encode · tomchristie · Jun 7, 2023 · May 24, 2023 · May 24, 2023 · May 25, 2023
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## unreleased
+
+- The networking backend interface has [been added to the public API](https://www.encode.io/httpcore/network-backends). Some classes which were previously private implementation detail are now part of the top-level public API. (#699)
+
 ## 0.17.2 (May 23th, 2023)
 
 - Add `socket_options` argument to `ConnectionPool` and `HTTProxy` classes. (#668)

diff --git a/docs/extensions.md b/docs/extensions.md
@@ -200,6 +200,8 @@ The interface provided by the network stream:
 
 This API can be used as the foundation for working with HTTP proxies, WebSocket upgrades, and other advanced use-cases.
 
+See the [network backends documentation](network-backends.md) for more information on working directly with network streams.
+
 ##### `CONNECT` requests
 
 A proxy CONNECT request using the network stream:

diff --git a/docs/network-backends.md b/docs/network-backends.md
@@ -1,3 +1,279 @@
 # Network Backends
 
-TODO
+The API layer at which `httpcore` interacts with the network is described as the network backend. Various backend implementations are provided, allowing `httpcore` to handle networking in different runtime contexts.
+
+## Working with network backends
+
+### The default network backend
+
+Typically you won't need to specify a network backend, as a default will automatically be selected. However, understanding how the network backends fit in may be useful if you want to better understand the underlying architecture. Let's start by seeing how we can explicitly select the network backend.
+
+First we're making a standard HTTP request, using a connection pool:
+
+```python
+import httpcore
+
+with httpcore.ConnectionPool() as http:
+    response = http.request('GET', 'https://www.example.com')
+    print(response)
+```
+
+We can also have the same behavior, but be explicit with our selection of the network backend:
+
+```python
+import httpcore
+
+network_backend = httpcore.SyncBackend()
+with httpcore.ConnectionPool(network_backend=network_backend) as http:
+    response = http.request('GET', 'https://www.example.com')
+    print(response)
+```
+
+The `httpcore.SyncBackend()` implementation handles the opening of TCP connections, and operations on the socket stream, such as reading, writing, and closing the connection.
+
+We can get a better understanding of this by using a network backend to send a basic HTTP/1.1 request directly:
+
+```python
+import httpcore
+
+# Create an SSL context using 'certifi' for the certificates.
+ssl_context = httpcore.default_ssl_context()
+
+# A basic HTTP/1.1 request as a plain bytestring.
+request = b'\r\n'.join([
+    b'GET / HTTP/1.1',
+    b'Host: www.example.com',
+    b'Accept: */*',
+    b'Connection: close',
+    b''
+])
+
+# Open a TCP stream and upgrade it to SSL.
+network_backend = httpcore.SyncBackend()
+network_stream = network_backend.connect_tcp("www.example.com", 443)
+network_stream = network_stream.start_tls(ssl_context, server_hostname="www.example.com")
+
+# Send the HTTP request.
+network_stream.write(request)
+
+# Read the HTTP response.
+while True:
+    response = network_stream.read(max_bytes=4096)
+    if response == b'':
+        break
+    print(response)
+
+# The output should look something like this:
+#
+# b'HTTP/1.1 200 OK\r\nAge: 600005\r\n [...] Content-Length: 1256\r\nConnection: close\r\n\r\n'
+# b'<!doctype html>\n<html>\n<head>\n    <title>Example Domain</title> [...] </html>\n'
+```
+
+### Async network backends
+
+If we're working with an `async` codebase, then we need to select a different backend.
+
+The `httpcore.AnyIOBackend` is suitable for usage if you're running under `asyncio`. This is a networking backend implemented using [the `anyio` package](https://anyio.readthedocs.io/en/3.x/).
+
+```python
+import httpcore
+import asyncio
+
+async def main():
+    network_backend = httpcore.AnyIOBackend()
+    async with httpcore.AsyncConnectionPool(network_backend=network_backend) as http:
+        response = await http.request('GET', 'https://www.example.com')
+        print(response)
+
+asyncio.run(main())
+```
+
+The `AnyIOBackend` will work when running under either `asyncio` or `trio`. However, if you're working with async using the [`trio` framework](https://trio.readthedocs.io/en/stable/), then we recommend using the `httpcore.TrioBackend`.
+
+This will give you the same kind of networking behavior you'd have using `AnyIOBackend`, but there will be a little less indirection so it will be marginally more efficient and will present cleaner tracebacks in error cases.
+
+```python
+import httpcore
+import trio
+
+async def main():
+    network_backend = httpcore.TrioBackend()
+    async with httpcore.AsyncConnectionPool(network_backend=network_backend) as http:
+        response = await http.request('GET', 'https://www.example.com')
+        print(response)
+
+trio.run(main)
+```
+
+### Mock network backends
+
+There are also mock network backends available that can be useful for testing purposes.
+These backends accept a list of bytes, and return network stream interfaces that return those byte streams.
+
+Here's an example of mocking a simple HTTP/1.1 response...
+
+```python
+import httpcore
+
+network_backend = httpcore.MockBackend([
