enchannel

Enchannel is a lightweight spec for flexible communications between a frontend, like the notebook or jupyter-sidecar, and a backend kernel (the runtime, like Python, Julia, or R). Enchannel does not specify the implementation or how the communications are constructed or destructed.

Motivation

Background

The core functionality of the notebook is to send messages from a frontend to a backend, and from a backend to a frontend (or many frontends). In the case of the Jupyter/IPython notebook, it communicates over websockets (which in turn reach out to ØMQ on the backend).

What if...?

What if you want to serve the same HTML and Javascript for the notebook application itself while being able to work in a native ØMQ environment? What if websockets are fairly restricted in your working *ahem* corporate environment and you need to send data via POST and receive streaming updates using server-sent events?

Solutions

Well, we'd need a nice, clean way to abstract the transport layer. As Jupyter is messages all the way down, hooking up a series of event emitters, all with the same interface, is one abstraction. That's definitely do-able.

Instead, let's rely on Observables: asynchronous data streams, from the future. Observables, as flexible transport, are the multi-valued promise we've all been waiting for:

	Single return value	Mutiple return values
Pull/Synchronous/Interactive	Object	Iterables (Array, Set, Map, Object)
Push/Asynchronous/Reactive	Promise	Observable

Note: The enchannel spec uses RxJS's observables implementation.

---

enchannel your data

The spec

Kernel communications are described by a single object containing subjects, each corresponding to a communication channel of the kernel instance. There will be between four and five channels:

const {
    shell,
    stdin,
    control,
    iopub,
    heartbeat, // (optional) 
} = channelsObject;  

For more information see the Jupyter client docs.

Relying on RxJS's implementation of subjects means the streams can be handled like so:

    iopub.filter(msg => msg.header.msg_type === 'execute_result')
    .map(msg => msg.content.data)
.subscribe(x => { console.log(`DATA: ${util.inspect(x)}`)})
    ```
 
    As a benefit of subjects, we can go ahead and submit messages to the
    underlying transport:
 
    ```javascript
    var message = {
header: {
msg_id: `execute_${uuid.v4()}`,
        username: '',
        session: '00000000-0000-0000-0000-000000000000',
        msg_type: 'execute_request',
        version: '5.0',
        },
content: {
code: 'print("woo")',
      silent: false,
      store_history: true,
      user_expressions: {},
      allow_stdin: false,
         },
    };
 
shell.next(message); // send the message

Messages observed from these Subjects are all immutable, not by convention but through a recursive Object.freeze.

Note that heartbeat is not included in the spec above primarily because it's an implementation by-product and may end up being deprecated based on the chosen development approach.

Mutliplexed enchannel

enchannel-zmq-backend exposes a multiplexed subject that allows the developer communicate with all four channels via a single interface.

const channel = e.createMainChannel(identity, kernel.config);
const body = {
    header: {
        msg_id: `execute_9ed11a0f-707e-4f71-829c-a19b8ff8eed8`,
        username: "rgbkrk",
        session: "00000000-0000-0000-0000-000000000000",
        msg_type: "execute_request",
        version: "5.0"
    },
    content: {
        code: 'print("woo")',
        silent: false,
        store_history: true,
        user_expressions: {},
        allow_stdin: false
    }
};
const message = { type: "shell", body };
channel.subscribe(console.log);
channel.next(message);

Develop with us

To contribute to the spec or convenience functions, clone this repo and install it by running the following from the repo root:

npm install

Before contributing changes to the utility functions, be kind to your peers and check if the unit tests pass locally by running:

npm test

Implementations

• enchannel-zmq-backend
• enchannel-socketio-backend

References

• The introduction to Reactive Programming you've been missing
• RxJS (5.0)
• RxJS (4.0)