Changed to the propagator API

Adding a UnifiedContext, composing DistributedContext and SpanContext.
This will enable propagators to extract and inject values from either system,
enabling more sophisticated schemes and standards to propagate data.

This also removes the need for generics and propagators that only
consume one or the other, requiring integrators to do extra work to
wire propagators appropriately.

Modifying the API of the propagators to consume the context as a mutable
argument. By passing in the context rather than returning, this enables the
chained use of propagators, allowing for situations such as supporting
multiple trace propagation standards simulatenously.

opentelemetry-api/src/opentelemetry/context/__init__.py

@@ -11,25 +11,22 @@
 # 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.
-
-
 """
 The OpenTelemetry context module provides abstraction layer on top of
 thread-local storage and contextvars. The long term direction is to switch to
 contextvars provided by the Python runtime library.
 
 A global object ``Context`` is provided to access all the context related
-functionalities::
+functionalities:
 
     >>> from opentelemetry.context import Context
     >>> Context.foo = 1
     >>> Context.foo = 2
     >>> Context.foo
     2
 
-When explicit thread is used, a helper function
-``Context.with_current_context`` can be used to carry the context across
-threads::
+When explicit thread is used, a helper function `Context.with_current_context`
+can be used to carry the context across threads:
 
     from threading import Thread
     from opentelemetry.context import Context
@@ -62,7 +59,7 @@ def work(name):
 
         print('Main thread:', Context)
 
-Here goes another example using thread pool::
+Here goes another example using thread pool:
 
     import time
     import threading
@@ -97,7 +94,7 @@ def work(name):
         pool.join()
         println('Main thread: {}'.format(Context))
 
-Here goes a simple demo of how async could work in Python 3.7+::
+Here goes a simple demo of how async could work in Python 3.7+:
 
     import asyncio
 
@@ -141,9 +138,9 @@ async def main():
 import typing
 
 from .base_context import BaseRuntimeContext
+from .unified_context import UnifiedContext
 
-__all__ = ["Context"]
-
+__all__ = ["Context", "UnifiedContext"]
 
 Context = None  # type: typing.Optional[BaseRuntimeContext]
 

opentelemetry-api/src/opentelemetry/context/base_context.py

@@ -37,7 +37,7 @@ def set(self, value: "object") -> None:
             raise NotImplementedError
 
     _lock = threading.Lock()
-    _slots = {}  # type: typing.Dict[str, 'BaseRuntimeContext.Slot']
+    _slots: typing.Dict[str, Slot] = {}
 
     @classmethod
     def clear(cls) -> None:
@@ -48,9 +48,7 @@ def clear(cls) -> None:
             slot.clear()
 
     @classmethod
-    def register_slot(
-        cls, name: str, default: "object" = None
-    ) -> "BaseRuntimeContext.Slot":
+    def register_slot(cls, name: str, default: "object" = None) -> "Slot":
         """Register a context slot with an optional default value.
 
         :type name: str

opentelemetry-api/src/opentelemetry/context/propagation/binaryformat.py

@@ -13,9 +13,8 @@
 # limitations under the License.
 
 import abc
-import typing
 
-from opentelemetry.trace import SpanContext
+from opentelemetry.context import UnifiedContext
 
 
 class BinaryFormat(abc.ABC):
@@ -27,14 +26,14 @@ class BinaryFormat(abc.ABC):
 
     @staticmethod
     @abc.abstractmethod
-    def to_bytes(context: SpanContext) -> bytes:
+    def to_bytes(context: UnifiedContext) -> bytes:
         """Creates a byte representation of a SpanContext.
 
         to_bytes should read values from a SpanContext and return a data
         format to represent it, in bytes.
 
         Args:
-            context: the SpanContext to serialize
+            context: the SpanContext to serialize.
 
         Returns:
             A bytes representation of the SpanContext.
@@ -43,15 +42,17 @@ def to_bytes(context: SpanContext) -> bytes:
 
     @staticmethod
     @abc.abstractmethod
-    def from_bytes(byte_representation: bytes) -> typing.Optional[SpanContext]:
-        """Return a SpanContext that was represented by bytes.
+    def from_bytes(context: UnifiedContext,
+                   byte_representation: bytes) -> None:
+        """Populate UnifiedContext that was represented by bytes.
 
-        from_bytes should return back a SpanContext that was constructed from
-        the data serialized in the byte_representation passed. If it is not
+        from_bytes should populated UnifiedContext with data that was
+        serialized in the byte_representation passed. If it is not
         possible to read in a proper SpanContext, return None.
 
         Args:
-            byte_representation: the bytes to deserialize
+            context: The UnifiedContext to populate.
+            byte_representation: the bytes to deserialize.
 
