Cnet

Cnet is an easy-to-use wrapper around the high-performance Chromium HTTP network stack and file cache. It includes Chromium's SPDY support and experimental QUIC support.

Cnet has language bindings for C and Java. See android/java for the complete Java API. See cnet.h for the complete C API. An application can use the Java and C bindings simultaneously.

Example Java Fetch

// Initialize the library.
final Context appContext = getApplicationContext();
Cnet.loadLibraries(appContext);
Cnet.initLibraryOnUiThread(appContext);

// Establish a Cnet Pool for connection reuse.
final CnetPool.Config poolConfig = new CnetPool.Config();
poolConfig.userAgent = "cnet-test";
poolConfig.enableSpdy = true;
poolConfig.enableSslFalseStart = true;
final CnetPool pool = new CnetPool(poolConfig);

// Create an HTTP request.
final Fetcher fetcher = pool.createFetcher("https://yahoo.com", "GET", new ResponseCompletion() {
    @Override
    public boolean onBackgroundComplete(Fetcher fetcher, Response response) {
        Log.d(LOG_TAG, "request completed, " + response.getHttpResponseCode() + 
              ", " + response.getOriginalUrl() + ", " + response.getFinalUrl() +
              ", " + response.getBodyLength() + ", " +
              (new String(response.getBody())));
        return RELEASE_NOW; // immediately release native resources
   }
});
fetcher.start();

Note that I've released the native resources used by the response and fetcher (by returning RELEASE_NOW from the onBackgroundComplete() callback). This ensures immediate recovery of native memory, instead of waiting for the Java garbage collector that is unaware of the amount of memory consumed by the native representation. This is really important for the response object, which might be large. If you wanted to keep the response data for use at a later time, then return RELEASE_LATER, and manually release its resources by invoking response.release().

Library Initialization

For Java, you should load the libraries using Cnet.loadLibraries(Context). This works around dynamic linking bugs in several versions of Android.

For Java, initialize the library on the main UI thread (Cnet.initLibraryOnUiThread(Context)), so that it can grab a reference to the UI thread's dispatch loop. Complete the initialization before interacting with Cnet on background threads.

For the C API, while on the UI thread, grab a reference to the UI's dispatch loop via CnetMessageLoopForUiGet(), and initialize the library (CnetInitialize(false)). Complete the initialization before interacting with Cnet on background threads.

Pools

A pool provides shared resources for a sequence of HTTP requests, such as reused sockets and a file cache. Avoid enabling the file cache for pools dedicated to API requests.

Each pool can create three threads:

1. a networking thread that runs all network logic;
2. a work thread that runs all callbacks (so that callbacks do not block the networking thread), one at a time;
3. and a file thread (created on demand), for reading files for uploads and writing files for downloads.

You can adjust several settings on pools:

• SSL false start: enable this to reduce SSL-connection times by 1/3.
• Proxy config: by default, Cnet uses the system's proxy settings (e.g., on Android, it registers a broadcast listener for proxy changes), but you may override these for debugging with Charles Proxy. The override expects a rule similar to http://10.73.218.92:8888.
• Trust self-signed certificate authorities: by default, and always in release builds, Cnet will reject self-signed certificate authorities. When debugging with Charles Proxy, you'll probably want to enable self-signed certificate authorities.

Fetching

The fetcher is a simple abstraction that, upon completion, provides a buffer or a file with the entire response --- it doesn't use a streaming abstraction. Thus the fetcher is convenient for JSON and images, which typically we want only when completely downloaded. It always provides the response on a background thread.

To stop a fetcher, you must invoke its cancel method --- trying to delete the fetcher will not stop its execution (it retains a reference to itself, to provide deterministic behavior in garbage-collected environments). A fetcher always executes its completion callback, even when cancelled.

The fetcher makes it easy to submit request parameters, whether as part of the query in the URL, or encoded as a multi-part form. It complements this with Oauth v1 support and file-upload support.

When creating a fetcher, you provide the following required parameters:

• the URL,
• the HTTP method (GET, POST, PUT, etc.),
• and the response/completion callback.

