From 384496cb1d69273b7ce3df3b7b74e51c92ade0bb Mon Sep 17 00:00:00 2001
From: Yusuke Tsutsumi <yusuke@tsutsumi.io>
Date: Sat, 14 Sep 2019 11:58:22 -0700
Subject: [PATCH 1/6] Adding a working propagator, adding to integrations and
 example

Adding a full, end-to-end example of propagation at work in the
example application, including a test.

Adding the use of propagators into the integrations.
---
 .../ext/http_requests/__init__.py             |  7 ++
 .../src/opentelemetry/ext/wsgi/__init__.py    | 25 ++++--
 .../tests/test_wsgi_middleware.py             |  4 +-
 .../propagation/tracestatehttptextformat.py   | 20 +++++
 .../src/opentelemetry/propagator/__init__.py  | 77 +++++++++++++++++++
 .../flask_example.py                          | 18 +++--
 .../tests/test_flask_example.py               | 49 +++++++++++-
 .../sdk/context/propagation/b3_format.py      |  1 -
 8 files changed, 184 insertions(+), 17 deletions(-)
 create mode 100644 opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py
 create mode 100644 opentelemetry-api/src/opentelemetry/propagator/__init__.py

diff --git a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
index 2689b8ea2d..6be8bc6519 100644
--- a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
+++ b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
@@ -22,6 +22,8 @@
 
 from requests.sessions import Session
 
+import opentelemetry.propagator as propagator
+
 
 # NOTE: Currently we force passing a tracer. But in turn, this forces the user
 # to configure a SDK before enabling this integration. In turn, this means that
@@ -72,6 +74,11 @@ def instrumented_request(self, method, url, *args, **kwargs):
             # TODO: Propagate the trace context via headers once we have a way
             # to access propagators.
 
+            headers = kwargs.get("headers", {})
+            propagator.get_global_propagator().inject(
+                tracer, type(headers).__setitem__, headers
+            )
+            kwargs["headers"] = headers
             result = wrapped(self, method, url, *args, **kwargs)  # *** PROCEED
 
             span.set_attribute("http.status_code", result.status_code)
diff --git a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
index 974a6998a8..1a8a6e0799 100644
--- a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
+++ b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
@@ -19,9 +19,10 @@
 """
 
 import functools
+import typing
 import wsgiref.util as wsgiref_util
 
-from opentelemetry import trace
+from opentelemetry import propagator, trace
 from opentelemetry.ext.wsgi.version import __version__  # noqa
 
 
@@ -35,12 +36,9 @@ class OpenTelemetryMiddleware:
         wsgi: The WSGI application callable.
     """
 
-    def __init__(self, wsgi, propagators=None):
+    def __init__(self, wsgi):
         self.wsgi = wsgi
 
-        # TODO: implement context propagation
-        self.propagators = propagators
-
     @staticmethod
     def _add_request_attributes(span, environ):
         span.set_attribute("component", "http")
@@ -87,8 +85,11 @@ def __call__(self, environ, start_response):
 
         tracer = trace.tracer()
         path_info = environ["PATH_INFO"] or "/"
+        parent_span = propagator.get_global_propagator().extract(
+            get_header_from_environ, environ
+        )
 
-        with tracer.start_span(path_info) as span:
+        with tracer.start_span(path_info, parent_span) as span:
             self._add_request_attributes(span, environ)
             start_response = self._create_start_response(span, start_response)
 
@@ -99,3 +100,15 @@ def __call__(self, environ, start_response):
             finally:
                 if hasattr(iterable, "close"):
                     iterable.close()
+
+
+def get_header_from_environ(
+    environ: dict, header_name: str
+) -> typing.Optional[str]:
+    """Retrieve the header value from the wsgi environ dictionary.
+
+    Returns:
+        A string with the header value if it exists, else None.
+    """
+    environ_key = "HTTP_" + header_name.upper().replace("-", "_")
+    return [environ.get(environ_key)]
diff --git a/ext/opentelemetry-ext-wsgi/tests/test_wsgi_middleware.py b/ext/opentelemetry-ext-wsgi/tests/test_wsgi_middleware.py
index b423224ade..c69c1bdc22 100644
--- a/ext/opentelemetry-ext-wsgi/tests/test_wsgi_middleware.py
+++ b/ext/opentelemetry-ext-wsgi/tests/test_wsgi_middleware.py
@@ -125,7 +125,9 @@ def validate_response(self, response, error=None):
             self.assertIsNone(self.exc_info)
 
