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JSChannel - The Protocol

This document describes the structure and semantics of messages sent by jschannel.

Notational Conventions

Throughout this document when documenting data structures both example objects (in JSON) and schema are provided. All schema are in Orderly, which is a textual shorthand for JSONSchema.

Underlying Technologies

JSChannel is built on top of several technologies: cross document messaging provides the raw mechanism to exchange messages between documents from within the browser on the client side. JSON gives us a concise and convenient means of serializing data that can be represented in JavaScript. Finally, JSON-RPC provides a representation and set of semantics for messages. While jschannel as presented here does not leverage JSON-RPC precisely, the protocol they leverage is heavily influenced by JSON-RPC.

"Wire Format" and Message Types

The jschannel protocol involves 5 different kinds of messages:

Requests

Request messages are the query half of a query/response transaction. All requests must conform to the following schema:

object {
  integer id;
  string method;
  any params?;
  array { string; } callbacks?;
};

An example request might look something like:

{
  "id": 72650,
  "method": "search::run",
  "params": {
    "term": "open"
  },
  "callbacks": [
    "results"
  ]
}

id is a unique integer selected by the endpoint who is sending the request.

method is a required method name, indicating which service or function should be executed on the receiving end.

params can be any data that is possible to represent in JSON. The precise contents are method dependant and are documented in a subsequent section of this document.

callbacks, like params, are method dependent. These are an array of strings which name "callbacks" that can be invoked during the execution of a method. That is, a recipient of a request may invoke any number of callbacks before returning completing the invocation (by returning a result or an error).

Callback Invocations

Callback invocations can occur after requests, but before responses. They invoke a "callback" named in the initial request message. Any number of callback messages may be sent before a response. It is an error to send a callback message after a response has been sent and the recipient should drop the message and may emit an error. Callback invocation messages must conform to the following schema:

object {
  integer id;
  string callback;
  any params?;
};

An example callback invocation looks like:

{
  "id": 72650,
  "callback": "results",
  "params": [
    {
      "title": "I like to open cans of worms"
      "link": "http://somesi.te/432521232"
    },
    {
      "title": "The open web is eye-opening"
      "link": "http://somesi.te/878235425"
    }
  ]
}

id the integer id from the request to which this callback invocation is a response.

callback the string identifier of a callback to invoke. The original request must have included this same string in its callbacks array.

params can be any data that is possible to represent in JSON. The precise contents are method dependent and are documented in a subsequent section of this document.

Error Responses

Error messages may be sent in response to any request that may not be fulfilled. The presence of both an id and an error property uniquely identifies error messages, which must conform to the following schema.

object {
  integer id;
  string error;
  string message?;
};

id the integer id from the request to which this error is a response.

error a textual error code which may be both a visual hint to developers as well as meaningful programatically.

Responses

Responses are sent when the action (or method) specified in a request is complete.

object {
  integer id;
  any result?;
};

id the integer id from the request to which this messge is a response.

result can be any data that is possible to represent in JSON. The precise contents are method dependent and are documented in a subsequent section of this document.

Notifications

Notifications are different from the other 4 message types in that they stand alone. Notifications are not required to have any response at the protocol level, and typically deliver information about asynchronous events. Notification messages must conform to the following schema:

object {
  string method;
  any params?;
};

method is a required method name, indicating the nature of the notification.

params can be any data that is possible to represent in JSON. The precise contents are method dependent and are documented in a subsequent section of this document.

Connection Setup

When a channel is first established the two endpoints become ready at different times. "Readiness" in this case is the act of establishing an event listener to receive messages and setting up whatever application level structures are required to handle messages.

To deal with this race condition, a simple application level handshake is employed. Each endpoint must obey the following:

  • Once ready, each endpoint should emit a "ping" notification.
  • Upon receipt of a "ping" notification an endpoint should assume that the other endpoint is ready and return a "pong" notification.
  • Upon receipt of a "pong" notification an endpoint should assume that the other endpoint is ready.

In the above set of rules a ping notification is simply a notification with method name "__ready", and the string "ping" as its param value. A "pong" notification is identical, but with "pong" as its param value.

The following is a typical message flow at message startup:

>> { "method": "conduit::__ready", "params": "ping" }
(message lost)
<< { "method": "conduit::__ready", "params": "ping" }
>> { "method": "conduit::__ready", "params": "pong" }
(application handshake complete)

All About Message IDs

XXX

Method names and scoping

XXX

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