spec/ddtrace/contrib/sinatra/tracer_spec.rb
@@ -9,7 +9,7 @@
 RSpec.describe 'Sinatra instrumentation' do
   include Rack::Test::Methods
 
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
   let(:options) { { tracer: tracer } }
 
   let(:span) { spans.first }
@@ -386,7 +386,7 @@
     include_context 'app with simple route'
 
     subject(:response) { get '/' }
-    let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new, enabled: false) }
+    let(:tracer) { get_test_tracer(enabled: false) }
 
     it do
       is_expected.to be_ok
diff --git a/spec/ddtrace/integration_spec.rb b/spec/ddtrace/integration_spec.rb
index 901e874126d..750c0a1cb0d 100644
--- a/spec/ddtrace/integration_spec.rb
+++ b/spec/ddtrace/integration_spec.rb
@@ -148,14 +148,8 @@ def agent_receives_span_step3(previous_success)
   end
 
   describe 'sampling priority metrics' do
-    let(:tracer) do
-      get_test_tracer.tap do |t|
-        t.configure(priority_sampling: true)
-        t.writer = writer
-      end
-    end
-
-    let(:writer) { FauxWriter.new(priority_sampler: Datadog::PrioritySampler.new) }
+    # Sampling priority is enabled by default
+    let(:tracer) { get_test_tracer }
 
     context 'when #sampling_priority is set on a child span' do
       let(:parent_span) { tracer.start_span('parent span') }
@@ -168,7 +162,7 @@ def agent_receives_span_step3(previous_success)
           end.finish
         end.finish
 
-        try_wait_until { writer.spans(:keep).any? }
+        try_wait_until { tracer.writer.spans(:keep).any? }
       end
 
       it do
diff --git a/spec/ddtrace/pin_spec.rb b/spec/ddtrace/pin_spec.rb
index c6d1d80182a..41cbb08e621 100644
--- a/spec/ddtrace/pin_spec.rb
+++ b/spec/ddtrace/pin_spec.rb
@@ -23,7 +23,7 @@
 
     context 'when given sufficient info' do
       let(:options) { { app: 'test-app', app_type: 'test-type', tracer: tracer } }
-      let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+      let(:tracer) { get_test_tracer }
 
       it 'sets the service info' do
         expect(tracer.services.key?(service_name)).to be true
@@ -35,7 +35,7 @@
 
     context 'when given insufficient info' do
       let(:options) { { app_type: 'test-type', tracer: tracer } }
-      let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+      let(:tracer) { get_test_tracer }
 
       it 'does not sets the service info' do
         expect(tracer.services).to be_empty
diff --git a/spec/ddtrace/propagation/http_propagator_spec.rb b/spec/ddtrace/propagation/http_propagator_spec.rb
index 1bbf84db715..5a868ddc782 100644
--- a/spec/ddtrace/propagation/http_propagator_spec.rb
+++ b/spec/ddtrace/propagation/http_propagator_spec.rb
@@ -4,7 +4,7 @@
 require 'ddtrace/propagation/http_propagator'
 
 RSpec.describe Datadog::HTTPPropagator do
-  let(:tracer) { ::Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   describe '#inject!' do
     let(:env) { { 'something' => 'alien' } }
diff --git a/spec/ddtrace/propagation/propagation_integration_spec.rb b/spec/ddtrace/propagation/propagation_integration_spec.rb
index 51197163e8e..72c7384d44d 100644
--- a/spec/ddtrace/propagation/propagation_integration_spec.rb
+++ b/spec/ddtrace/propagation/propagation_integration_spec.rb
@@ -4,7 +4,7 @@
 require 'ddtrace/propagation/http_propagator'
 
 RSpec.describe 'Context propagation' do
-  let(:tracer) { Datadog::Tracer.new(writer: FauxWriter.new) }
+  let(:tracer) { get_test_tracer }
 
   describe 'when max context size is exceeded' do
     let(:max_size) { 3 }
diff --git a/spec/support/tracer_helpers.rb b/spec/support/tracer_helpers.rb
index 706a22ee4ce..3d87f1606dd 100644
--- a/spec/support/tracer_helpers.rb
+++ b/spec/support/tracer_helpers.rb
@@ -4,8 +4,39 @@
 
 module TracerHelpers
   # Return a test tracer instance with a faux writer.
-  def get_test_tracer
-    Datadog::Tracer.new(writer: FauxWriter.new)
+  def get_test_tracer(options = {})
+    options = { writer: FauxWriter.new }.merge(options)
+    Datadog::Tracer.new(options).tap do |tracer|
+      # TODO: Let's try to get rid of this override, which has too much
+      #       knowledge about the internal workings of the tracer.
+      #       It is done to prevent the activation of priority sampling
+      #       from wiping out the configured test writer, by replacing it.
+      tracer.define_singleton_method(:configure) do |opts = {}|
+        super(opts)
+
+        # Re-configure the tracer with a new test writer
+        # since priority sampling will wipe out the old test writer.
+        unless @writer.is_a?(FauxWriter)
+          @writer = if @sampler.is_a?(Datadog::PrioritySampler)
+                      FauxWriter.new(priority_sampler: @sampler)
+                    else
+                      FauxWriter.new
+                    end
+
+          hostname = opts.fetch(:hostname, nil)
+          port = opts.fetch(:port, nil)
+
+          @writer.transport.hostname = hostname unless hostname.nil?
+          @writer.transport.port = port unless port.nil?
+
+          statsd = opts.fetch(:statsd, nil)
+          unless statsd.nil?
+            @writer.statsd = statsd
+            @writer.transport.statsd = statsd
+          end
+        end
+      end
+    end
   end
 
   # Return some test traces