         # Verify that start_span has been called
-        self.start_span.assert_called_once_with("/")
+        self.start_span.assert_called_once_with(
+            "/", trace_api.INVALID_SPAN_CONTEXT
+        )
 
     def test_basic_wsgi_call(self):
         app = OpenTelemetryMiddleware(simple_wsgi)
diff --git a/opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py b/opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py
new file mode 100644
index 0000000000..dbc3451408
--- /dev/null
+++ b/opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py
@@ -0,0 +1,20 @@
+import opentelemetry.trace as trace
+from opentelemetry.context.propagation import httptextformat
+
+
+class TraceStateHTTPTextFormat(httptextformat.HTTPTextFormat):
+    """TODO: a propagator that extracts and injects tracestate.
+    """
+
+    def extract(
+        self, _get_from_carrier: httptextformat.Getter, _carrier: object
+    ) -> trace.SpanContext:
+        return trace.INVALID_SPAN_CONTEXT
+
+    def inject(
+        self,
+        context: trace.SpanContext,
+        set_in_carrier: httptextformat.Setter,
+        carrier: object,
+    ) -> None:
+        pass
diff --git a/opentelemetry-api/src/opentelemetry/propagator/__init__.py b/opentelemetry-api/src/opentelemetry/propagator/__init__.py
new file mode 100644
index 0000000000..0c7cf5b299
--- /dev/null
+++ b/opentelemetry-api/src/opentelemetry/propagator/__init__.py
@@ -0,0 +1,77 @@
+import typing
+
+import opentelemetry.context.propagation.httptextformat as httptextformat
+import opentelemetry.trace as trace
+from opentelemetry.context.propagation.tracestatehttptextformat import (
+    TraceStateHTTPTextFormat,
+)
+
+
+class Propagator:
+    """Class which encapsulates propagation of values to and from context.
+
+    In contrast to using the formatters directly, a propagator object can
+    help own configuration around which formatters to use, as well as
+    help simplify the work require for integrations to use the intended
+    formatters.
+    """
+
+    def __init__(self, httptextformat_instance: httptextformat.HTTPTextFormat):
+        self._httptextformat = httptextformat_instance
+
+    def extract(
+        self, get_from_carrier: httptextformat.Getter, carrier: object
+    ) -> typing.Union[trace.SpanContext, trace.Span, None]:
+        """Load the parent SpanContext from values in the carrier.
+
+        Using the specified HTTPTextFormatter, the propagator will
+        extract a SpanContext from the carrier. If one is found,
+        it will be set as the parent context of the current span.
+
+        Args:
+            get_from_carrier: a function that can retrieve zero
+                or more values from the carrier. In the case that
+                the value does not exist, return an empty list.
+            carrier: and object which contains values that are
+                used to construct a SpanContext. This object
+                must be paired with an appropriate get_from_carrier
+                which understands how to extract a value from it.
+        """
+        span_context = self._httptextformat.extract(get_from_carrier, carrier)
+        return span_context if span_context else trace.Tracer.CURRENT_SPAN
+
+    def inject(
+        self,
+        tracer: trace.Tracer,
+        set_in_carrier: httptextformat.Setter,
+        carrier: object,
+    ) -> None:
+        """Inject values from the current context into the carrier.
+
+        inject enables the propagation of values into HTTP clients or
+        other objects which perform an HTTP request. Implementations
+        should use the set_in_carrier method to set values on the
+        carrier.
+
+        Args:
+            set_in_carrier: A setter function that can set values
+                on the carrier.
+            carrier: An object that a place to define HTTP headers.
+                Should be paired with set_in_carrier, which should
+                know how to set header values on the carrier.
+        """
+        self._httptextformat.inject(
+            tracer.get_current_span().get_context(), set_in_carrier, carrier
+        )
+
+
+_PROPAGATOR = Propagator(TraceStateHTTPTextFormat())
+
+
+def get_global_propagator() -> Propagator:
+    return _PROPAGATOR
+
+
+def set_global_propagator(propagator: Propagator) -> None:
+    global _PROPAGATOR  # pylint:disable=global-statement
+    _PROPAGATOR = propagator
diff --git a/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py b/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py
index 9568f270c9..71158a573d 100644
--- a/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py
+++ b/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py
@@ -16,13 +16,13 @@
 This module serves as an example to integrate with flask, using
 the requests library to perform downstream requests
 """
-import time
-
 import flask
+import requests
 
 import opentelemetry.ext.http_requests
-from opentelemetry import trace
+from opentelemetry import propagator, trace
 from opentelemetry.ext.wsgi import OpenTelemetryMiddleware
+from opentelemetry.sdk.context.propagation.b3_format import B3Format
 from opentelemetry.sdk.trace import Tracer
 