After you have created a fetcher, you can customize some of its behavior:

• Cache behavior: normal, validate, bypass, prefer, cache only, cache if offline, and cache disable (see the source files for documentation of each).
• Parameter encoding: the query string for the URL, multi-part body, or a query string in the body.
• Set parameters.
• Set the upload file.
• Set the Oauth v1 credentials.

The Fetcher Response

The response includes access to the timing (telemetry) data of different stages of the HTTP request. On Android, this data is printed to the log, where it can be extracted and converted to a HAR file.

The response includes the body of the response as an array of bytes. On Android, you can get the raw pointer to this (as a long), which you can pass to another native library (such as Ymagine to decode an image).

Related Work

The Chromium project already has a component, cronet, which packages the Chromium network stack for use in Android applications. Our Cnet wrapper gives us the ability to expose the Chromium features that we need. Alternatively, we could have modified Cronet, but then we'd amplify the task of merging our code with the evolving Chromium code.

Design

API

The fetcher simplifies the task of requesting composites such as JSON and images. It provides the response as a byte-array --- not a streaming API. It is not a general-purpose API to HTTP and all of its features --- it is a simplification for use by common mobile applications. When we need more features, we'll add a second request type that offers more generalized features.

Threading

Cnet generally supports multi-threading via message passing: one thread can own the Cnet objects at a time, but can transfer ownership of the objects to other threads. We try for additional safety to be defensive:

• The response object is immutable, and thus is safely accessed concurrently from multiple threads.
• Any thread can change a Cnet pool, for the pool will commit the changes on the network thread via message passing.

cnet-util Command-Line Utility

Example Flickr fetch of the interesting list, using Charles Proxy:

./out/Debug/cnet-util --method=POST --encoding=body-multipart \
     --proxy-rules=http://10.73.218.92:8888 \
     --oauth-app-key=${MY_OAUTH_APP_KEY} \
     --oauth-app-secret=${MY_OAUTH_APP_SECRET} \
     --oauth-token=${MY_OAUTH_TOKEN} \
     --oauth-token-secret=${MY_OAUTH_TOKEN_SECRET} \
     method=flickr.favorites.getList \
     page=1 per_page=10  format=json nojsoncallback=1 \
     api_key=${MY_FLICKR_API_KEY}  --trust-all-cert-authorities \
     https://api.flickr.com/services/rest

Example Flickr upload, using Charles Proxy:

./out/Debug/cnet-util --method=POST --encoding=body-multipart  \
    --proxy-rules=http://10.73.218.92:8888 \
    --oauth-app-key=${MY_OAUTH_APP_KEY} \
    --oauth-app-secret=${MY_OAUTH_APP_SECRET} \
    --oauth-token=${MY_OAUTH_TOKEN} \
    --oauth-token-secret=${MY_OAUTH_TOKEN_SECRET} \
    --upload=${HOME}/Pictures/test-photo.jpg --upload-key=photo \
    --upload-type=image/jpeg "title=testing upload" name=test.jpg \
    is_public=0 is_friend=0 is_family=0 async=0 \
    api_key=${MY_FLICKR_API_KEY}  --trust-all-cert-authorities \
    https://up.flickr.com/services/upload

Building Cnet from Source

Initial Checkout and Build

The Cnet code must be built within the Chromium environment. Android requires a Linux build machine.

1. For Android, review the system requirements for your build machine at https://code.google.com/p/chromium/wiki/AndroidBuildInstructions, but don't follow its checkout instructions. For iOS, review the system requirements for your build machine at http://www.chromium.org/developers/how-tos/build-instructions-ios, but don't follow its checkout instructions. In particular, you need Google's depot_tools somewhere in your path.

2. Create a top-level directory such as chromium.

3. Within your top-level directory, create a file called .gclient with the following contents:

solutions = [{ u'managed': False, u'name': u'src', u'url': u'https://chromium.googlesource.com/chromium/src.git', u'custom_deps': { "src/yahoo/cnet":"https://github.com/yahoo/cnet.git", }, u'custom_hooks': [ { "name": "cnet-gyp", "pattern": ".", "action": ["python", "src/build/gyp_chromium", "src/yahoo/cnet/cnet.gyp"], }, ], u'deps_file': u'.DEPS.git', }] target_os = ['ios', 'mac']

If using Android, change `target_os` to `['android']`.
4. Within your top-level directory, create a file called `chromium.gyp_env`.
For Android, the file should have:

{ 'GYP_DEFINES': 'OS=android enable_websockets=0 use_icu_alternatives_on_android=1 disable_ftp_support=1 disable_file_support=1', }

You have to choose the architecture by adding additional variables: `target_arch=arm arm_version=6`, or
`target_arch=arm arm_version=7`, or `target_arch=ia32`.

