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readme.md

Purpose

streaming-worker is designed to give you a simple interface for sending and receiving events/messages from a long running asynchronous C++ Node.js Addon.

Note - this library is really only for creating very specific types of Node.js C++ addons, ones that are relatively long running and either need to receive a continuous stream of inputs or will send a stream of outputs to/from your JavaScript code (or both). The addons operate in seperate worker threads, and use nan's AsyncProgressWorker to facilitate stream-like and event-like interfaces.

This library is based on a chapter in Node C++ Integration - which covers it's implementation in a lot more detail.

Usage

This library is actually two distinct modules - streaming-worker-sdk and streaming-worker. They can be used independently - however you'll probably use them together.

streaming-worker-sdk is a C++ SDK/headers for building streaming addons - it's not an addon itself!. You create addons by inheriting from the StreamingWorker abstract class defined in the SDK. Your addon, at a minimum, needs to implement a few virtual functions (most importantly, Execute), and can utilize standard methods to read and write Message objects to and from JavaScript. Message objects are name/value (string) pairs.

Once your C++ addon is built, you'll use streaming-worker in JavaScript to interface with the addon. This module exports a single factory method which accepts a path to the C++ addon. The second parameter (optional) is for any parameters you wish to send to C++ to initialize the addon.

Step 1: Setup addon dependencies and build

To start creating your streaming addon, first add a dependency to streaming-worker-sdk.

$ npm install --save streaming-worker-sdk

Next, make sure you've added the include directives to your binding.gyp file for both streaming-worker-sdk and nan.

"include_dirs" : [
    "<!(node -e \"require('nan')\")", 
    "<!(node -e \"require('streaming-worker-sdk')\")"
]

Note, to use nan and streaming-worker-sdk, you need to enable C++11 and use a fairly modern C++ compiler. Checkout the examples below for some actual working binding.gyp files.

Step 2: Build your C++ addon

You'll need to implement a class that inherits from the StreamingWorker abstract base class defined in /dist/streaming-worker.h. This class actually already extends nan's AsyncProgressWorker class. You also need to create a proper constructor in your addon class, along with a factory method to help the base class build your addon.

By example, here's a simple example that will send consecutive integers 0-99 to Node.js

#include "streaming-worker.h"

class Simple : public StreamingWorker {
   public:
   Simple(Callback *data, Callback *complete, Callback *error_callback, v8::Local<v8::Object> & options) 
          : StreamingWorker(data, complete, error_callback){

        // nothing needs to be here - just make sure you call base constructor
        
        if (options->IsObject() ) {
          // extract any options you want to work with. The user of the addon must supply these from JavaScript
          // The options parameter is for your JavaScript code to pass in 
          // an options object.  You can use this for whatever you want.
        }
    }
     
    // You must match the call signature here, `Execute` is a virtual function
    // defined on Streaing Worker.
    void Execute (const AsyncProgressWorker::ExecutionProgress& progress) {

      // Just send 100 integers and stop
      for (int i = 0; i < 100; i++ ) {
        Message tosend("integer", std::to_string(i));
        writeToNode(progress, tosend);
      }
    }
};

Your constructor must, at a minimum, accept three callback objects and an options object. You do not need to directly deal with these callbacks in anyway - other than passing them along to the base class constructor. They are specified by the streaming-worker - the JavaScript adapter/module that you'll use to access this addon later.

Addon Execute method

Once the addon is created, the base class will queue your addon up as a worker thread, hooked into lib_uv's event loop. Your addon will be invoked within it's Execute function. Beware, this function is executed in a worker thread, not the Node.js event loop thread!. Synchronization when accessing any state variables (member variables) is your responsibility. The Execute function must accept a progress object, which is defined by nan's AsyncProgressWorker class. You won't use it directly, but you'll pass it along to the sending method when you want to send messages to Node.js.

You communicate with Node.js through Message objects - which are simple name value pairs (values are strings, feel free to extend the implementation to accomodate other types/templates!).