 
@@ -39,7 +39,6 @@ def configure_opentelemetry(flask_app: flask.Flask):
 
     * processors?
     * exporters?
-    * propagators?
     """
     # Start by configuring all objects required to ensure
     # a complete end to end workflow.
@@ -47,6 +46,13 @@ def configure_opentelemetry(flask_app: flask.Flask):
     # as the opentelemetry-api defines the interface with a no-op
     # implementation.
     trace.set_preferred_tracer_implementation(lambda _: Tracer())
+    # Next, we need to configure how the values that are used by
+    # traces and metrics are propagated (such as what specific headers
+    # carry this value).
+
+    # TBD: can remove once default TraceState propagators are installed.
+    propagator.set_global_propagator(propagator.Propagator(B3Format()))
+
     # Integrations are the glue that binds the OpenTelemetry API
     # and the frameworks and libraries that are used together, automatically
     # creating Spans and propagating context as appropriate.
@@ -61,8 +67,8 @@ def configure_opentelemetry(flask_app: flask.Flask):
 def hello():
     # emit a trace that measures how long the
     # sleep takes
-    with trace.tracer().start_span("sleep"):
-        time.sleep(0.001)
+    with trace.tracer().start_span("example-request"):
+        requests.get("http://www.example.com")
     return "hello"
 
 
diff --git a/opentelemetry-example-app/tests/test_flask_example.py b/opentelemetry-example-app/tests/test_flask_example.py
index ca2a237a60..82bfa725ff 100644
--- a/opentelemetry-example-app/tests/test_flask_example.py
+++ b/opentelemetry-example-app/tests/test_flask_example.py
@@ -1,6 +1,13 @@
 import unittest
+from unittest import mock
+
+import requests
+from werkzeug.test import Client
+from werkzeug.wrappers import BaseResponse
 
 import opentelemetry_example_app.flask_example as flask_example
+from opentelemetry.sdk import trace
+from opentelemetry.sdk.context.propagation import b3_format
 
 
 class TestFlaskExample(unittest.TestCase):
@@ -8,7 +15,43 @@ class TestFlaskExample(unittest.TestCase):
     def setUpClass(cls):
         cls.app = flask_example.app
 
+    def setUp(self):
+        mocked_response = requests.models.Response()
+        mocked_response.status_code = 200
+        mocked_response.reason = "Roger that!"
+        self.send_patcher = mock.patch.object(
+            requests.Session,
+            "send",
+            autospec=True,
+            spec_set=True,
+            return_value=mocked_response,
+        )
+        self.send = self.send_patcher.start()
+
+    def tearDown(self):
+        self.send_patcher.stop()
+
     def test_full_path(self):
-        with self.app.test_client() as client:
-            response = client.get("/")
-            assert response.data.decode() == "hello"
+        trace_id = trace.generate_trace_id()
+        # We need to use the Werkzeug test app because
+        # The headers are injected at the wsgi layer.
+        # The flask test app will not include these, and
+        # result in the values not propagated.
+        client = Client(self.app.wsgi_app, BaseResponse)
+        # emulate b3 headers
+        client.get(
+            "/",
+            headers={
+                "x-b3-traceid": b3_format.format_trace_id(trace_id),
+                "x-b3-spanid": b3_format.format_span_id(
+                    trace.generate_span_id()
+                ),
+                "x-b3-sampled": "1",
+            },
+        )
+        # assert the http request header was propagated through.
+        prepared_request = self.send.call_args[0][1]
+        headers = prepared_request.headers
+        for required_header in {"x-b3-traceid", "x-b3-spanid", "x-b3-sampled"}:
+            assert required_header in headers
+        assert headers["x-b3-traceid"] == b3_format.format_trace_id(trace_id)
diff --git a/opentelemetry-sdk/src/opentelemetry/sdk/context/propagation/b3_format.py b/opentelemetry-sdk/src/opentelemetry/sdk/context/propagation/b3_format.py
index 72d02d6070..9e8f7d3f19 100644
--- a/opentelemetry-sdk/src/opentelemetry/sdk/context/propagation/b3_format.py
+++ b/opentelemetry-sdk/src/opentelemetry/sdk/context/propagation/b3_format.py
@@ -91,7 +91,6 @@ def extract(cls, get_from_carrier, carrier):
         # header is set to allow.
         if sampled in cls._SAMPLE_PROPAGATE_VALUES or flags == "1":
             options |= trace.TraceOptions.RECORDED
-
         return trace.SpanContext(
             # trace an span ids are encoded in hex, so must be converted
             trace_id=int(trace_id, 16),