+    b"HTTP/1.1 200 OK\r\n",
+    b"Content-Type: plain/text\r\n",
+    b"Content-Length: 13\r\n",
+    b"\r\n",
+    b"Hello, world!",
+])
+with httpcore.ConnectionPool(network_backend=network_backend) as http:
+    response = http.request("GET", "https://example.com/")
+    print(response.extensions['http_version'])
+    print(response.status)
+    print(response.content)
+```
+
+Mocking a HTTP/2 response is more complex, since it uses a binary format...
+
+```python
+import hpack
+import hyperframe.frame
+import httpcore
+
+content = [
+    hyperframe.frame.SettingsFrame().serialize(),
+    hyperframe.frame.HeadersFrame(
+        stream_id=1,
+        data=hpack.Encoder().encode(
+            [
+                (b":status", b"200"),
+                (b"content-type", b"plain/text"),
+            ]
+        ),
+        flags=["END_HEADERS"],
+    ).serialize(),
+    hyperframe.frame.DataFrame(
+        stream_id=1, data=b"Hello, world!", flags=["END_STREAM"]
+    ).serialize(),
+]
+# Note that we instantiate the mock backend with an `http2=True` argument.
+# This ensures that the mock network stream acts as if the `h2` ALPN flag has been set,
+# and causes the connection pool to interact with the connection using HTTP/2.
+network_backend = httpcore.MockBackend(content, http2=True)
+with httpcore.ConnectionPool(network_backend=network_backend) as http:
+    response = http.request("GET", "https://example.com/")
+    print(response.extensions['http_version'])
+    print(response.status)
+    print(response.content)
+```
+
+### Custom network backends
+
+The base interface for network backends is provided as public API, allowing you to implement custom networking behavior.
+
+You can use this to provide advanced networking functionality such as:
+
+* Network recording / replay.
+* In-depth debug tooling.
+* Handling non-standard SSL or DNS requirements.
+
+Here's an example that records the network response to a file on disk:
+
+```python
+import httpcore
+
+
+class RecordingNetworkStream(httpcore.NetworkStream):
+    def __init__(self, record_file, stream):
+        self.record_file = record_file
+        self.stream = stream
+
+    def read(self, max_bytes, timeout=None):
+        data = self.stream.read(max_bytes, timeout=timeout)
+        self.record_file.write(data)
+        return data
+
+    def write(self, buffer, timeout=None):
+        self.stream.write(buffer, timeout=timeout)
+
+    def close(self) -> None:
+        self.stream.close()
+
+    def start_tls(
+        self,
+        ssl_context,
+        server_hostname=None,
+        timeout=None,
+    ):
+        self.stream = self.stream.start_tls(
+            ssl_context, server_hostname=server_hostname, timeout=timeout
+        )
+        return self
+
+    def get_extra_info(self, info):
+        return self.stream.get_extra_info(info)
+
+
+class RecordingNetworkBackend(httpcore.NetworkBackend):
+    """
+    A custom network backend that records network responses.
+    """
+    def __init__(self, record_file):
+        self.record_file = record_file
+        self.backend = httpcore.SyncBackend()
+
+    def connect_tcp(
+        self,
+        host,
+        port,
+        timeout=None,
+        local_address=None,
+        socket_options=None,
+    ):
+        # Note that we're only using a single record file here,
+        # so even if multiple connections are opened the network
+        # traffic will all write to the same file.
+
+        # An alternative implementation might automatically use
+        # a new file for each opened connection.
+        stream = self.backend.connect_tcp(
+            host,
+            port,
+            timeout=timeout,
+            local_address=local_address,
+            socket_options=socket_options
+        )
+        return RecordingNetworkStream(self.record_file, stream)
+
+
+# Once you make the request, the raw HTTP/1.1 response will be available
+# in the 'network-recording' file.
+#
+# Try switching to `http2=True` to see the difference when recording HTTP/2 binary network traffic,
+# or add `headers={'Accept-Encoding': 'gzip'}` to see HTTP content compression.
+with open("network-recording", "wb") as record_file:
+    network_backend = RecordingNetworkBackend(record_file)
+    with httpcore.ConnectionPool(network_backend=network_backend) as http:
+        response = http.request("GET", "https://www.example.com/")
+        print(response)
+```
+
+---
+
+## Reference
+
+### Networking Backends
+
+* `httpcore.SyncBackend`
+* `httpcore.AnyIOBackend`
+* `httpcore.TrioBackend`
+
+### Mock Backends
+
+* `httpcore.MockBackend`
+* `httpcore.MockStream`
+* `httpcore.AsyncMockBackend`
+* `httpcore.AsyncMockStream`
+
+### Base Interface
+
+* `httpcore.NetworkBackend`
+* `httpcore.NetworkStream`
+* `httpcore.AsyncNetworkBackend`
+* `httpcore.AsyncNetworkStream`
diff --git a/httpcore/__init__.py b/httpcore/__init__.py
@@ -8,6 +8,15 @@
     AsyncHTTPProxy,
     AsyncSOCKSProxy,
 )
+from ._backends.base import (
+    SOCKET_OPTION,
+    AsyncNetworkBackend,
+    AsyncNetworkStream,
+    NetworkBackend,
+    NetworkStream,
+)
+from ._backends.mock import AsyncMockBackend, AsyncMockStream, MockBackend, MockStream
+from ._backends.sync import SyncBackend
 from ._exceptions import (
     ConnectError,
     ConnectionNotAvailable,
@@ -37,6 +46,30 @@
     SOCKSProxy,
 )
 