/// defined in streaming-worker.h
class Message {
public:
  string name;
  string data;
  Message(string name, string data) : name(name), data(data){}
};

Your addon inherits a thread-safe queue, fromNode, for reading messages sent to it from JavaScript (see below for the JavaScript API). You can extract Message objects from the queue at any time by calling read() - but keep in mind this is a blocking call.

Your addon also inherits a thread-safe method for sending messages back to JavaScript, called writeToNode. This call requires two parameters, the progress object passed into your Execute method, and the message to send.

Below is a simple Execute function that receives a message from Node and just returns the square. It stops if it receives -1.

// Member method of another Addon class
void Execute (const AsyncProgressWorker::ExecutionProgress& progress) {
  int value;
  do {
    Message m = fromNode.read();  // get a message from node
    value = std::stoi(m.data);
    Message tosend("square", std::to_string(value*value));
    writeToNode(progress, tosend);
 } while (value > 0);
}

Addon setup code

You need to add just a little bit of boilerplate C++ code to properly bootstrap your addon. The following functions are defined outside of your addon class.

The first is a factory method named create_worker. You MUST include this function, and you cannot alter the signature at all. The base wrapper class calls this to build your particular worker. The prototype for this function is defined in addon-streams.h

StreamingWorker * create_worker(Callback *data
    , Callback *complete
    , Callback *error_callback, v8::Local<v8::Object> & options) {
 
 return new Simple(data, complete, error_callback, options);
}

Lastly, you need to initialize your module using the standard Node.js convention. Replace <ADDON_NAME> with whatever you named your module (target_name in binding.gyp). Keep StreamWorkerWrapper::Init unchanged, it's the bootstrap method defined in streaming-worker.h. It ultimately calls the create_worker method defined above.

NODE_MODULE(simple_streample, StreamWorkerWrapper::Init)

At this point, you can build the addon with node-gyp.

Step 3: Write your JavaScript program

A Node.js program that uses the addon must require the streaming-worker module:

const worker = require("streaming-worker");

All you need to do is give it the file path where your addon is located:

const path = require("path");
const addon_path = path.join(__dirname, "PATH TO ADDON");
const streaming_addon = worker(addon_path, {foo: "bar"});

The optional second parameter is for initialization variables - see the examples below for usage.

Using the Event Emitter API

The streaming_addon object represents your C++ addon, and streaming-worker has added two EventEmitter-like interfaces on it - to and from. As you might expect, to allows you to emit events to your addon - your addon will read the messages using the fromNode.read() call. The from object lets you listen for events sent from your addon - which is done in C++ using the writeToNode method.

streaming_addon.to.emit("input", "here's something");
streaming_addon.to.emit("input", "here's something else");

streaming_addon.from.on('event', function(message){
    console.log("Got something from the addon!");
});

streaming_addon.from.on('error', function(e) {
    console.error("Something has gone wrong in the addon");
});

streaming_addon.from.on('close', function() {
    console.error("The addon has terminated (natural causes)");
});

When you send an event to your addon, the C++ code will read a Message object whose name is set to the event name (first parameter of emit) and data is set to the actual message (second parameter of emit). Likewise, when sending a Message object from C++, it's name will be the name of the event that gets emitted.

To capture the output of the simple example from above (sending integers 0-99 to Node.js), you'd just have the following JS code:

const path = require('path');
const worker = require("streaming-worker");
const addon_path = path.join(__dirname, "../addon/build/Release/simple_stream");
const simple_stream = worker(addon_path);

simple_stream.from.on('integer', function(value){
    console.log(value);
});

Using the Streaming API

Ok, so that's not exactly "streaming"... the streaming-worker adapter also adds two methods for creating actual streams to and from your C++ code too though. To capture the output of your C++ via a stream, you can use the stream() method on the from object:

const str_out = streaming_addon.from.stream();

It's likely you'll want to pipe this to something, I recommend getting familiar with through.

str_out.pipe(
    through(function(message) { 
      	this.queue(JSON.parse(message));
    })).pipe(
    through(function(message) {
        console.log(message);
    }));