From 4e9f07551401e62a1e61f6611dd5daa86c9e6aa1 Mon Sep 17 00:00:00 2001
From: Yusuke Tsutsumi <yusuke@tsutsumi.io>
Date: Mon, 16 Sep 2019 16:26:37 -0700
Subject: [PATCH 2/6] Addressing review feedback

Fixing incorrect type annotation for WSGI getter for
HTTPTextFormatter.

Using setdefautl to enable more atomic and simple
setting of headers.

Using UnitTest style assertions vs PyTest.
---
 .../src/opentelemetry/ext/http_requests/__init__.py        | 3 +--
 .../src/opentelemetry/ext/wsgi/__init__.py                 | 7 +++++--
 opentelemetry-example-app/tests/test_flask_example.py      | 6 ++++--
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
index 6be8bc6519..576d13d922 100644
--- a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
+++ b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
@@ -74,11 +74,10 @@ def instrumented_request(self, method, url, *args, **kwargs):
             # TODO: Propagate the trace context via headers once we have a way
             # to access propagators.
 
-            headers = kwargs.get("headers", {})
+            headers = kwargs.setdefault("headers", {})
             propagator.get_global_propagator().inject(
                 tracer, type(headers).__setitem__, headers
             )
-            kwargs["headers"] = headers
             result = wrapped(self, method, url, *args, **kwargs)  # *** PROCEED
 
             span.set_attribute("http.status_code", result.status_code)
diff --git a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
index 1a8a6e0799..1577ae546c 100644
--- a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
+++ b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
@@ -104,11 +104,14 @@ def __call__(self, environ, start_response):
 
 def get_header_from_environ(
     environ: dict, header_name: str
-) -> typing.Optional[str]:
+) -> typing.List[str]:
     """Retrieve the header value from the wsgi environ dictionary.
 
     Returns:
         A string with the header value if it exists, else None.
     """
     environ_key = "HTTP_" + header_name.upper().replace("-", "_")
-    return [environ.get(environ_key)]
+    value = environ.get(environ_key)
+    if value:
+        return [value]
+    return []
diff --git a/opentelemetry-example-app/tests/test_flask_example.py b/opentelemetry-example-app/tests/test_flask_example.py
index 82bfa725ff..d33e2b9ac0 100644
--- a/opentelemetry-example-app/tests/test_flask_example.py
+++ b/opentelemetry-example-app/tests/test_flask_example.py
@@ -53,5 +53,7 @@ def test_full_path(self):
         prepared_request = self.send.call_args[0][1]
         headers = prepared_request.headers
         for required_header in {"x-b3-traceid", "x-b3-spanid", "x-b3-sampled"}:
-            assert required_header in headers
-        assert headers["x-b3-traceid"] == b3_format.format_trace_id(trace_id)
+            self.assertIn(required_header, headers)
+        self.assertEqual(
+            headers["x-b3-traceid"], b3_format.format_trace_id(trace_id)
+        )

From d856f9d1810d8077be7ef654f6273f120c3485e8 Mon Sep 17 00:00:00 2001
From: Chris Kleinknecht <libc@google.com>
Date: Mon, 16 Sep 2019 16:43:56 -0700
Subject: [PATCH 3/6] Make getter/setter types specific to context

---
 .../context/propagation/httptextformat.py             | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py b/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py
index f3823a86d1..ffde2f1c25 100644
--- a/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py
+++ b/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py
@@ -14,11 +14,14 @@
 
 import abc
 import typing
+from typing import Type, TypeVar
 
 from opentelemetry.trace import SpanContext
 
-Setter = typing.Callable[[object, str, str], None]
-Getter = typing.Callable[[object, str], typing.List[str]]
+_T = typing.TypeVar("_T")
+
+Setter = typing.Callable[[Type[_T], str, str], None]
+Getter = typing.Callable[[Type[_T], str], typing.List[str]]
 
 
 class HTTPTextFormat(abc.ABC):
@@ -70,7 +73,7 @@ def example_route():
 
     @abc.abstractmethod
     def extract(
-        self, get_from_carrier: Getter, carrier: object
+        self, get_from_carrier: Getter[_T], carrier: _T
     ) -> SpanContext:
         """Create a SpanContext from values in the carrier.
 