         Returns:
             A bytes representation of the SpanContext if it is valid.

opentelemetry-api/src/opentelemetry/context/propagation/httptextformat.py

@@ -15,6 +15,7 @@
 import abc
 import typing
 
+from opentelemetry.context import UnifiedContext
 from opentelemetry.trace import SpanContext
 
 Setter = typing.Callable[[object, str, str], None]
@@ -35,11 +36,12 @@ class HTTPTextFormat(abc.ABC):
         import flask
         import requests
         from opentelemetry.context.propagation import HTTPTextFormat
+        from opentelemetry.trace import tracer
+        from opentelemetry.context import UnifiedContext
 
         PROPAGATOR = HTTPTextFormat()
 
 
-
         def get_header_from_flask_request(request, key):
             return request.headers.get_all(key)
 
@@ -48,15 +50,17 @@ def set_header_into_requests_request(request: requests.Request,
             request.headers[key] = value
 
         def example_route():
-            span_context = PROPAGATOR.extract(
-                get_header_from_flask_request,
+            span = tracer().create_span("")
+            context = UnifiedContext.create(span)
+            PROPAGATOR.extract(
+                context, get_header_from_flask_request,
                 flask.request
             )
             request_to_downstream = requests.Request(
                 "GET", "http://httpbin.org/get"
             )
             PROPAGATOR.inject(
-                span_context,
+                context,
                 set_header_into_requests_request,
                 request_to_downstream
             )
@@ -70,15 +74,20 @@ def example_route():
 
     @abc.abstractmethod
     def extract(
-        self, get_from_carrier: Getter, carrier: object
-    ) -> SpanContext:
-        """Create a SpanContext from values in the carrier.
+        self,
+        context: UnifiedContext,
+        get_from_carrier: Getter,
+        carrier: object,
+    ) -> None:
+        """Extract values from the carrier into the context.
 
         The extract function should retrieve values from the carrier
-        object using get_from_carrier, and use values to populate a
-        SpanContext value and return it.
+        object using get_from_carrier, and use values to populate
+        attributes of the UnifiedContext passed in.
 
         Args:
+            context: A UnifiedContext instance that will be
+                populated with values from the carrier.
             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.

opentelemetry-api/src/opentelemetry/context/unified_context.py

@@ -0,0 +1,65 @@
+# 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.
+
+from opentelemetry.distributedcontext import DistributedContext
+from opentelemetry.trace import SpanContext
+
+
+class UnifiedContext:
+    """A unified context object that contains all context relevant to
+    telemetry.
+
+    The UnifiedContext is a single object that composes all contexts that
+    are needed by the various forms of telemetry. It is intended to be an
+    object that can be passed as the argument to any component that needs
+    to read or modify content values (such as propagators). By unifying
+    all context in a composed data structure, it expands the flexibility
+    of the APIs that modify it.
+
+    As it is designed to carry context specific to all telemetry use
+    cases, it's schema is explicit. Note that this is not intended to
+    be an object that acts as a singleton that returns different results
+    based on the thread or coroutine of execution. For that, see `Context`.
+
+
+    Args:
+        distributed: The DistributedContext for this instance.
+        span: The SpanContext for this instance.
+    """
+    __slots__ = ["distributed", "span"]
+
+    def __init__(self, distributed: DistributedContext, span: SpanContext):
+        self.distributed = distributed
+        self.span = span
+
+    @staticmethod
+    def create(span: SpanContext) -> "UnifiedContext":
+        """Create an unpopulated UnifiedContext object.
+
+        Example:
+
+            from opentelemetry.trace import tracer
+            span = tracer.create_span("")
+            context = UnifiedContext.create(span)
+
+
+        Args:
+            parent_span: the parent SpanContext that will be the
+                parent of the span in the UnifiedContext.
+        """
+        return UnifiedContext(DistributedContext(), span)
+
+    def __repr__(self) -> str:
+        return "{}(distributed={}, span={})".format(
+            type(self).__name__, repr(self.distributed), repr(self.span))

opentelemetry-sdk/src/opentelemetry/sdk/context/propagation/b3_format.py

@@ -32,7 +32,7 @@ class B3Format(HTTPTextFormat):
     _SAMPLE_PROPAGATE_VALUES = set(["1", "True", "true", "d"])
 
     @classmethod
-    def extract(cls, get_from_carrier, carrier):
+    def extract(cls, context, get_from_carrier, carrier):
         trace_id = format_trace_id(trace.INVALID_TRACE_ID)
         span_id = format_span_id(trace.INVALID_SPAN_ID)
         sampled = 0
@@ -57,7 +57,7 @@ def extract(cls, get_from_carrier, carrier):
             elif len(fields) == 4:
                 trace_id, span_id, sampled, _parent_span_id = fields
             else:
-                return trace.INVALID_SPAN_CONTEXT
+                return
         else:
             trace_id = (
                 _extract_first_element(
@@ -92,7 +92,7 @@ def extract(cls, get_from_carrier, carrier):
         if sampled in cls._SAMPLE_PROPAGATE_VALUES or flags == "1":
             options |= trace.TraceOptions.RECORDED
 
-        return trace.SpanContext(
+        context.span = trace.SpanContext(
             # trace an span ids are encoded in hex, so must be converted
             trace_id=int(trace_id, 16),
             span_id=int(span_id, 16),