The websocket protocol x-afb-ws-json1

The WebSocket protocol x-afb-ws-json1 is used to communicate between an application and a binder. It allows access to all registered apis of the binder.

This protocol is inspired from the protocol OCPP - SRPC as described for example here: OCPP transport specification - SRPC over WebSocket.

The registration to the IANA is still to be done, see: WebSocket Protocol Registries

This document gives a short description of the protocol x-afb-ws-json1. A more formal description has to be done.

Architecture

The protocol is intended to be symmetric. It allows:

  • to CALL a remote procedure that returns a result
  • to push and receive EVENT

Messages

Valid messages are made of text frames that are all valid JSON.

Valid messages are:

Calls:

[ 2, ID, PROCN, ARGS ]
[ 2, ID, PROCN, ARGS, TOKEN ]

Replies (3: OK, 4: ERROR):

[ 3, ID, RESP ]
[ 3, ID, RESP, TOKEN ]
[ 4, ID, RESP ]
[ 4, ID, RESP, TOKEN ]

Events:

[ 5, EVTN, OBJ ]

Where:

Field Type Description
ID string A string that identifies the call. A reply to that call use the ID of the CALL.
PROCN string The procedure name to call of the form “api/verb”
ARGS any Any argument to pass to the call (see afb_req_json that returns it)
RESP any The response to the call
TOKEN string The token in case of refresh
EVTN string Name of the event in the form “api/event”
OBJ any The companion object of the event

Below, an example of exchange:

C->S:   [2,"156","hello/ping",null]
S->C:   [3,"156",{"response":"Some String","jtype":"afb-reply","request":{"status":"success","info":"Ping Binder Daemon tag=pingSample count=1 query=\"null\"","uuid":"ec30120c-6997-4529-9d63-c0de0cce56c0"}}]

Future

Here are the planned extensions:

  • add binary messages with cbor data
  • add calls with unstructured replies

This could be implemented by extending the current protocol or by allowing the binder to accept either protocol including the new ones.

Javascript implementation

The file AFB.js is a javascript implementation of the protocol.

Here is that code:

/*
 * Copyright (C) 2017, 2018 "IoT.bzh"
 * Author: José Bollo <jose.bollo@iot.bzh>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
AFB = function(base, initialtoken){

if (typeof base != "object")
	base = { base: base, token: initialtoken };

var initial = {
	base: base.base || "api",
	token: base.token || initialtoken || "HELLO",
	host: base.host || window.location.host,
	url: base.url || undefined
};

var urlws = initial.url || "ws://"+initial.host+"/"+initial.base;

/*********************************************/
/****                                     ****/
/****             AFB_context             ****/
/****                                     ****/
/*********************************************/
var AFB_context;
{
	var UUID = undefined;
	var TOKEN = initial.token;

	var context = function(token, uuid) {
		this.token = token;
		this.uuid = uuid;
	}

	context.prototype = {
		get token() {return TOKEN;},
		set token(tok) {if(tok) TOKEN=tok;},
		get uuid() {return UUID;},
		set uuid(id) {if(id) UUID=id;}
	};

	AFB_context = new context();
}
/*********************************************/
/****                                     ****/
/****             AFB_websocket           ****/
/****                                     ****/
/*********************************************/
var AFB_websocket;
{
	var CALL = 2;
	var RETOK = 3;
	var RETERR = 4;
	var EVENT = 5;

	var PROTO1 = "x-afb-ws-json1";

	AFB_websocket = function(on_open, on_abort) {
		var u = urlws;
		if (AFB_context.token) {
			u = u + '?x-afb-token=' + AFB_context.token;
			if (AFB_context.uuid)
				u = u + '&x-afb-uuid=' + AFB_context.uuid;
		}
		this.ws = new WebSocket(u, [ PROTO1 ]);
		this.url = u;
		this.pendings = {};
		this.awaitens = {};
		this.counter = 0;
		this.ws.onopen = onopen.bind(this);
		this.ws.onerror = onerror.bind(this);
		this.ws.onclose = onclose.bind(this);
		this.ws.onmessage = onmessage.bind(this);
		this.onopen = on_open;
		this.onabort = on_abort;
	}

	function onerror(event) {
		var f = this.onabort;
		if (f) {
			delete this.onopen;
			delete this.onabort;
			f && f(this);
		}
		this.onerror && this.onerror(this);
	}

	function onopen(event) {
		var f = this.onopen;
		delete this.onopen;
		delete this.onabort;
		f && f(this);
	}

	function onclose(event) {
		for (var id in this.pendings) {
			try { this.pendings[id][1](); } catch (x) {/*TODO?*/}
		}
		this.pendings = {};
		this.onclose && this.onclose();
	}

	function fire(awaitens, name, data) {
		var a = awaitens[name];
		if (a)
			a.forEach(function(handler){handler(data);});
		var i = name.indexOf("/");
		if (i >= 0) {
			a = awaitens[name.substring(0,i)];
			if (a)
				a.forEach(function(handler){handler(data);});
		}
		a = awaitens["*"];
		if (a)
			a.forEach(function(handler){handler(data);});
	}

	function reply(pendings, id, ans, offset) {
		if (id in pendings) {
			var p = pendings[id];
			delete pendings[id];
			try { p[offset](ans); } catch (x) {/*TODO?*/}
		}
	}

	function onmessage(event) {
		var obj = JSON.parse(event.data);
		var code = obj[0];
		var id = obj[1];
		var ans = obj[2];
		AFB_context.token = obj[3];
		switch (code) {
		case RETOK:
			reply(this.pendings, id, ans, 0);
			break;
		case RETERR:
			reply(this.pendings, id, ans, 1);
			break;
		case EVENT:
		default:
			fire(this.awaitens, id, ans);
			break;
		}
	}

	function close() {
		this.ws.close();
		this.ws.onopen =
		this.ws.onerror =
		this.ws.onclose =
		this.ws.onmessage =
		this.onopen =
		this.onabort = function(){};
	}

	function call(method, request, callid) {
		return new Promise((function(resolve, reject){
			var id, arr;
			if (callid) {
				id = String(callid);
				if (id in this.pendings)
					throw new Error("pending callid("+id+") exists");
			} else {
				do {
					id = String(this.counter = 4095 & (this.counter + 1));
				} while (id in this.pendings);
			}
			this.pendings[id] = [ resolve, reject ];
			arr = [CALL, id, method, request ];
			if (AFB_context.token) arr.push(AFB_context.token);
			this.ws.send(JSON.stringify(arr));
		}).bind(this));
	}

	function onevent(name, handler) {
		var id = name;
		var list = this.awaitens[id] || (this.awaitens[id] = []);
		list.push(handler);
	}

	AFB_websocket.prototype = {
		close: close,
		call: call,
		onevent: onevent
	};
}
/*********************************************/
/****                                     ****/
/****                                     ****/
/****                                     ****/
/*********************************************/
return {
	context: AFB_context,
	ws: AFB_websocket
};
};