@@ -93,7 +96,7 @@ def extract(
 
     @abc.abstractmethod
     def inject(
-        self, context: SpanContext, set_in_carrier: Setter, carrier: object
+        self, context: SpanContext, set_in_carrier: Setter[_T], carrier: _T
     ) -> None:
         """Inject values from a SpanContext into a carrier.
 

From 9789e581da3d7c0646161d576e84e0cd45ba2d99 Mon Sep 17 00:00:00 2001
From: Yusuke Tsutsumi <yusuke@tsutsumi.io>
Date: Tue, 17 Sep 2019 21:31:12 -0700
Subject: [PATCH 4/6] Addressing feedback

Renaming propagator to plural, to match other top level module names.

Using static functions for setting / getting global httptextformat,
and making propagator methods static.

Renaming "TraceStateHTTP" to "TraceContextHTTP", since
the latter is the correct name for the w3c spec.
---
 .../ext/http_requests/__init__.py             |  6 +-
 .../src/opentelemetry/ext/wsgi/__init__.py    |  6 +-
 .../propagation/tracecontexthttptextformat.py | 35 +++++++++
 .../propagation/tracestatehttptextformat.py   | 20 -----
 .../src/opentelemetry/propagator/__init__.py  | 77 -------------------
 .../src/opentelemetry/propagators/__init__.py | 66 ++++++++++++++++
 .../flask_example.py                          |  6 +-
 7 files changed, 108 insertions(+), 108 deletions(-)
 create mode 100644 opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py
 delete mode 100644 opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py
 delete mode 100644 opentelemetry-api/src/opentelemetry/propagator/__init__.py
 create mode 100644 opentelemetry-api/src/opentelemetry/propagators/__init__.py

diff --git a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
index 576d13d922..d904a85f90 100644
--- a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
+++ b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
@@ -22,7 +22,7 @@
 
 from requests.sessions import Session
 
-import opentelemetry.propagator as propagator
+from opentelemetry import propagators
 
 
 # NOTE: Currently we force passing a tracer. But in turn, this forces the user
@@ -75,9 +75,7 @@ def instrumented_request(self, method, url, *args, **kwargs):
             # to access propagators.
 
             headers = kwargs.setdefault("headers", {})
-            propagator.get_global_propagator().inject(
-                tracer, type(headers).__setitem__, headers
-            )
+            propagators.inject(tracer, type(headers).__setitem__, headers)
             result = wrapped(self, method, url, *args, **kwargs)  # *** PROCEED
 
             span.set_attribute("http.status_code", result.status_code)
diff --git a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
index 1577ae546c..1f428fb9ab 100644
--- a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
+++ b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
@@ -22,7 +22,7 @@
 import typing
 import wsgiref.util as wsgiref_util
 
-from opentelemetry import propagator, trace
+from opentelemetry import propagators, trace
 from opentelemetry.ext.wsgi.version import __version__  # noqa
 
 
@@ -85,9 +85,7 @@ def __call__(self, environ, start_response):
 
         tracer = trace.tracer()
         path_info = environ["PATH_INFO"] or "/"
-        parent_span = propagator.get_global_propagator().extract(
-            get_header_from_environ, environ
-        )
+        parent_span = propagators.extract(get_header_from_environ, environ)
 