To create an input stream, us the stream method on the to object:

const str_in = streaming_addon.to.stream("value",
	function () {
		str_in.to.emit('value', -1);
	});

Unlike the output stream, you need to specify a few more things when creating the input stream, since streaming-worker needs to know how to turn the streamed data into Message objects for your addon to read. The first parameter specifies the name that should be attached to each data put into the stream. The second parameter is a callback that will be invoked when the stream is closed. Under normal circumstances, your C++ code is likely to be looking for some sort of sentinal value - and this is your opportunity to send it.

const streamify = require('stream-array');

// the addon will get -1 when the array is drained, 
// because we specified the end callback above 
streamify([1, 2, 3]).pipe(input); 

If the stream is expected to receive the sentinal anyway, then you can just leave the callback undefined.

const str_in = streaming_addon.to.stream("value");
streamify([1, 2, 3, -1]).pipe(input); // the addon receives the -1 it is looking for...

To print the numbers flowing from the original simple example (numbers 0-99 from C++ to Node), you could do somethign like this:

const through = require('through');
var output = simple_stream.from.stream();
output
   .pipe(
        through(function (data) {
            // the data coming in is an array, 
           	// Element 0 is the name of the event emitted by the addon ("integer")
           	// Element 1 is the data - which in this case is just the integer 
           	this.queue(data[1]);
        }))
   .pipe(process.stdout);

Examples

Below are four examples designed to give you an overview of some of the ways you can interact with addons written using the streaming-worker API. The first two use the event emitter interface to communicate with the addons. The third and fourth examples show you how to use the stream interface.

To get started:

git clone https://github.com/freezer333/streaming-worker

Prime Factorization

This example uses a C++ addon to compute the prime factorization of larg(ish) numbers. The number to be factored is passed in as options to the addon, which immediately begins emitting prime factors as it finds them. The JavaScript code listens to the event emitter interface exposed by the C++ worker and simply prints the factors to the screen.

The demo is found in /examples/factorization. Do an npm install followed by an npm start to run it.

Even/Odd Generator

This example shows you how to send input to your addons via the event emitter interface. The even_odd C++ code sits in a loop waiting for integer input (N). Once received, if N is not -1, it emits even_event and odd_event messages for each number from 0...N.

This example also accepts optional parameters on startup, in this case which number to start from (instead of 0).

The example itself emits a few inputs (10, then 5 seconds later, 20) to the addon and closes it by emitting a -1 at the end.

The demo is found in /examples/even_odd. Do an npm install followed by an npm start to run it.

Sensor Data (simulation)

This example demonstrates creating a streaming interface to capture output from a C++ addon. It's meant to mimick what you might see from a positional tracker, like something reporting the position of a head mounted display in a VR application. The C++ addon emits sensor data at a regular interval. The sensor data emitted is an object, which is serialized to JSON strings using the JSON library for C++.

The JavaScript instantiates the addon, which immediately starts emitting the sensor data. In the example, to interfaces to capture the output are used.

The first method is the same event emitter interface from the previous examples. In the second, an output stream is created (using emit-stream internally). That stream is subsequently piped through some transforms to deserialize the JSON and pull out the data (not the message name) using [through]. The result is streaming sensor data printed to the console.

The demo is found in /examples/sensor_sim. Do an npm install in both the addon and the js directories followed by an npm start in the js directory to run it.

Accumulation in C++

This example demonstrates streaming data into your addon. The addon is a simple accumulator - it receives events, assuming the event data are integers. The addon sums up all the events it reads from the stream and emits a single "sum" event once the stream has been closed.

In this example, stream closing is propogated to the addon via a single message whose data is -1. This is achieved by specifying a custom end callback when creating the addon's input stream. This callback is invoked when the stream is closed.

The accumulator C++ addon also allows filtering of messages, allowing it to selectively choose which events it adds to the "sum". This demonstrates the use of initialization options again, and is helpfulf or the final example (below).

The demo is found in /examples/accumulation. Do an npm install followed by an npm start to run it.