For iOS, the file should have:

{ 'GYP_DEFINES': 'OS=ios', # use space to delimit additional defines. 'GYP_GENERATOR_FLAGS': 'xcode_project_version=3.2', 'GYP_GENERATORS': 'ninja,xcode', }

For Mac, the file should have:

{ 'GYP_DEFINES': 'OS=mac', # use space to delimit additional defines. 'GYP_GENERATOR_FLAGS': 'xcode_project_version=3.2', 'GYP_GENERATORS': 'ninja,xcode', }

5. Within your top-level directory, checkout the release code:
run `gclient sync --with_branch_heads --jobs 16`.
This may fail with an error about duplicate IDs, which you can ignore.
6. Within the `src` directory, determine the newest Chromium release
version: run `git show-ref --tags` and find the newest, e.g., 39.0.2130.1.
7. Within the `src` directory, checkout the latest release version:
`git checkout -b rel-39.0.2130.1 39.0.2130.1`
8. Re-sync to update all dependencies to the versions that correspond
to the latest Chromium release:
run `gclient sync --with_branch_heads --jobs 16`
9. If the `gclient sync` command failed, then you'll have to manually generate
the build files: in the source directory,
run `./build/gyp_chromium yahoo/cnet/cnet.gyp`.
10. If you are developing for iOS or Mac, you can then open `src/yahoo/cnet/cnet.xcodeproj`.
11. To build the cnet-util, from the command line, run: `ninja -C out/Debug cnet-util`.
 Or to build the Android cnet package, run: `ninja -C out/Debug cnet_package`.

## Building for Android

You probably want to support multiple Android architectures, such as
`armeabi` (arm6), `armeabi-v7a` (arm7), and `x86` (ia32).  This
requires maintaining multiple, concurrent Chromium build trees.

The following instructions will create three build trees with output
in `out/arm6/out/Debug`, `out/arm7/out/Debug`, and
`out/ia32/out/Debug`:

1. Comment out the `GYP_DEFINES` in the `chromium.gyp_env` file in
your Chromium root directory.  You're going to specify them on the
command line instead.
2. Generate the build files (in the top-level `src` directory):

GYP_DEFINES='target_arch=arm arm_version=7 OS=android enable_websockets=0 use_icu_alternatives_on_android=1 disable_ftp_support=1 disable_file_support=1'
GYP_GENERATOR_OUTPUT='out/arm7' ./build/gyp_chromium yahoo/cnet/cnet.gyp GYP_DEFINES='target_arch=arm arm_version=6 OS=android enable_websockets=0 use_icu_alternatives_on_android=1 disable_ftp_support=1 disable_file_support=1'
GYP_GENERATOR_OUTPUT='out/arm6' ./build/gyp_chromium yahoo/cnet/cnet.gyp GYP_DEFINES='target_arch=ia32 OS=android enable_websockets=0 use_icu_alternatives_on_android=1 disable_ftp_support=1 disable_file_support=1'
GYP_GENERATOR_OUTPUT='out/ia32' ./build/gyp_chromium yahoo/cnet/cnet.gyp

3. Build for each architecture:

make -C out/arm7/out/Debug cnet_package make -C out/arm6/out/Debug cnet_package make -C out/ia32/out/Debug cnet_package

4. The `cnet_package` generates a variety of outputs: `libcnet.so`,
`cnet.jar`, `cnet-src.jar`, and `java/*`; all under the `cnet`
subdirectory of the output directory.  Only the `libcnet.so` files
should be architecture dependent.  You can consume the Java
via a compiled jar, a source jar, or the raw source files
themselves (under `java/*`).  You may need the symbols for the dynamic
link library: they are in the output directory as `lib/libcnet.so`.