         with tracer.start_span(path_info, parent_span) as span:
             self._add_request_attributes(span, environ)
diff --git a/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py b/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py
new file mode 100644
index 0000000000..35591635f9
--- /dev/null
+++ b/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py
@@ -0,0 +1,35 @@
+# Copyright 2019, OpenTelemetry Authors
+#
+# Licensed 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.
+#
+
+import opentelemetry.trace as trace
+from opentelemetry.context.propagation import httptextformat
+
+
+class TraceContextHTTPTextFormat(httptextformat.HTTPTextFormat):
+    """TODO: extracts and injects using w3c TraceContext's headers.
+    """
+
+    def extract(
+        self, _get_from_carrier: httptextformat.Getter, _carrier: object
+    ) -> trace.SpanContext:
+        return trace.INVALID_SPAN_CONTEXT
+
+    def inject(
+        self,
+        context: trace.SpanContext,
+        set_in_carrier: httptextformat.Setter,
+        carrier: object,
+    ) -> None:
+        pass
diff --git a/opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py b/opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py
deleted file mode 100644
index dbc3451408..0000000000
--- a/opentelemetry-api/src/opentelemetry/context/propagation/tracestatehttptextformat.py
+++ /dev/null
@@ -1,20 +0,0 @@
-import opentelemetry.trace as trace
-from opentelemetry.context.propagation import httptextformat
-
-
-class TraceStateHTTPTextFormat(httptextformat.HTTPTextFormat):
-    """TODO: a propagator that extracts and injects tracestate.
-    """
-
-    def extract(
-        self, _get_from_carrier: httptextformat.Getter, _carrier: object
-    ) -> trace.SpanContext:
-        return trace.INVALID_SPAN_CONTEXT
-
-    def inject(
-        self,
-        context: trace.SpanContext,
-        set_in_carrier: httptextformat.Setter,
-        carrier: object,
-    ) -> None:
-        pass
diff --git a/opentelemetry-api/src/opentelemetry/propagator/__init__.py b/opentelemetry-api/src/opentelemetry/propagator/__init__.py
deleted file mode 100644
index 0c7cf5b299..0000000000
--- a/opentelemetry-api/src/opentelemetry/propagator/__init__.py
+++ /dev/null
@@ -1,77 +0,0 @@
-import typing
-
-import opentelemetry.context.propagation.httptextformat as httptextformat
-import opentelemetry.trace as trace
-from opentelemetry.context.propagation.tracestatehttptextformat import (
-    TraceStateHTTPTextFormat,
-)
-
-
-class Propagator:
-    """Class which encapsulates propagation of values to and from context.
-
-    In contrast to using the formatters directly, a propagator object can
-    help own configuration around which formatters to use, as well as
-    help simplify the work require for integrations to use the intended
-    formatters.
-    """
-
-    def __init__(self, httptextformat_instance: httptextformat.HTTPTextFormat):
-        self._httptextformat = httptextformat_instance
-
-    def extract(
-        self, get_from_carrier: httptextformat.Getter, carrier: object
-    ) -> typing.Union[trace.SpanContext, trace.Span, None]:
-        """Load the parent SpanContext from values in the carrier.
-
-        Using the specified HTTPTextFormatter, the propagator will
-        extract a SpanContext from the carrier. If one is found,
-        it will be set as the parent context of the current span.
-
-        Args:
-            get_from_carrier: a function that can retrieve zero
-                or more values from the carrier. In the case that
-                the value does not exist, return an empty list.
-            carrier: and object which contains values that are
-                used to construct a SpanContext. This object
-                must be paired with an appropriate get_from_carrier
-                which understands how to extract a value from it.
-        """
-        span_context = self._httptextformat.extract(get_from_carrier, carrier)
-        return span_context if span_context else trace.Tracer.CURRENT_SPAN
-
-    def inject(
-        self,
-        tracer: trace.Tracer,
-        set_in_carrier: httptextformat.Setter,
-        carrier: object,
-    ) -> None:
-        """Inject values from the current context into the carrier.
-
-        inject enables the propagation of values into HTTP clients or
-        other objects which perform an HTTP request. Implementations
-        should use the set_in_carrier method to set values on the
-        carrier.
-
-        Args:
-            set_in_carrier: A setter function that can set values
-                on the carrier.
-            carrier: An object that a place to define HTTP headers.
-                Should be paired with set_in_carrier, which should
-                know how to set header values on the carrier.
-        """
-        self._httptextformat.inject(
-            tracer.get_current_span().get_context(), set_in_carrier, carrier
-        )
-
-
-_PROPAGATOR = Propagator(TraceStateHTTPTextFormat())
-
-
-def get_global_propagator() -> Propagator:
-    return _PROPAGATOR
-
-
-def set_global_propagator(propagator: Propagator) -> None:
-    global _PROPAGATOR  # pylint:disable=global-statement
-    _PROPAGATOR = propagator
diff --git a/opentelemetry-api/src/opentelemetry/propagators/__init__.py b/opentelemetry-api/src/opentelemetry/propagators/__init__.py
new file mode 100644
index 0000000000..1fb8551ab5
--- /dev/null
+++ b/opentelemetry-api/src/opentelemetry/propagators/__init__.py
@@ -0,0 +1,66 @@
+import opentelemetry.context.propagation.httptextformat as httptextformat
+import opentelemetry.trace as trace
+from opentelemetry.context.propagation.tracecontexthttptextformat import (
+    TraceContextHTTPTextFormat,
+)
+
+
+def extract(
+    get_from_carrier: httptextformat.Getter, carrier: object
+) -> trace.SpanContext:
+    """Load the parent SpanContext from values in the carrier.
+
+    Using the specified HTTPTextFormatter, the propagator will
+    extract a SpanContext from the carrier. If one is found,
+    it will be set as the parent context of the current span.
+
+    Args:
+        get_from_carrier: a function that can retrieve zero
+            or more values from the carrier. In the case that
+            the value does not exist, return an empty list.
+        carrier: and object which contains values that are
+            used to construct a SpanContext. This object
+            must be paired with an appropriate get_from_carrier
+            which understands how to extract a value from it.
+    """
+    return get_global_httptextformat().extract(get_from_carrier, carrier)
+
+
+def inject(
+    tracer: trace.Tracer,
+    set_in_carrier: httptextformat.Setter,
+    carrier: object,
+) -> None:
+    """Inject values from the current context into the carrier.
+
+    inject enables the propagation of values into HTTP clients or
+    other objects which perform an HTTP request. Implementations
+    should use the set_in_carrier method to set values on the
+    carrier.
+
+    Args:
+        set_in_carrier: A setter function that can set values
+            on the carrier.
+        carrier: An object that contains a representation of HTTP
+            headers. Should be paired with set_in_carrier, which
+            should know how to set header values on the carrier.
+    """
+    get_global_httptextformat().inject(
+        tracer.get_current_span().get_context(), set_in_carrier, carrier
+    )
+
+
+_HTTP_TEXT_FORMAT = (
+    TraceContextHTTPTextFormat()
+)  # type: httptextformat.HTTPTextFormat
+
+
+def get_global_httptextformat() -> httptextformat.HTTPTextFormat:
+    return _HTTP_TEXT_FORMAT
+
+
+def set_global_httptextformat(
+    http_text_format: httptextformat.HTTPTextFormat
+) -> None:
+    global _HTTP_TEXT_FORMAT  # pylint:disable=global-statement
+    _HTTP_TEXT_FORMAT = http_text_format
diff --git a/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py b/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py
index 71158a573d..229acdfb43 100644
--- a/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py
+++ b/opentelemetry-example-app/src/opentelemetry_example_app/flask_example.py
@@ -20,7 +20,7 @@
 import requests
 