+# The 'httpcore.AnyIOBackend' class is conditional on 'anyio' being installed.
+try:
+    from ._backends.anyio import AnyIOBackend
+except ImportError:  # pragma: nocover
+
+    class AnyIOBackend:  # type: ignore
+        def __init__(self, *args, **kwargs):  # type: ignore
+            msg = (
+                "Attempted to use 'httpcore.AnyIOBackend' but 'anyio' is not installed."
+            )
+            raise RuntimeError(msg)
+
+
+# The 'httpcore.TrioBackend' class is conditional on 'trio' being installed.
+try:
+    from ._backends.trio import TrioBackend
+except ImportError:  # pragma: nocover
+
+    class TrioBackend:  # type: ignore
+        def __init__(self, *args, **kwargs):  # type: ignore
+            msg = "Attempted to use 'httpcore.TrioBackend' but 'trio' is not installed."
+            raise RuntimeError(msg)
+
+
 __all__ = [
     # top-level requests
     "request",
@@ -62,8 +95,23 @@
     "HTTP2Connection",
     "ConnectionInterface",
     "SOCKSProxy",
+    # network backends, implementations
+    "SyncBackend",
+    "AnyIOBackend",
+    "TrioBackend",
+    # network backends, mock implementations
+    "AsyncMockBackend",
+    "AsyncMockStream",
+    "MockBackend",
+    "MockStream",
+    # network backends, interface
+    "AsyncNetworkStream",
+    "AsyncNetworkBackend",
+    "NetworkStream",
+    "NetworkBackend",
     # util
     "default_ssl_context",
+    "SOCKET_OPTION",
     # exceptions
     "ConnectionNotAvailable",
     "ProxyError",

diff --git a/httpcore/_async/connection.py b/httpcore/_async/connection.py
@@ -4,13 +4,13 @@
 from types import TracebackType
 from typing import Iterable, Iterator, Optional, Type
 
+from .._backends.auto import AutoBackend
+from .._backends.base import SOCKET_OPTION, AsyncNetworkBackend, AsyncNetworkStream
 from .._exceptions import ConnectError, ConnectionNotAvailable, ConnectTimeout
 from .._models import Origin, Request, Response
 from .._ssl import default_ssl_context
 from .._synchronization import AsyncLock
 from .._trace import Trace
-from ..backends.auto import AutoBackend
-from ..backends.base import SOCKET_OPTION, AsyncNetworkBackend, AsyncNetworkStream
 from .http11 import AsyncHTTP11Connection
 from .interfaces import AsyncConnectionInterface