 import opentelemetry.ext.http_requests
-from opentelemetry import propagator, trace
+from opentelemetry import propagators, trace
 from opentelemetry.ext.wsgi import OpenTelemetryMiddleware
 from opentelemetry.sdk.context.propagation.b3_format import B3Format
 from opentelemetry.sdk.trace import Tracer
@@ -50,8 +50,8 @@ def configure_opentelemetry(flask_app: flask.Flask):
     # traces and metrics are propagated (such as what specific headers
     # carry this value).
 
-    # TBD: can remove once default TraceState propagators are installed.
-    propagator.set_global_propagator(propagator.Propagator(B3Format()))
+    # TBD: can remove once default TraceContext propagators are installed.
+    propagators.set_global_httptextformat(B3Format())
 
     # Integrations are the glue that binds the OpenTelemetry API
     # and the frameworks and libraries that are used together, automatically

From 105654517f57e1318dd3d8c7dcf59473c87ac6de Mon Sep 17 00:00:00 2001
From: Yusuke Tsutsumi <yusuke@tsutsumi.io>
Date: Tue, 17 Sep 2019 22:13:53 -0700
Subject: [PATCH 5/6] Fixing type signatures

---
 .../context/propagation/httptextformat.py              |  5 ++---
 .../context/propagation/tracecontexthttptextformat.py  | 10 +++++++---
 .../src/opentelemetry/propagators/__init__.py          | 10 +++++++---
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py b/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py
index ffde2f1c25..35bdfbb3fe 100644
--- a/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py
+++ b/opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py
@@ -14,14 +14,13 @@
 
 import abc
 import typing
-from typing import Type, TypeVar
 
 from opentelemetry.trace import SpanContext
 
 _T = typing.TypeVar("_T")
 
-Setter = typing.Callable[[Type[_T], str, str], None]
-Getter = typing.Callable[[Type[_T], str], typing.List[str]]
+Setter = typing.Callable[[typing.Type[_T], str, str], None]
+Getter = typing.Callable[[typing.Type[_T], str], typing.List[str]]
 
 
 class HTTPTextFormat(abc.ABC):
diff --git a/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py b/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py
index 35591635f9..575644a91f 100644
--- a/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py
+++ b/opentelemetry-api/src/opentelemetry/context/propagation/tracecontexthttptextformat.py
@@ -13,23 +13,27 @@
 # limitations under the License.
 #
 
+import typing
+
 import opentelemetry.trace as trace
 from opentelemetry.context.propagation import httptextformat
 
+_T = typing.TypeVar("_T")
+
 
 class TraceContextHTTPTextFormat(httptextformat.HTTPTextFormat):
     """TODO: extracts and injects using w3c TraceContext's headers.
     """
 
     def extract(
-        self, _get_from_carrier: httptextformat.Getter, _carrier: object
+        self, _get_from_carrier: httptextformat.Getter[_T], _carrier: _T
     ) -> trace.SpanContext:
         return trace.INVALID_SPAN_CONTEXT
 
     def inject(
         self,
         context: trace.SpanContext,
-        set_in_carrier: httptextformat.Setter,
-        carrier: object,
+        set_in_carrier: httptextformat.Setter[_T],
+        carrier: _T,
     ) -> None:
         pass
diff --git a/opentelemetry-api/src/opentelemetry/propagators/__init__.py b/opentelemetry-api/src/opentelemetry/propagators/__init__.py
index 1fb8551ab5..aa59669407 100644
--- a/opentelemetry-api/src/opentelemetry/propagators/__init__.py
+++ b/opentelemetry-api/src/opentelemetry/propagators/__init__.py
@@ -1,12 +1,16 @@
+import typing
+
 import opentelemetry.context.propagation.httptextformat as httptextformat
 import opentelemetry.trace as trace
 from opentelemetry.context.propagation.tracecontexthttptextformat import (
     TraceContextHTTPTextFormat,
 )
 
+_T = typing.TypeVar("_T")
+
 
 def extract(
-    get_from_carrier: httptextformat.Getter, carrier: object
+    get_from_carrier: httptextformat.Getter[_T], carrier: _T
 ) -> trace.SpanContext:
     """Load the parent SpanContext from values in the carrier.
 
@@ -28,8 +32,8 @@ def extract(
 
 def inject(
     tracer: trace.Tracer,
-    set_in_carrier: httptextformat.Setter,
-    carrier: object,
+    set_in_carrier: httptextformat.Setter[_T],
+    carrier: _T,
 ) -> None:
     """Inject values from the current context into the carrier.
 

From 607a7943fc7dce105b16b8304c97785a21286578 Mon Sep 17 00:00:00 2001
From: Yusuke Tsutsumi <yusuke@tsutsumi.io>
Date: Wed, 18 Sep 2019 09:53:27 -0700
Subject: [PATCH 6/6] fixing formatting.

---
 .../src/opentelemetry/ext/http_requests/__init__.py           | 1 +
 .../src/opentelemetry/ext/wsgi/__init__.py                    | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
index 7971b0e128..994968e196 100644
--- a/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
+++ b/ext/opentelemetry-ext-http-requests/src/opentelemetry/ext/http_requests/__init__.py
@@ -25,6 +25,7 @@
 from opentelemetry import propagators
 from opentelemetry.trace import SpanKind
 
+
 # NOTE: Currently we force passing a tracer. But in turn, this forces the user
 # to configure a SDK before enabling this integration. In turn, this means that
 # if the SDK/tracer is already using `requests` they may, in theory, bypass our
diff --git a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
index 6974a77df3..28caa77e52 100644
--- a/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
+++ b/ext/opentelemetry-ext-wsgi/src/opentelemetry/ext/wsgi/__init__.py
@@ -87,7 +87,9 @@ def __call__(self, environ, start_response):
         path_info = environ["PATH_INFO"] or "/"
         parent_span = propagators.extract(get_header_from_environ, environ)
 
-        with tracer.start_span(path_info, parent_span, kind=trace.SpanKind.SERVER) as span:
+        with tracer.start_span(
+            path_info, parent_span, kind=trace.SpanKind.SERVER
+        ) as span:
             self._add_request_attributes(span, environ)
             start_response = self._create_start_response(span, start_response)